Moon at Last Quarter

2017 May 18

Dominic Ford – originally published on In-The-Sky.org

From London (click to change), the Moon will be difficult to observe as it will appear no higher than 17° above the horizon. It will be visible in the dawn sky. It will rise at 02:11 (BST), 2 hours and 51 minutes before the Sun, and attain an altitude of 17° above the south-eastern horizon before fading from view as dawn breaks at around 04:35.

The Moon‘s orbital motion carries it around the Earth once every four weeks, and as a result its phases cycle from new moon, through first quarter, full moon and last quarter, back to new moon once every 29.5 days.

This motion also means that the Moon travels more than 12° across the sky from one night to the next, causing it to rise and set nearly an hour later each day. Click here for more information about the Moon’s phases.

The Moon’s path in coming days

Over the next few days, the distance between the Moon and the Sun will decrease each night, and it will rise later in the night each day. Eventually it will only be visible very shortly before sunrise and it will sink into the Sun’s glare as it approaches new moon (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
14 May 2017 20:38 07:46 15° south-west
15 May 2017 20:39 08:32 17° south
16 May 2017 20:41 09:24 19° south
17 May 2017 20:42 10:21 20° south
18 May 2017 20:44 11:22 20° south
19 May 2017 20:45 12:29 20° south-east
20 May 2017 20:46 13:38 18° south-east
21 May 2017 20:48 14:49 15° south-east
22 May 2017 20:49 16:03 12° east
23 May 2017 20:51 17:22 east
24 May 2017 20:52 18:41 east
25 May 2017 20:53 20:01 -1° north-east

Observing the Moon at last quarter

The period when the Moon shows half phase is ideal for observing the Moon with a pair of binoculars or a small telescope, because the mountains and craters on its surface are presented very clearly. Even though only half of the Moon’s face is illuminated, this is a time when the terminator line which divides the illuminated and unilluminated portions of the Moon’s disk is clearly visible.

Along this line, an observer on the Moon would see the Sun rising above the horizon. As it does so, it illuminates the lunar landscape at a low angle, making mountains and crater rims cast long shadows which are easy to see from Earth, even through a modest pair of binoculars.

Seasonal variations

Although the Moon is always separated from the Sun by the same amount – 90° – when it passes last quarter, it is more favourably placed in the pre-dawn sky at some times of year than others.

Specifically, 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 is the line through the zodiacal constellations that the Sun follows through the year, and marks the flat plane in space in which all of the planets circle the Sun.

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 May 19, the ecliptic is inclined at 21° to the eastern dawn horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion the Moon is poorly 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 389,000 km. Its exact position will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 22h00m30s -12°07' Aquarius 30'41"

The coordinates above are given in J2000.0.


The details of this observing event were provided courtesy of In-The-Sky.org

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