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BAA Observing calendar

Moon at Last Quarter

Thursday, 2019, July 25 - 02:19

The Moon will be prominent in the dawn sky, rising at around midnight.

From London, it will be visible in the dawn sky, rising at 00:14 (BST) – 4 hours and 58 minutes before the Sun – and reach an altitude of 37° above the south-eastern horizon before fading from view as dawn breaks at around 04:44.

The Moon's path in coming days

Over coming days, the Moon will rise later each day, so that it is visible for less time before sunrise and it less far above the eastern horizon before dawn. By the time it reaches new moon, it will rise at around dawn and set at around dusk, making it visible only during the daytime.

Its day-by-day progress is charted below, with all times are given below in London local time. Over the next few days, the distance between the Moon and the Sun will decrease and it will rise later each day. By the time it disappears into the Sun's glare as it approaches new moon, it will only be visible very shortly before sunrise.

All times given below in London local time.

Date Sun
sets at
Moon
sets at
Altitude of Moon
at sunset
Direction of Moon
at sunset
20 Jul 2019 21:02 08:20 20° south-west
21 Jul 2019 21:01 09:26 26° south
22 Jul 2019 20:59 10:30 31° south
23 Jul 2019 20:58 11:34 36° south
24 Jul 2019 20:57 12:39 39° south
25 Jul 2019 20:56 13:48 40° south-east
26 Jul 2019 20:54 14:56 40° south-east
27 Jul 2019 20:53 16:07 38° south-east
28 Jul 2019 20:51 17:19 33° east
29 Jul 2019 20:50 18:31 27° east
30 Jul 2019 20:48 19:36 19° east
31 Jul 2019 20:47 20:32 10° east

Observing the Moon at last quarter

The Moon orbits the Earth once every four weeks, causing its phases to cycle through new moon, first quarter, full moon, last quarter, and back to new moon once every 29.5 days.

As it progresses through this cycle, it is visible at different times of day. At last quarter, it rises at around midnight, appears high in the sky by dawn, and sets at around midday. Click here for more information about the Moon's phases.

The period when the Moon shows half phase is ideal for observing the Moon with a pair of binoculars or a small telescope. The border between the light and dark portions of the Moon's disk is the best place to look for detail on its surface, because along this line, the Moon's surface is illuminated at a very shallow angle. As a result, mountains and crater rims cast long shadows which are very easy to see. An observer on the Moon would see the Sun on the horizon, casting long shadows just like the ones we see on Earth at sunrise and sunset.

At first quarter and last quarter, when the terminator line is down the middle of the Moon, it is best presented for view, without any foreshortening.

Seasonal variations

Although the Moon passes last quarter every month, it is more favourably placed in the pre-dawn sky at some times of year than others.

It appears high up in the pre-dawn sky around the autumn equinox, but much lower towards the horizon around the spring equinox.

This is because it always lies close to a line across the sky called the ecliptic. This marks the flat plane in space in which all of the planets circle the Sun. It is the line through the zodiacal constellations that the Sun follows through the year.

The altitude at which the Moon appears above the horizon at sunrise depends how steeply the line of the ecliptic is inclined to the horizon. If the plane of the ecliptic meet the horizon at a very shallow angle, the Moon will rise or set along a line which is almost parallel to the horizon, and a large separation from the Sun along this line would still only correspond to a very low altitude in the sky.

The inclination of the ecliptic plane to the horizon at London varies between 61° (sunrise at the autumn equinox) and 15° (sunrise at the spring equinox). On July 25, the ecliptic is inclined at 46° to the eastern dawn horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion the Moon is very favourably placed for viewing from London.

The Moon's position

At the moment it reaches last quarter, the Moon's distance from the Earth will be 396,000 km. Its exact position will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 02h04m50s +07°13' Pisces 30'07"

The coordinates above are given in J2000.0.

This entry in the observing calendar was provided by In-The-Sky.org

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