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Moon at Last Quarter

Mon, 27 Dec 2010 at23:20 EST(2612 days ago)
04:20 UTC

Dominic Ford, Editor
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Ashburn
The sky at

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

From Ashburn (click to change), it will be visible in the morning sky, becoming accessible at around 01:06, when it rises 7° above your eastern horizon. It will then reach its highest point in the sky at 06:00, 43° above your southern horizon. It will be lost to dawn twilight at around 07:10, 41° above your southern horizon.

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 Ashburn varies between 74° (sunrise at the autumn equinox) and 27° (sunrise at the spring equinox). On December 27, the ecliptic is inclined at 43° 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 Ashburn.

The Moon's position

At the moment it reaches last quarter, the Moon's distance from the Earth will be 370,000 km. Its exact position will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 12h14m10s -07°17' Virgo 32'12"

The coordinates above are given in J2000.0.

The sky on 20 February 2018
Sunrise 06:54
Sunset 17:51
Twilight ends
19:20
Twilight begins
05:24

5-day old moon
Age of Moon
5 days

All times shown in EST.
Rise Culm. Set
Mercury 07:10 12:35 18:00
Venus 07:27 13:03 18:38
Moon 09:43 16:08 22:34
Mars 02:17 07:03 11:49
Jupiter 00:25 05:30 10:35
Saturn 03:50 08:35 13:21

Source

The circumstances of this event were computed using the DE405 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 Dec 2010, 23:20 EST  –  Moon at Last Quarter
04 Jan 2011, 04:04 EST  –  New Moon
12 Jan 2011, 06:33 EST  –  Moon at First Quarter
19 Jan 2011, 16:23 EST  –  Full Moon

Image credit

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Ashburn

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Longitude:
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39.04°N
77.49°W
EST

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