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 sky at

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

From Fairfield , it will become visible around 20:37 (EDT) as the dusk sky fades, 33° above your south-western horizon. It will then sink towards the horizon, setting 3 hours and 58 minutes after the Sun at 00:15.

Begin typing the name of a town near to you, and then select the town from the list of options which appear below.

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 Fairfield local time.

Date Sun
sets at
Moon
sets at
Altitude of Moon
at sunset
Direction of Moon
at sunset
22 Jul 202020:2122:0418°west
23 Jul 202020:2122:3624°west
24 Jul 202020:1723:0630°south-west
25 Jul 202020:1723:3834°south-west
26 Jul 202020:17--:--35°south-west
27 Jul 202020:1700:0635°south
28 Jul 202020:1700:3732°south
29 Jul 202020:1301:0927°south
30 Jul 202020:1301:4722°south-east
31 Jul 202020:1302:3416°south-east
01 Aug 202020:1303:27south-east
02 Aug 202020:0804:25south-east

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 Fairfield varies between 72° (sunset at the spring equinox) and 25° (sunset at the autumn equinox). On July 27, the ecliptic is inclined at 32° to the western sunset 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 Fairfield.

The Moon's position

At the moment it reaches first quarter, the Moon's distance from the Earth will be 370,000 km. Its celestial coordinates will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 14h15m10s -09°07' Virgo 32'16"

The coordinates above are given in J2000.0.

The sky on 27 July 2020
Sunrise
05:45
Sunset
20:17
Twilight ends
22:09
Twilight begins
03:49

7-day old moon
Waxing Crescent

49%

7 days old

Planets
Rise Culm. Set
Mercury 04:12 11:35 18:59
Venus 02:36 09:48 17:01
Moon 13:33 19:10 00:06
Mars 23:24 05:37 11:49
Jupiter 19:23 00:03 04:43
Saturn 19:48 00:34 05:20
All times shown in EDT.

Source

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

27 Jul 2020  –  Moon at First Quarter
02 Aug 2020  –  The Moon at aphelion
03 Aug 2020  –  Full Moon
09 Aug 2020  –  The Moon at apogee

Image credit

Simulated image courtesy of Tom Ruen.

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