Simulated image courtesy of Tom Ruen.

Moon at First Quarter

Dominic Ford, Editor
From the Moon feed

Objects: The Moon
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The Moon will pass first quarter phase, appearing prominent in the evening sky and setting in the middle of the night.

From Ashburn , it will become visible around 17:40 (EDT) as the dusk sky fades, 58° above your southern horizon. It will then reach its highest point in the sky at 18:12, 59° above your southern horizon. It will continue to be observable until around 23:58, when it sinks below 7° above your western horizon.

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At this time in its monthly cycle of phases, it appears almost exactly half illuminated.

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 first quarter, it appears high in the sky at sunset before sinking towards the horizon and setting in the middle of the night. More information about the Moon's phases is available here.

Observing the Moon at first quarter

Over coming days, the Moon will set later each day, becoming visible for more of the night. Within a few days, it will not make it very far above the eastern horizon before nightfall. By the time it reaches full phase, it will be visible for much of the night, rising at around dusk and setting at around dawn.

Its day-by-day progress is charted below, with all times are given in Ashburn local time.

Date Sun
sets at
sets at
Altitude of Moon
at sunset
Direction of Moon
at sunset
19 Jan 201817:1519:5125°south-west
20 Jan 201817:2020:5133°south-west
21 Jan 201817:2021:5041°south-west
22 Jan 201817:2022:5049°south
23 Jan 201817:2023:5354°south
24 Jan 201817:2400:5757°south
25 Jan 201817:2500:5755°south-east
26 Jan 201817:2502:0650°south-east
27 Jan 201817:2503:1341°east
28 Jan 201817:2904:2331°east
29 Jan 201817:2905:2620°east
30 Jan 201817:2906:28east

Seasonal variation

Although the Moon passes first quarter every month, it is more favourably placed in the early evening sky at some times of year than others.

The first quarter moon appears high in the evening sky around the spring equinox, but much lower towards the horizon around the autumn 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 sunset 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 Ashburn varies between 74° (sunset at the spring equinox) and 27° (sunset at the autumn equinox). On January 24, the ecliptic is inclined at 61° to the western sunset horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion the Moon is favourably placed for viewing from Ashburn.

The Moon's position

At the moment it reaches first quarter, the Moon's distance from the Earth will be 378,000 km. Its celestial coordinates will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 02h16m30s +08°09' Cetus 31'33"

The coordinates above are given in J2000.0.

The sky on 24 January 2018
Twilight ends
Twilight begins

7-day old moon
Waxing Crescent


7 days old

Rise Culm. Set
Mercury 06:34 11:15 15:57
Venus 07:41 12:37 17:33
Moon 11:38 18:14 00:57
Mars 02:43 07:41 12:39
Jupiter 02:00 07:07 12:14
Saturn 05:27 10:12 14:57
All times shown in EST.


The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

Related news

24 Jan 2018  –  Moon at First Quarter
30 Jan 2018  –  The Moon at perigee
31 Jan 2018  –  Blue Moon
01 Feb 2018  –  The Moon at aphelion

Image credit

Simulated image courtesy of Tom Ruen.






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