Simulated image courtesy of Tom Ruen.

Moon at Last Quarter

Dominic Ford, Editor
From the Moon feed

Objects: The Moon
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The sky at

The Moon will pass last quarter phase, rising in the middle of the night and appearing prominent in the pre-dawn sky.

From Fairfield, it will be visible in the dawn sky, rising at 23:47 (EDT) and reaching an altitude of 59° above the south-eastern horizon before fading from view as dawn breaks around 05:39.

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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 last quarter, it rises in the middle of the night and appears high in the sky by dawn. It sets at around lunchtime. More information about the Moon's phases is available here.

Observing the Moon at last quarter

Over coming days, the Moon will rise later each day, so that it is visible for less time before sunrise and rises 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 in Fairfield local time.

Date Sun
rises at
Moon
rises at
Altitude of Moon
at sunrise
Direction of Moon
at sunrise
06 Aug 202005:5222:0326°south-west
07 Aug 202005:5622:2535°south-west
08 Aug 202005:5522:4943°south-west
09 Aug 202005:5523:0951°south
10 Aug 202005:5923:3557°south
11 Aug 202005:5923:3761°south
12 Aug 202005:5900:0461°south-east
13 Aug 202005:5800:3657°south-east
14 Aug 202006:0201:1350°east
15 Aug 202006:0202:0041°east
16 Aug 202006:0202:5531°east
17 Aug 202006:0503:5820°east

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.

The last quarter moon appears high 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 Fairfield varies between 72° (sunrise at the autumn equinox) and 25° (sunrise at the spring equinox). On August 11, the ecliptic is inclined at 65° to the eastern dawn 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 Fairfield.

The Moon's position

At the moment it reaches last quarter, the Moon's distance from the Earth will be 401,000 km. Its celestial coordinates will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 03h10m20s +14°27' Aries 29'43"

The coordinates above are given in J2000.0.

The sky on 11 August 2020
Sunrise
05:59
Sunset
19:59
Twilight ends
21:47
Twilight begins
04:11

22-day old moon
Waning Crescent

49%

22 days old

Planets
Rise Culm. Set
Mercury 05:18 12:30 19:42
Venus 02:30 09:47 17:04
Moon 23:37 06:32 13:35
Mars 22:41 05:01 11:21
Jupiter 18:14 22:53 03:31
Saturn 18:40 23:25 04:10
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

11 Aug 2020  –  Moon at Last Quarter
18 Aug 2020  –  New Moon
20 Aug 2020  –  The Moon at perihelion
21 Aug 2020  –  The Moon at perigee

Image credit

Simulated image courtesy of Tom Ruen.

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