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Moon at First Quarter

Dominic Ford, Editor
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Ashburn
The sky at

The Moon will be prominent in the evening sky, setting around midnight.

From Ashburn (click to change), it will become visible at around 18:55 (EDT) as the dusk sky fades, 23° above your southern horizon. It will then reach its highest point in the sky at 19:10, 23° above your southern horizon. It will continue to be observable until around 22:36, when it sinks to 8° above your south-western horizon.

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

Observing the Moon at first 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 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.

It appears high up 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 October 10, the ecliptic is inclined at 28° 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 Ashburn.

The Moon's position

At the moment it reaches first quarter, the Moon's distance from the Earth will be 371,000 km. Its exact position will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 19h18m50s -27°31' Sagittarius 32'06"

The coordinates above are given in J2000.0.

The sky on 23 June 2018
Sunrise
05:43
Sunset
20:38
Twilight ends
22:37
Twilight begins
03:44

10-day old moon
Waxing Gibbous

82%

10 days old

Planets
Rise Culm. Set
Mercury 07:07 14:34 22:00
Venus 08:50 15:59 23:08
Moon 16:35 22:01 02:54
Mars 23:05 03:54 08:40
Jupiter 16:33 21:46 03:04
Saturn 20:40 01:30 06:15
All times shown in EDT.

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

10 Oct 2005, 15:02 EDT  –  Moon at First Quarter
17 Oct 2005, 08:15 EDT  –  Full Moon
24 Oct 2005, 21:18 EDT  –  Moon at Last Quarter
01 Nov 2005, 20:26 EST  –  New Moon

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Ashburn

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Longitude:
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39.04°N
77.49°W
EDT

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