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

From Seattle, it will be visible in the morning sky, becoming accessible around 01:40, when it reaches an altitude of 7° above your south-eastern horizon. It will then reach its highest point in the sky at 05:58, 30° above your southern horizon. It will be lost to dawn twilight around 07:35, 27° above your south-western horizon.

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

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 Seattle varies between 65° (sunrise at the autumn equinox) and 18° (sunrise at the spring equinox). On January 7, the ecliptic is inclined at 30° 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 Seattle.

The Moon's position

At the moment it reaches last quarter, the Moon's distance from the Earth will be 379,000 km. Its exact position will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 12h53m50s -11°30' Virgo 31'30"

The coordinates above are given in J2000.0.

The sky on 28 October 2020
Sunrise
07:48
Sunset
17:58
Twilight ends
19:41
Twilight begins
06:04

12-day old moon
Waxing Gibbous

91%

12 days old

Planets
Rise Culm. Set
Mercury 07:13 12:27 17:42
Venus 04:35 10:44 16:53
Moon 17:22 23:24 04:20
Mars 17:18 23:42 06:11
Jupiter 13:50 18:07 22:24
Saturn 14:07 18:30 22:53
All times shown in PDT.

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

07 Jan 2010  –  Moon at Last Quarter
14 Jan 2010  –  The Moon at perihelion
14 Jan 2010  –  New Moon
16 Jan 2010  –  The Moon at apogee

Image credit

Simulated image courtesy of Tom Ruen.

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