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Here Come the Geminids

The first members of this year's Geminid meteor shower are now beginning to appear. Active from December 6-17, but with a slow rise to maximum, the Geminids are currently the richest of the regular annual meteor showers, producing an abundance of bright meteors, with rates outstripping those of the August Perseids for a 24-hour interval centred on their 14 December maximum.

The Geminid shower radiant (at RA 07h 33m,  Dec +32o, just north of the first magnitude star Castor) rises early in the evening and reaches a respectable elevation above the horizon (> 40o) well before midnight, so observers who are unable to stay up late can still contribute very useful watches, although this year the peak occurs over a weekend which may be more convenient for many.

While it is true that the peak of the shower this year occurs only two days after Full Moon, and that at maximum the Moon will be on the Gemini/Cancer border just below Pollux, one should not look directly at the radiant itself to best observe the shower and Geminid meteors may appear in any part of the sky.  So visual observers may mitigate the effects of bright moonlight by positioning themselves so the Moon is behind them and hidden behind a wall or other suitable obstruction.
 
The shower maximum is rather broad and the early morning hours of Saturday, 14th and Sunday, 15th December are likely to yield the greatest Geminid activity for observers in the UK, when the radiant is high in the sky and rates are climbing towards the peak around dawn.  December nights can be quite chilly, especially in the early morning hours, so wrap up well with plenty of layers of warm, dry clothing and make sure that you wear a hat, gloves, thick socks and sensible waterproof footwear.

In spite of the moonlight, many Geminids are quite bright and past observations have shown that such meteors become more numerous some hours after the rates have peaked, a consequence of particle-sorting in the meteoroid stream.  Given that the peak is predicted for around 14hrs on December 14, this would favour the night of December 14/15 (Saturday night into Sunday morning) for observing the brightest members of the shower.

Geminid meteors enter the atmosphere at a relatively slow 35 kilometres per second, and thanks to their robust (presumably more rocky than dusty) nature tend to last longer than most in luminous flight. Unlike swift Perseid or Orionid meteors, which last only a couple of tenths of a second, Geminids may be visible for a second or longer, sometimes appearing to fragment into a train of ‘blobs’. Their low speed and abundance of bright events makes the Geminids a prime target for imaging, but individual exposures will have to be kept very short this year to prevent fogging by moonlight.

The Geminid shower has grown in intensity over the past 50 years as a result of the stream orbit being dragged gradually outwards across that of the Earth. A consequence is that we currently encounter the most densely-populated parts of the stream. This happy situation is unfortunately only temporary – in a few more decades, Geminid displays can be expected to diminish in intensity. Here we have an excellent opportunity to follow, year on year, the evolution of a meteoroid stream.

The BAA’s visual meteor report forms, available as downloads in both pdf and Excel formats, enable observers to record the details of each meteor seen. These include: time of appearance (UT); apparent magnitude (brightness); type (shower member, or random, ‘background’ sporadic); constellation in which seen; presence and duration of any persistent train. Other notes may mention flaring or fragmentation in flight, or marked colour. Watches should ideally be of an hour’s duration or longer (in multiples of 30 minutes). Observers are reminded to carefully record the observing conditions and the stellar limiting magnitude. Wrap up warmly and enjoy what should be a great show!

By whatever means you observe the Geminids this year, please submit your results to the BAA Meteor Section via meteor@britastro.org.

Dr John Mason
Director, BAA Meteor Section
email: docjohn@dircon.co.uk


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BAA Articles

Observer’s Challenge - Conjunction of Venus and Saturn

This months Observers' Challenge is the conjunction of Venus and Saturn that will occur on December 11th.

This will be a challenging target for three main reasons:

1. It will be visible for just a short time after sunset.
2. The pair will be very low in Sagittarius in the South-West. From Greenwich at 5pm Venus' altitude will be a mere 5.66 degrees, and will have set by 5:45 pm. You will need a good horizon.
3. There is a big disparity in the brightness of the two planets. Venus will be shining at around magnitude -3.9 (extincted to -2.69), and Saturn will be 0.58 (extincted to 1.59)

For the visual observer the brightness difference will not be too much of a hindrance, though Saturn may be easier to spot with binoculars against the twilight sky.

Photographers will find this rather more challenging though if using a single exposure. To make a nice photo try finding a location with some pictorial content in the foreground.

Please do submit any observations to your BAA Member Page and to the Mercury and Venus Section and Saturn Section.

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BAA Tutorials Intermediate

Filters for visual observing of the Moon and planets

Introduction

If you are new to astronomy and wish to see as much detail as possible on the planets of the solar system, or even if one has been observing solar system objects for some time, it may come as a surprise to find that coloured filters can make a world of difference to your observing clarity. Using filters can revolutionize your observing as coloured filters bring out additional detail from the subtle shadings found on solar system objects. This tutorial builds on the excellent tutorial by Paul G. Abel, and looks in more depth at the filters most commonly used by visual observers of solar system objects.

Many astronomical suppliers provide these filters, and all filters are identified firstly by their colour, and secondly by particular numbers or a # which are known as Wratten numbers. These allow the observer to choose which parts of the spectrum they are going to enhance in order to make planetary and lunar definition stand out. The principle of the filters come from black and white photography in which complementary or “opposite” colours enhance the contrast visible. When juxtaposed, complementary colours make each colour seem more vivid and defined, enabling particular coloured features to stand out against the background hues. So, a red or orange filter will enhance blue features and a blue filter will enhance red features.

The Wratten system was developed in Britain in the early 20th century by Frederick Wratten and Kenneth Mees who founded a company in 1906 that produced gelatin solutions for photography. Mees then developed gelatin filters dyed with tartrazine to produce a yellow filter, but soon developed other colours and a panchromatic process of photography. In 1912 they sold the company to the American company Kodak, with their British offices at Harrow in England and Mees moved to New York to found the Eastman-Kodak laboratories there. In honour of his partner and mentor, Kenneth Mees named the burgeoning number of coloured filters “Wratten” and introduced the complex numbering system that is still in use today. Not all the Wratten filters are suitable for astronomical use, but the main colours are still widely used in visual astronomy and are detailed in this tutorial.

These coloured filters are known as broadband or “longpass” in that they allow a wide range of wavelengths through but block wavelengths above or below a certain point in the electromagnetic spectrum. As the spectrum of visible light lies between 390 and 700 nanometers (nm), with the blue wavelengths being the shortest (~400nm) and the red being the longest (~700nm). Anything with a wavelength range above or below a particular filter will be blocked and increased contrast in compensating colours will be noticed.

Most astronomical suppliers sell complete sets of filters for solar system observing and naturally such sets are known as lunar and planetary filters. They generally have a range from red to blue across the spectrum and cover the broad bandwidths associated with such colours. A typical set will include a neutral density filter for lunar observing and a No. 25 red, No.12 yellow and No. 80A blue for as full coverage as possible. A typical filter set is shown here in figure 1. A more extensive set of astronomical filters with typical Wratten numbers can be seen here in figure 2.

This tutorial will introduce each filter and instruct the reader on which targets in the solar system each filter can be used and what features the filters will enhance Keep in mind that visual acuity does vary from observer to observer and that in the dark the sensitivity of the human eye shifts to the blue end of the spectrum. This is due to a phenomenon known as the Purkinje effect, named after the Czech doctor who discovered that the spectral sensitivity of the human eye does not enable red light to be seen clearly in the dark, but shorter blue wavelengths are detected.

Technical aspects of Filters
Filters can be separated into a few main groups that enable enhancement, lessened contrast or can be used for colour shift or balance. Colour subtraction filters work by absorbing certain colours of light, letting the remaining colours through. They can be used to demonstrate the primary colours that make up an image or can be seen in the features of our planetary neighbours. A colour correction filter makes a scene appear more natural by simulating the mix of colour temperatures that occur naturally, and subtly enhancing the middle ranges of the spectrum.

In addition to these filters, there are also colour temperature filters. Some filters change the correlated colour temperature of a light source. They can change the appearance of light from a bright white source so that it looks more yellow and natural to the eye. The term colour temperature comes from the natural phenomenon of coloured light emitted by warm objects. Warm objects, such as a flame from a fire, emit deep red and orange light. The temperature of such flames are roughly 1500K. If you increase that temperature the light emitted begins to look more blue as its wavelength changes to the shorter (hotter) or blue end of the spectrum.

Of course, optical filters don’t really change the temperature of the object emitting the light. Colour temperature filters simply remove some of the light of wavelengths of our choosing so we can absorb or reflect away some of the orange and red light emitted by the planets. This makes the remaining light look more blue and therefore has a higher colour temperature. Conversely, some filters can remove some of the blue light emitted by a planet, making the remaining light look more orange and thus apparently emitting a lower colour temperature.

Wratten filters and their uses
In the following tutorial, I have grouped the filters under their colour designation rather than put them in number order, as the colour of each filter is their most obvious feature when using them. All of these filters are available to purchase in 37.1mm (1.25”) or 50mm (2”) fittings and are commonly available from astronomical suppliers. For a fuller description of Wratten filters, please follow this link: https://en.wikipedia.org/wiki/Wratten_number. In this tutorial, only those filters useful to astronomers will be described.

One question commonly asked is "do filters block out too much light and make observing more difficult or less enjoyable?" It is true that filters do block out some light, but I hope you will see from this tutorial that by selectively blocking out certain wavelengths of light, and by altering the contrast of any surface features, the observer is often able to resolve finer or more subtle detail. In fact, in the case of bright objects the reduction in light transmission is an advantage. Let us examine this a little more technically.

The difference in contrast between the belts and zones on an object such as Jupiter can be so small that the human eye and brain just smear the whole and it can be difficult to discern details without a filtered system. Because Jupiter is a very bright object seen against a dark background, the differences in intensity of reflected light from light/dark zones on such planets is not really seen to advantage by the human eye.

Contrast in any system can be measured using the formula:

C = (b2 - b1) ÷ b2

Where C is the contrast and b1 and b2 are different areas of brightness on the surface of a planet. Bright areas on Jupiter have an intensity of 6 lumens  m-2 and the intensity of the darker zones have an intensity of 3 lumens m-2. This would give:

(6 – 3) ÷ 6 = 0.5

or a visual contrast 50% lower in the darker zones than in the brighter zones. A filter will enhance the contrast by permitting wavelengths representative of the redder or darker zones through whilst diminishing the blue contrast on the brighter zones. Surely a filter that would aid in the perception of subtle features is going to be a bonus to any observer?

This tutorial will convincingly show that the use of filters, despite their decrease in light transmission is actually very useful in visual astronomy. The use of filters assists primarily in enhancing contrast initially and although the reduction in light transmission is generally not favoured in astronomy, this is one area in which this general rule need not apply.

No. 25 Red
The No. 25 filter reduces blue and green wavelengths, which when used on Jupiter or Saturn, result in well-defined contrast between some cloud formations and the lighter surface features of these gas giants. However, it needs to be used judiciously as the light transmission is only 15% but for such bright planets this filter will enhance the observed detail even when used with small telescopes. This filter blocks light shorter than 580nm wavelength. This filter is also sometimes referred to as a Wratten 25A.

No. 23A Light Red
This is a good filter for use on Mars, Jupiter, and Saturn, and has proved useful for daylight observations of Venus as it has a 25% light transmission. The light red is an “opposite” colour to blue and therefore darkens the sky very effectively in daylight. Some astronomers report that it also works well on Mercury, but I would not recommend viewing this planet in general during daylight due to its proximity to the Sun. This filter blocks wavelengths of light shorter than 550nm.

No. 21 Orange
This orange filter reduces the transmission of blue and green wavelengths and increases contrast between red, yellow and orange areas on planets such as Jupiter, Saturn and Mars. It brings out the glories of the Great Red Spot on Jupiter very well under conditions of good seeing with a medium magnification (e.g. x100). It also blocks some glare from the bright planet and provides less of a contrast between a planet and the black background of space. A good all round planetary filter as it transmits about 50% of the light and blocks wavelengths short of 530nm.

No. 8 Light Yellow
This filter can be used for enhancing details in red and orange features in the belts of Jupiter. It is also useful in increasing the contrast on the surface of Mars, and can under good sky conditions aid the visual resolution on Uranus and Neptune in telescopes of 250mm of aperture or larger. The No. 8 cuts down glare from the Moon and works much better than the “moon filters” included with some cheaper telescopes. This filter allows 80% of the light through but blocks light short of 465nm.

No. 12 Yellow
This filter works on the principle of opposites described above, blocking the light in the blue and green region and making red and orange features on Jupiter and Saturn stand out clearly. Deeper in colour than the No. 8 filter, it is the filter most astronomers recommend for visual work on the gas giants. It has a 70% light transmission and cancels some of the glare on Jupiter when seen against a dark background sky. It blocks visible wavelengths short of 500nm.

No. 15 Deep Yellow
This filter can be used to bring out Martian surface features, especially the polar caps and can be used to bring out detail in the red areas of Jupiter and Saturn. Some astronomers also have reported some success using this filter to see low-contrast detail on Venus. I have used this filter on Venus during the day to add more contrast to the image and it generally works well. This filter is particularly useful for visual observations of Venus as it is a very bright object and the filter can considerably reduce the glare of this very bright planet in evening or morning apparitions despite its 65% light transmission. The No 15 blocks light short of 500nm.

Although at this point it may feel like every filter suits Jupiter and Saturn, the variegated nature of their surfaces and their extreme brightness at opposition or during favourable apparitions enables a wide range of filters to bring out different details. Some of the details may be subtle, but can be explored better by an experienced observer equipped with a range of filters.

No. 11 Yellow-Green
This darker filter is a good choice to enable the observer to directly see surface details on Jupiter and Saturn. It can also be useful on Mars if you are using a large aperture telescope in the 250mm range. At times of steady atmospheric seeing, this filter darkens the surface features and makes areas such as Acidalia and Syrtis Major stand out and the polar caps and occasional features such as clouds appear quite marked. The No. 11 filter allows 75% light transmission can be used to darken some features on the Moon.

No. 56 Light Green
I have used this filter for observing the ice caps of Mars during its close encounter in 2003 and found that despite the low altitude of Mars from the UK during that apparition the filter worked well in bringing out these features and even hinted at rocky features on the planet’s surface during periods of clear seeing. I have to admit that the orange No 21 filter did work surprisingly well in rendering colour and detail on the red planet, but the contrast with the No 56 filter was quite good. This filter allows most wavelengths through but does have a peak around 500nm.

With its 50% light transmission this filter is a favourite of lunar observers as it increases the contrast while reducing the glare. It is also a filter that is well tuned to the wavelengths of the human eye and the greenish cast can almost be ignored during visual observation. This is a colour correction filter with all wavelengths equally affected. The effect can be seen on the first quarter moon in figure 3 photographed here in ordinary white light and then through the Wratten No 56 filter.

No. 58 Green
This filter blocks red and blue wavelengths of light and many observers find that it slightly increases contrast on the lighter parts of the surface of Jupiter. I have also used it on Venus where it does add to the contrast and reduces glare a little but it must be admitted that it is not easy to visualize any detail in the clouds.

The No 58 filter has a 25% light transmission, and it is a colour correction filter rather than a longpass. Such filters alter the colour temperature of the incoming light, enhancing contrasting colours in the object by allowing specific wavelengths through that correspond to the temperature of the light. This is a complex subject but to put it simply, the colour of light not only corresponds to particular wavelengths but also to particular colours where blue is cooler and red is hotter. Note that this is more of a perception than anything else as in reality blue light is “hotter” (has a higher frequency and shorter wavelength) than red light.

No. 82A Light Blue
This is almost a multipurpose filter as it does enhance some features on Jupiter, Mars and Saturn and also works very well in enhancing some features on the Moon (figure 4). It is commonly referred to as a “warming” filter that increases the colour temperature slightly and allows the red wavelengths through due to the complementary colours that we discussed above. With a light transmission of 75% it can be used on any aperture telescope and can even make some difference to deep sky objects such as M42 and M8 though the effects can be quite subtle. 

No. 80A Blue
Although this is quite a dark filter, it is as versatile as the No 82A in that it enhances features in the red on planets such as Jupiter, Saturn and Mars. It is also good for lunar observation as it reduces the glare and provides good contrast for some features such as ejecta blankets, ray systems and lava fronts. Some astronomers report success in its use on binary star systems with red components such as Antares and α Herculis as the contrast enables the observer to split the two components well. The No. 80A filter has a 30% light transmission and also acts as a colour conversion filter enhancing wavelengths around 500nm.

No. 38A Dark Blue
Again, a good filter to use on a planet such as Jupiter because it blocks red and orange wavelengths in such features as the belts and in the Great Red Spot. Some astronomers report that it also adds contrast to Martian surface phenomena, such as dust storms, and makes a better contrast for observing the rings of Saturn. Try using it for observations of Venus as some report that using this filter increases the contrast, leading to the visual observation of some dusky cloud features. This filter has about a 15% light transmission. It absorbs red, green and UV light and is commonly referred to as a minus green, plus blue filter. The difference can be gauged in figure 5.

No. 47 Violet
A very dark filter which strongly blocks the red, yellow, and green wavelengths. I would highly recommend it for Venus observation due to its low light transmission of about 5% providing great contrast and enhancing cloud features. Try using it on the Moon to decrease the glare when observing features at a 10-12 day old phase. Some observers report that features in the Schroeter Valley and Aristarchus crater are clearer due to the lack of glare. Recommended for the Moon, especially if you are using a large aperture telescope! This is another colour separation filter that enhances the blue or shorter wavelengths of the spectrum at 450nm.

Additional filters that also are helpful in visual observing are the polarizing filter and the neutral density filter. Both are longpass filters that usually transmit all wavelengths of light but can cut down on glare and contrast.

Non Wratten Filters
Neutral Density Filters
A neutral density (ND) filter transmits light uniformly across the entire visible spectrum and is an excellent filter to use to reduce glare in such objects as the Moon and planets, but especially the Moon. Due to its bright glare many lunar and planetary astronomers keep an ND filter on their favourite eyepiece and add on other filters as necessary. Neutral density filters come in a variety of densities that reduce the glare in the image based upon the amount of light transmission each ND filter allows. Commonly they come in numbers such as 50, 25 and 13 that signify the amount of light they transmit; 50%, 25% or 13%.

Polarizing filters
Although it does not work at any specific wavelength, the polarizing filter allows light of any wavelength through but blocks those with random scattering patterns allowing only light waves in a flat “plane” through, which has the effect of increasing the contrast, reducing glare and slightly enhancing the saturation of colour in an object. Such filters are very useful on bright objects such as the Moon and some planets.

Neodymium Filter
The Neodymium filter is an interesting addition to the filter armoury as it filters the yellow light of the spectrum, rendering most objects a faint blue colour. It is useful for observations of Venus and for Jupiter and Saturn too. Some astronomers report that this is a useful filter for observing in light polluted areas as it cuts through sodium light pollution somewhat, although it is not as effective as a Light Pollution Reduction filter.

Summary

Many planetary observers rely on filters and they report that they really do make a difference in seeing faint details. Filters also reduce the glare of objects like Jupiter, Saturn and the Moon and render a better contrast between their sunlit surface and the dark background sky.

Observers can also be affected by a phenomenon known as prismatic or atmospheric dispersion. This is most evident when a star or planet is seen near the horizon. It results from atmospheric refraction occurring less for the longer wavelength red light where the red appears clearer nearer the horizon and the light shifts to the violet toward the zenith. This is the reason that most astronomers prefer to observe an object when it is near or at culmination (the highest point in the sky as seen from an observers latitude) so that this effect is lessened. Use of red or blue filters on an ascending object may make the difference between seeing details such as the Great Red Spot for example.

I hope that this tutorial shows that coloured filters are a very useful tool in visual astronomy. Using such, I hope that this tutorial gives the reader some tips on which filters to use to observe any of the planets of the solar system and our moon. Most features on planetary surfaces may be quite subtle and filters can make a great difference between seeing or recording a feature or missing it completely in the sky background. For more information on using filters for visual observing or for astrophotography, please see my book Choosing and Using Astronomical Filters.

Martin Griffiths

Martin is the Director of the Brecon Beacons Observatory and an astronomer at Dark Sky Wales

[The graphical plots in this tutorial have been prepared using copyright free spectroscopic data]

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BAA Observing Sections Comet

Comet 21P approaches the North America Nebula

It will be a challenging observation due to midsummer skies and a First Quarter Moon but the periodic comet 21P/Giacobini-Zinner will be passing the bright emission nebula NGC7000 (the North America Nebula) in Cygnus between June 19 and June 21. The comet is currently around 13th magnitude but will brighten over the summer to become a potentially 6th magnitude object by September. At present the comet is a small fuzzy spot with a short tail to the south west and it will be completely overwhelmed by the large nebula but it will be interesting to compare the two objects. A chart showing the encounter is here. Please send any observations to the Comet Section.

The image at left shows the comet on the morning of 2018 June 13. The field of view is around 11 arcmin square. More images of this comet are available in the Section's archive here.

Update on 2018-06-18 - Here is an image of the comet approaching the nebula taken from New Mexico.

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BAA Tutorials Starting out

Noctilucent Clouds – a beginners guide

Noctilucent clouds (popularly referred to by the abbreviation “NLC”) are high atmosphere clouds which occur over summertime at mid latitude locations. They form at very high altitudes – around 82 km above sea level – and are, thus, a quite separate phenomena from normal weather or tropospheric cloud. They appear as thin streaks of “cloud”, often a pearly-blue colour, reminiscent of “mares-tail” cirrus cloud formations.

NLCs can be seen from around mid May to early August during the darkest part of a summer’s night when the Sun is between 6 – 16 degrees below the horizon. Typically, they will occupy the northern horizon, along the twilight arch, extending to an altitude of 10 – 15 degrees. Over the NLC “season” the bright star Capella dominates this part of the sky and serves as a good marker for the NLC observer. They used to be associated with northern UK but have been seen as far south as central France and they seem to be spreading further south with each season.

Observations of NLC remain of great value to professional scientists studying upper-atmosphere phenomena. Useful observations are very easy to make and require no special equipment.

The following information lists the important details you should include in your report:

LOCATION: Give the latitude and longitude of the place observations were made. Alternatively, give the name of the nearest town or city.
DATE: Use the “double-date” convention as used in reporting aurorae. That is, “June 21-22″ would refer to the night of the 21st and the early hours of the 22nd.
TIME: Try to use universal time (UT) even though British Summer Time (BST) will be in civil use for UK observers. Remember, UT = GMT = (BST – 1 hour).

Observations
The following features and details should be recorded at 15 minute intervals (i.e. on the hour, quarter past, half past and so on):

AZIMUTHS If you see NLC measure the left (western) and right hand (eastern) extent of the display. This is measured in degrees with west = 270, north = 000, east = 90 and south = 180. Polaris defines the northern point of your horizon. Azimuths can be gauged by using a clenched fist, held at arms length, as a measure of 10 degrees.
ELEVATION If possible, measure the angle subtended by the uppermost part of the display. A simple alidade can be made from a protractor and plumb line for this purpose.
BRIGHTNESS NLC brightness is measured on a three point scale with 1 = faint; 2 = moderate; 3 = very bright.

Structure
NLC forms are classified into 5 easily identified structures. Any combination of the following is possible:

Type 1: Veil – A simple structureless sheet, sometimes as background to other forms.
Type 2: Bands – Lines or streaks, parallel or crossing at small angles.
Type 3: Waves – Fine herring-bone structure like the sand ripples on a beach at low tide. Very characteristic of NLC.
Type 4: Whirls – Large-scale looped or twisted structures.
Type 5: Amorphous – Isolated patches of NLC with no definite structure.

Drawings
Simple sketches of the NLC can be very useful. These are best made in negative form with the darker parts of the sketch corresponding to the brighter NLC.

Photography
Photographs of NLC can easily be taken with a digital camera firmly fixed to a tripod; using 400 ISO gives good results. An exposure of 3-6 seconds with a lens aperture setting of f3.4 will normally suffice. However, it is always best to take several shots of different exposures, and pick the best exposure. Once this is done you can try a panorama by taking several overlapping photos. Make sure the camera is level, then move it about 20 degrees after each shot, starting just beyond one end. This makes sure that you will get it all, because the camera will see more than you can.

Sandra Brantingham

Sandra is the Director of the Aurora and Noctilucent Cloud Section of the BAA.

A selection of observatorions of noctilucent clouds which BAA members have uploaded to their Member pages can be found here.

[Thumbnail image by Gordon Mackie]

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BAA Gallery Solar
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Sun_Samworth_091219 g10_08_36

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About this observation
Observer
Roger Samworth
Time of observation
09/12/2019 - 10:08
Object
Sun
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Copyright of all images and other observations submitted to the BAA remains with the owner of the work. Reproduction of the work by third-parties is expressly forbidden without the consent of the copyright holder. For more information, please contact the webmaster.
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About this observation
Observer
Roger Samworth
Time of observation
08/12/2019 - 10:41
Object
Sun
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BAA Tutorials Starting out

The Beginner's Sky

Introduction
The night sky seen looking over the Solent (image courtesy Andrew Paterson).Go out on a dark, cloudless night, preferably away from any bright sources of artificial light and look up at the sky. What do you see? The Moon perhaps, stars certainly, some bright some faint. Maybe one of them is a planet but which one and what is the difference between a star and a planet anyway? A brief flash of light streaks across the sky – a meteor! What is it? Where did it come from? Towards the west a point of light moves slowly across the sky before fading in the east, an artificial satellite, perhaps the International Space Station.  Are there other things in the night sky that can only be seen with a telescope? What about unpredictable phenomena such as bright comets and aurorae?

This article is a brief guide for the curious beginner to the inhabitants of the night sky and seeks to answer these questions.


The stars
The Sun imaged through a specialist, safe, solar filter showing a row of sunspots (image courtesy James Dawson).There is one star that can never be seen at night – the Sun, for indeed the Sun is just another star, basically similar to all the others in the night sky but more impressive because it is many times closer. It lights up our Solar System and is the ultimate energy source of virtually all life on Earth.

Although seemingly a perfect unblemished disk, the Sun’s surface can be marked by cooler areas, sunspots, whose numbers wax and wane on a roughly 11 year cycle.

WARNING: Never stare at the Sun either with or without a telescope, binoculars or other optical aid. Severe eye damage or even blindness can result.

Examining our star more closely we find it is a hot ball of gas, mostly hydrogen, with a surface temperature of about 6000oC and a diameter of nearly 1,400,000km. At its centre nuclear reactions are fusing hydrogen into helium and releasing huge amounts of energy while doing so. The Sun has been shining for 4.6 billion years. After another 5 billion years the Sun’s character will change, swelling up to become what is known as a Red Giant so large that it will likely engulf the Earth’s orbit.

As stars go, the Sun is very ordinary and run of the mill. The one thing that distinguishes it from the other stars for us is distance. Our Sun is ‘only’ 149,600,000km away. The next nearest star in the night sky is over 40,000,000,000,000km distant or over 267,000 times further away. Most stars are much further away than this. Once we start talking about distances to the stars these numbers become very big indeed and we need something better to use than kilometres. Astronomers often use the term ‘light year’, the distance light travels in one year or 9,500,000,000,000km. So the nearest star to Earth is 4.25 light years distant.

The stars come in many different levels of apparent brightness. (Image courtesy Andrew Paterson).Looking at the stars in the night sky one thing is clear; they come in all shades of brightness from the very obvious to those at the very limit of visibility. Why is this? Are some stars truly brighter than others or do they just look brighter because they are nearer to us? The answer is both. The closest star to us, Proxima Centauri, is 4.25 light years distant and has an absolute brightness of only 0.005% that of the Sun. Sirius, the apparent brightest star in the sky is 8.6 light years distant and 24 times the brightness of the Sun. Lastly consider Deneb the brightest star in the constellation of Cygnus the swan. This is roughly 2,600 light years distant and has a brightness of perhaps 196,000 suns!

The stars vary greatly in size too. Proxima Centauri is only one third of the Sun’s diameter, Sirius 3.5 times larger and Deneb a full 410 times.


The constellations
The stars of the night sky are divided up into patterns known as constellations (Stellarium).Thousands of years ago our ancestors looked up at the sky and tried to make sense of what they saw. Cultures around the world drew on their myths and legends and the environment around them to join the dots of the stars and turn them into patterns with meaning to them – the constellations. The constellations we know today, visible from temperate northern latitudes, come down to us from the classical worlds of ancient Greece and Rome with many originating in Mesopotamia as long as five thousand years ago. The constellations of the southern sky, not visible from Europe, were created as European voyages of discovery brought knowledge of southern stars to northern people. While we continue to use the European names, indigenous peoples invented their own patterns and names thousands of years ago many of which have been passed on and continue to be used in parallel.

It is important to remember that in most cases the patterns are entirely arbitrary and there are no actual relationships between the included stars. Indeed the stars are usually at completely different distances to each other.

Learning the constellations and finding your way around the night sky can be an interesting project and if you would like to give it a go then a pair of linked tutorials starting here may be of help.

The turning sky
The summer Milky Way. (Image courtesy James Dawson).Just as the Sun moves across the sky, rising in the east and setting in the west, so do the Moon, stars and planets. The cause of this is simple, the turning of the Earth on its axis. More than this, as the Earth moves around the Sun in its orbit, so different patterns of stars become visible from season to season. In winter, looking south, we find the constellation of Orion the mighty hunter. With the spring comes Leo the lion. Summer finds Cygnus the swan nearly overhead and the autumn skies feature Pegasus the flying horse. Of course, these are just the most obvious constellations of each season, there are many others all around.

The Milky Way
Go outside on a dark clear night in late summer or autumn, look up at the sky and you should see a dim band of light stretching from overhead down towards the south or south western horizon. This is the Milky Way. Seen through a good pair of binoculars or a telescope it is resolved into innumerable faint stars. What we are seeing here is a view of our home galaxy, a giant star city at least 100,000 light years in diameter and containing something like 100 billion stars. Its shape is that of a flat disk with a central bulge and spiral arms. The Sun is embedded in the disc part way from the centre to the edge. From this position looking along the line of the disk we see countless stars. At right angles to this we are looking out of the plane of the galaxy into the spaces beyond and see far fewer stars.

Clusters and nebulae
Is our galaxy just composed of isolated stars or does it have other occupants? The answer is yes it does. Many stars are not alone in space but exist as doubles, triples or even more. Beyond this there are also real clusters of stars. There are two types, open clusters and globular clusters. A good example of an open cluster is the Pleiades or Seven Sisters, visible in the eastern part of the evening sky in autumn as a hazy spot. People with good eyesight can separate this haze into individual stars and with binoculars it is a splendid sight. Open clusters are generally found not far from the plane of the galaxy hence their alternate name of galactic clusters.
The Pleiades; an example of an open cluster (Image courtesy Graham Roberts). The globular cluster known as M13 containing perhaps 300,000 stars (Image courtesy Geof Lewis). The Orion Nebula (Image courtesy Callum Scott Wingrove).
The second type of cluster is the globular cluster. These are giant balls of stars with numbers running into hundreds of thousands. They form a halo around the outside of our galaxy.

Lastly there are the nebulae, clouds of gas or dust. These may become visible either by reflecting the light of nearby stars, being excited to glow themselves by stellar radiation or appear as dark clouds silhouetted against a bright starry background. Looking at the picture of the Pleiades, the hazy patches are clouds of dust. The best known gaseous nebula is the Great Nebula in Orion. Roughly 24 light years across, new stars are being born there.

The wider universe
The galaxy M31 in Andromeda (Image courtesy Callum Scott Wingrove).We live in a galaxy composed of stars, gas and dust. A reasonable question is whether that is the entire universe or is there more? The answer is yes, much more. Our galaxy is just one of perhaps 100 billion in the observable universe. They come in all sizes some much larger than our Milky Way, some much smaller. Not all are grand spirals like ours some are elliptical, some irregular. The nearest large galaxy is known as M31, found in the constellation of Andromeda it is some 2.5 million light years away.

Of course the observable universe is much bigger than this; current ideas place its size at around 92 billion light years across.

The Solar System
Having plumbed the depths of the universe let us now return home to our own backyard and look at the Sun’s family.

Our star is orbited by eight major planets and numerous smaller bodies including minor planets and comets. Several of these bodies are orbited by satellites of their own, their moons. For example our own moon orbits the Earth at an average distance of 384,000km.

The planets are often easily visible. Venus is seen here right of centre with Jupiter at upper right (Image courtesy Andrew Paterson).Before we take a closer look at the solar system’s individual members we can now answer the question posed at the beginning of this article. What is the difference between a star and a planet? A star is a luminous body generating heat and light through the processes of nuclear fusion at its core. A planet however is a much smaller body orbiting our sun and not generating its own light. While thousands of ‘planets’ have now been discovered around other stars these are technically not known as planets but exoplanets. One last point, the only body emitting light in the solar system is the Sun, everything else, planets, moons, comets, etc shine in the sky only by reflecting the Sun’s light. They have no light of their own and were the Sun to be extinguished they too would cease to shine, only the stars would remain to light the sky.

The planets themselves all orbit the Sun in roughly the same plane and as a result appear projected against the background stars in a band of constellations known as the Zodiac. How the planets move is described by Paul Abel here. Many of the planets are visible to the naked eye although some require binoculars or a telescope.

It is important to realise that to the naked eye the planets never appear as disks but as points of light, the images in this article were all made using telescopes.

Should you wish to locate a planet for yourself, the British Astronomical Association regularly publishes “Sky Notes” including details of which planets are visible. In addition there are magazines and software available that will help point you in the right direction.

The inner Solar System
Let’s take a very brief tour of the Solar System. Starting from the Sun the first planet we encounter is tiny Mercury at 4879km in diameter. Being so near to the Sun its temperature can exceed 400oC. Being closer to the Sun than we are, Mercury like Venus alternates between appearances in the evening and morning skies. However because Mercury orbits so close to our star it is never very high up in the morning or evening twilight and can be challenging to find.

Next up is Venus, slightly smaller than our Earth with a diameter of 12,104km. Permanently shrouded in dense clouds, a runaway greenhouse effect has lifted the temperature on its surface to over 400oC. Like Mercury, Venus oscillates back and forth between the morning and evening skies and in classical times it was often believed to be two different objects. Apart from the Moon Venus can be the brightest object in the night sky. It is often visible in the twilight hanging like a lamp in the sky well before any stars are visible. Indeed it can become so bright that at times it is visible in broad daylight.

The Moon seen through a small telescope, note the profusion of craters towards the bottom (Image courtesy John Hughes).The third planet from the Sun is our own Earth, most important to humanity but in reality only a minor body. It has a diameter of 12,756km and is the only planet known to harbour life.

The Earth has one natural satellite, the Moon which orbits the Earth in just under a month. Its changing apparent shape or phases are a caused by this orbital motion. The Moon’s surface is heavily cratered in many places the result of massive bombardments of meteors, comets and asteroids early in its history. There are also darker areas, less cratered, the result of lava flooding low lying areas and solidifying. These dark areas together with the lighter heavily cratered regions combine to create the familiar “Man in the Moon” appearance seen near full moon.

Beyond the Earth we encounter Mars, the red planet. Only slightly more than half the size of the Earth at 6,792km and with a negligible atmosphere it endures temperatures mostly below freezing. Once thought to be the abode of intelligent life, it is now considered most likely sterile.
 

A drawing of Mars and its two moons (Image courtesy Paul Abel).The surface is heavily cratered, with towering inactive volcanoes and numerous valleys, one as long as the United States is wide. There is strong evidence that water once flowed on its surface but that time is long gone. Mars has two tiny moons, Phobos and Deimos considered to be captured from elsewhere in the solar system.

Orbiting the Sun outside of the Earth, Mars can sometimes be visible all night. Occasionally it can be very bright while at other times it appears as a very ordinary star although its marked ruddy colour can give it away.

After Mars we reach the asteroid belt. Here there are a vast number of pieces of rocky debris ranging in size from tiny grains up to the dwarf planet Ceres which is 950km in diameter. The smaller asteroids can be very irregular in shape. One of the asteroids, Vesta, gets bright enough to be seen with the naked eye but then only with difficulty and at infrequent intervals. Binoculars will show many more but because they look like faint stars knowing exactly where to look is vital.

The giant planets
Jupiter with Ganymede to its upper right (Image courtesy Geof Lewis).So far all the objects in the solar system have been rocky worlds but now there is a step change as we reach the realm of the giant planets. Far from having solid surfaces these planets are giant balls of gas possibly overlying a much smaller icy or rocky core.

First up is Jupiter. By far the largest planet in the solar system its diameter is over 11 times larger than that of the Earth. Comprised mainly of hydrogen its atmosphere is in constant turmoil with wind speeds often reaching 360km/hr. Jupiter has an extensive family of moons of which four are bright enough to be seen with binoculars. These four, named Io, Europa, Ganymede and Callisto, were discovered by Galileo when he turned his telescope to the sky in 1609/10. Ganymede is the largest, not only bigger than our own moon but even larger than the planet Mercury!

Jupiter is always bright and is easy to spot as long as it is not too close to the Sun. If you do locate it and have a pair of binoculars then try for the planet’s four brightest moons. They appear as ‘stars’ very close to Jupiter itself. You may not see all of them every time but looking from night to night you will find their positions will have changed.

 

A drawing of Saturn made in 2019 (Image courtesy Paul Abel).Beyond Jupiter we come to Saturn, probably the Solar System’s most instantly recognizable planet with its iconic system of rings. Despite appearances, the rings are not a solid sheet but are comprised of innumerable small particles all orbiting around Saturn. Like Jupiter, Saturn has a large number of moons some large, some small. The largest of these, Titan, is unique in having a substantial atmosphere albeit one comprised mainly of nitrogen. Titan, like Jupiter’s Ganymede is also larger than Mercury.

Saturn is not difficult to find in the night sky if you know where to look as it is moderately bright.

Moving out from Saturn we reach the first of the so called ice giants: Uranus. Uranus is four times larger than the Earth and has a small family of moons although all are faint. The planet is in principle visible to the naked eye however it calls for good eyesight, a very dark sky and a precise knowledge of where to look. A pair of binoculars makes the job much easier.

The last of the major planets is Neptune which is slightly smaller than Uranus and has one major moon, Triton. Neptune is never visible to the naked eye and a pair of binoculars or a telescope is a must as is a good star chart.

Beyond Neptune we come to a belt of icy bodies, the Trans-Neptunian Objects, of which the largest is the former planet Pluto now demoted to the status of a dwarf planet.

Comets
A bright naked eye comet (Image courtesy Peter Anderson).A comet hanging in the sky like a flaming sword can be a most impressive sight but sadly the majority of comets never become this prominent and are only visible as diffuse smudges in a telescope. Most comets are visitors from the far reaches of the Solar System. They swing in on very elongated orbits, pass close to the Sun and then recede once more into the depths of space.

A comet can be described as a ‘dirty snowball’ comprised of ‘dust’ and ices. When far from the Sun in its orbit it is very cold and effectively frozen solid. Closing in on the Sun it warms up and begins to give off gas and dust. It is these emissions that cause the comet to grow and can create a prominent tail depending on the comet and how close it comes to the Sun. Once past the Sun it cools down once more, enters a state of deep freeze and becomes dormant until it next approaches our star.


Meteors
A meteor flashes across the sky near the Plough (Image courtesy Alan C. Tough).If you see a brief flash of light streaking across the sky you have almost certainly seen a meteor. A meteor originates as a particle in size from a tiny grain upwards (a meteoroid) orbiting the Sun whose path intercepts the orbit of the Earth. On encountering our atmosphere heat is generated, this not only affects the meteoroid but also the atmosphere causing it to glow in the meteor’s wake. It is this that we see as a meteor in the sky.

The vast majority of meteoroids are tiny and never reach the Earth’s surface. Occasionally a larger one will survive its passage through the atmosphere and reach the Earth’s surface. The resultant rock is now known as a meteorite.

Many meteors result from the debris shed by comets in their journey around the Sun. When the Earth encounters this trail a meteor shower can result. Two of the best are the Perseids seen every August and the Geminids every December.

Satellites
Up until 1957 the Earth had only one satellite, the Moon. Then, with the launch of Sputnik, the first artificial satellite, everything changed. Satellites in orbit around the Earth now number in thousands and several of these may be visible to the naked eye on any given clear night. Generally they appear as a ‘star’ moving steadily across the sky. Some will flash, many will not. The easiest to see is the International Space Station (ISS) which is usually quite bright. Like many satellites, because of the way the orbit works, there will often be a run of several days when the ISS is visible in the evening sky, then a period of invisibility followed by several days of visibility in the morning sky before sunrise. If you want to see the ISS go to heavens-above.com, enter your location and predictions will be presented for a number of satellites including the ISS. There is also an Android app if you prefer and similar apps are available for Apple devices.

Glows in the sky
The aurora seen from Scotland (Image courtesy Alan C. Tough). Display of Noctilucent Clouds seen from Lossiemouth (Image courtesy Alan C. Tough).
The most famous and spectacular of the glows in the sky is the aurora. Best seen from higher latitudes and reasonably common in northern Scotland it is caused by matter ejected by the Sun at great speed channelled by the Earth’s magnetic field towards the poles and hitting the upper atmosphere causing it to glow in a variety of different colours. On rare occasions the strength of the Sun’s emission is such that the aurora is driven down to lower latitudes and a grand display can result.

A less spectacular but perhaps more predictable glow comes from Noctilucent Clouds (NLCs). These are clouds that form very high in the atmosphere during the summer months and are still illuminated by the Sun even though it had set a considerable time before. NLCs can be identified by their pearly blue colour, their position above the northern section of the horizon and by the fact that if ordinary clouds are present these show up as dark shapes being too low to be lit by the Sun.

Conclusion
If you have reached this far and were a complete beginner you hopefully now know more about the night sky than when you started and possibly more than most people.

Astronomy can be a very fulfilling and lifetime hobby at whatever level you chose. Maybe you just want to be an armchair astronomer and read about the wonders of the sky. Perhaps you aspire to a telescope and see or photograph the wonders of the universe for yourself. All the images in this article with the exception of the Stellarium constellation chart were made by amateur astronomers who are also members of the British Astronomical Association (BAA). Astronomy remains one of the few sciences where dedicated amateurs can still make a real contribution to scientific knowledge if they choose.

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David Basey is an amateur astronomer living in semi-rural East Anglia. Primarily a visual observer he has been scanning the skies for over fifty years. He still remembers the excitement of finding his way around the night sky, not to mention the first sight of Saturn through a telescope. The excitement continues to this day!!

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Mercury

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About this observation
Observer
Leo Aerts
Time of observation
30/11/2019 - 08:11
Object
Mercury
Observing location
Heist op den Berg, Belgium
Equipment
C14 scope
ASI 290MM webcam
Baader Ir filter
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Mercury, at 6”4 apparent diameter.  The planet was at an altitude of 21° and seeing was excellent for the altitude.  Nonetheless, a difficult observation.

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