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Partial Solar Eclipse 10 June 2021: viewing it safely


A partial solar eclipse. Originally taken on 30 May 1984 when there was a partial eclipse visible from London. It has been rotated to simulate what will be seen in the UK on 10 June 2021.
Credit - Robin Scagell - All Rights Reserved


On Thursday, 10 June, the whole of the British Isles will see a partial eclipse of the Sun. These are quite rare, and this one will be a significant event. That morning, the Moon will pass in front of the Sun, its magnitude (the proportion of the Sun’s diameter which is obscured at greatest eclipse) ranging from 30% along the Kent coast to just over 50% in Shetland. For further details do read the article by Peter Macdonald in the April issue of the BAA Journal. In Edinburgh maximum eclipse takes place at 10:18 UT (11:18 BST) with a magnitude of 43%, while at Greenwich greatest phase occurs at 10:13 UT (11:13 BST), the magnitude being 31%.

However, it is extremely dangerous to just go out and look up. The Sun is so bright that just looking at it can result in serious eye damage or blindness. The danger is NOT because of the eclipse – it is dangerous to look at the Sun at any time. So you’ll need to prepare beforehand. There are various ways to observe the eclipse safely, using both everyday materials and telescopes or binoculars. This guide tells you what will happen and how you can safely view all the stages of the event.

What will happen

A solar eclipse occurs when the Moon comes directly between the Sun and the Earth. Anyone lucky enough to be within a certain area will witness the Moon crossing directly in front of the Sun. On 10 June, people in a very narrow band across the Earth’s surface, extending from Ontario in Canada, across northern Quebec, Baffin Island, the north-western tip of Greenland, and the Arctic Ocean to far eastern Russia will see the Moon passing centrally in front of the Sun. But on this occasion the Moon’s disc will appear slightly smaller than that of the Sun, leaving a bright ring of light around the lunar silhouette. Eclipses like this are called annular after the ring, or 'annulus', of sunlight seen around the dark disc of the New Moon at mid-eclipse. It is interesting to note that the annular phase of the 10 June eclipse is visible at the North Pole.

Note that this eclipse won’t be as spectacular as a total solar eclipse where the Sun is completely covered by the Moon, the sky goes dark and one can see streamers in the Sun's outer atmosphere, the corona, extending outwards from the Moon's dark silhouette


The annular solar eclipse of 20 May 2012 as seen from Tokyo. A bright ring of sunlight is visible around the silhouette of the Moon.
Credit - Marek Okon / Unsplash


How an annular eclipse appears depending on whether your location is in the antumbra or penumbra.
Credit - RAS/Lucinda Offer


From the UK, well away from this track, the Earth, Moon and Sun are less precisely aligned. This means that the Moon appears to only take a fairly small bite out of the edge of the Sun’s disc. The eclipse will take place over a couple of hours, with mid-eclipse occurring soon after 11 am BST. It’s not as spectacular as the view from the track where the eclipse is annular, but If you take the proper precautions it’s a great thing to watch, and something you’ll remember for many years to come.


The partial phase of the annular eclipse of 20 May 2012.
Credit - Photo by Marek Okon on Unsplash

How to observe

Viewing a solar eclipse is potentially hazardous and should only be attempted with caution. The Sun is the brightest object in the sky. In addition to the visible light, it sends out huge amounts of invisible infrared and ultraviolet rays which can harm your sight. You should never, ever – under any circumstances – look directly at the Sun without proper eye protection!

DON'T ever look at the uneclipsed, partially-eclipsed or annularly-eclipsed Sun through an unfiltered camera, telescope, binoculars or other optical instrument.
Suitable filters have to be so dense that you can’t normally see through them.

DON'T try black plastic such as bin liners, no matter how thick. They may look dense, but they transmit infrared (heat) radiation, which can seriously damage your eyesight even though the Sun might look dim.

DON’T view the Sun through sunglasses of any type (single or multiple pairs, no matter how dark), or filters made of black & white or colour photographic film, or any combination of photographic filters, crossed polarisers or gelatin filters, CDs, CD-ROMs, or smoked glass. These are NOT safe.

DO view the Sun ONLY through special-purpose solar filters, such as so-called “eclipse glasses” or handheld solar viewers, made for safe solar viewing, e.g. aluminised mylar filters, or black polymer filters, identified as suitable for direct viewing of the Sun. If you purchased these filters a few years ago, perhaps before the partial solar eclipse visible from the British Isles in March 2015, these will bear the CE mark AND a statement that it conforms to European Community Directive 89/686/EEC. However, the international standard for filters that are suitable for direct viewing of the Sun is now ISO 12312-2, which supersedes CE and other standards, that were regional not global. All modern eclipse glasses or viewers should carry a statement that they are compliant with the ISO 12312-2 standard and should only be obtained from a reputable supplier. You could also use a welder’s glass rated at No. 14 or higher. Always read and follow the manufacturer’s instructions carefully.

DO check filters thoroughly for any damage BEFORE use. DON’T use them if they are wrinkled, scuffed, scratched or there are pinholes in them.

DO place the special filter firmly over your eyes BEFORE looking up at the Sun, and DON’T remove it until AFTER looking away.

DO use the special filters to view the eclipse while any part of the Sun’s brilliant disk, however small, is still visible. This will apply at ALL times when you are viewing an annular or partial eclipse as on 10 June. Intermittent use of the filter for the duration of the partial eclipse is the best way to view the event.

DO supervise children closely at all times when using solar filters.

Even with the special filter placed firmly over your eyes, DON’T ever look at the Sun through any optical instrument, e.g. telescope, binoculars or camera. Such devices concentrate the Sun’s harmful radiation and will cause severe eye damage in a fraction of a second. Filters identified as suitable for direct viewing of the Sun are NOT safe for use in conjunction with any optical instrument.

If you are not certain that a filter is approved and safe, or you have any other doubts, DON’T USE IT.


A woman using eclipse shades, certified to safety standards, to view a solar eclipse.
Credit - Robin Scagell - All Rights Reserved


Mirror projection

If you don’t have any “eclipse glasses” or handheld solar viewers, made for safe solar viewing, there is a good way to view the progress of the eclipse using nothing more elaborate than a small mirror. A compact or small shaving mirror is ideal, particularly if it has a stand to hold it in position. Use the flat side, not the concave or magnifying side.

All you need to do is to cover the mirror with paper in which you have cut a hole about 4 mm or an eighth of an inch across. The hole doesn’t have to be neat, or even round. Then shine the Sun’s reflection from the mirror into a room or onto any surface that’s in shade. You’ll see a circular spot of light which is in fact an image of the Sun. A projection distance of about 5 metres (15 feet) works perfectly, giving an image about 50 mm (2 inches) across, and you can view the image on a wall or a white piece of paper or whatever you have to hand.


Use a covered mirror with a 5mm hole in the cover to reflect the image of the eclipse onto a wall. Never look directly at the Sun.
Credit - RAS/Lucinda Offer


Cover the mirror with paper and create a 4-5mm hole in the centre of the cover.
Credit - Robin Scagell - All Rights Reserved

As the eclipse progresses you’ll see in perfect safety the Moon progressively taking a bite out of the edge of the Sun. It will be a little fuzzy around the edges, but it will be obvious to see.

Other simple methods of projection, such as using a pinhole in the side of a cereal packet, are possible but the image size is very small – only about 1 mm. So the fairly small bite from the Sun in the partial eclipse of 10 June as seen from the UK will hardly be visible.


Projection using binoculars or telescope

You can use a small telescope or binoculars with great care to project the Sun’s image for a very sharp view. Without looking through it, point the instrument at the Sun, judging from its shadow when it is in line. Hold a sheet of white paper about 30 cm (12 inches) away and you should see a bright spot appear when everything is lined up. You will probably need to focus to get a sharp image.


An image of the Sun projected using binoculars to simulate the view of the 10 June partial solar eclipse from the UK.
Credit Robin Scagell - All Rights Reserved

If you use a tripod make sure that there’s no chance that someone can accidentally look into the eyepiece of the instrument, This is the main danger with this method. The lesser danger is ruining the interior of the eyepiece. If the Sun’s bright image moves out of the field of view it will be focused on the interior of the eyepiece, which these days is often made of plastic even if the barrel is metal. The result will be very obvious damage to the field stop, which is what gives you the well-defined circle when you view a scene. You have been warned!

If you have a telescope and are projecting the image, or are viewing it using a safe solar filter, you may notice that there are dark spots on the Sun: sunspots. These are regions where relatively cool gas, that doesn't glow as brightly as the hotter gas around it, is trapped by strong magnetic fields. Although sunspots look black, they are still part of the Sun and if the Moon’s disc happens to cross one you could notice that the Moon’s disc is darker than the sunspot. At the moment the Sun is not very active, but there are usually small spots visible.

Also look at the edge of the Moon (known as the limb) and you might see that it isn’t completely smooth, as you can see mountains or valleys on the limb silhouetted against the Sun. You’ll also probably see a lot of shimmering at the limb, which is caused by turbulence in our own atmosphere rather than anything astronomical.

When to look

If you live in the UK you will see a partial eclipse with between one and two fifths of the Sun covered by the Moon. The farther north you are located, the more of the Sun will be obscured at maximum eclipse. If the weather is favourable, the Sun will appear to have an obvious bite taken out of it around the time of maximum eclipse.

The precise times of the eclipse and its appearance vary with location. Observers in Lerwick in the Shetland Islands will see the eclipse begin at 1015 BST, mid-eclipse will be at 1127 BST when 38% of the Sun will appear to be covered by the Moon, and the eclipse will end at 1244 BST. In London the eclipse starts at 1009 BST, mid-eclipse is at 1113 BST when 20% of the Sun is obscured by the Moon, and the eclipse ends at 1222 BST.


Credit - Robin Scagell - All Rights Reserved

The eclipse begins with first contact. You can’t see details on the disc of the Moon, or the whole of the disc, during a partial eclipse – all you see is that a bit of the bright Sun is missing. It may take a minute or so before you notice it.

The Moon continues to move ever so slowly across the Sun until after about 70 minutes (on this occasion) it has reached its maximum, and there will be an obvious bite taken out of the Sun. The exact timings depend on where you are in the UK. Then the Moon moves off again, and after roughly another 70 minutes it’s all over, with fourth contact.

What happened to second and third contacts? They only apply during a total solar eclipse, which we won’t see from anywhere this time because the Moon is too far away from the Earth to completely cover the Sun. You’ll either have to travel abroad or wait until 2090 to see a total eclipse from the mainland UK.

Resources

For teacher resources or to learn more about eclipses, please take a look at these links:
Role play a solar eclipse: Years 2 -5
Solar Eclipse Worksheet: Years 7-9
The Mathematics of a Total Solar Eclipse: Years 10-11
Solar Eclipse News Article

The British Astronomical Association (BAA)would like to thank the Royal Astronomical Society (RAS) and the Society for Popular Astronomy (SPA) for their valuable collaboration on this article.

Cover image: Eclipse of the Sun, November 3rd., 2013 by Colin Henshaw

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