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Moon at First Quarter

Sun, 30 Jul 2017 at11:25 EDT(110 days ago)
15:25 UTC

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 20:40 (EST) as the dusk sky fades, 36° above your south-western horizon. It will then sink towards the horizon, setting 4 hours and 18 minutes after the Sun at 00:38.

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

The Moon's path in coming days

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 below in Ashburn local time.

Date Sun
sets at
Moon
sets at
Altitude of Moon
at sunset
Direction of Moon
at sunset
25 Jul 201720:2422:0214°west
26 Jul 201720:2322:3921°west
27 Jul 201720:2223:1127°west
28 Jul 201720:2123:4332°south-west
29 Jul 201720:2000:1436°south-west
30 Jul 201720:1900:1438°south-west
31 Jul 201720:1800:4738°south
01 Aug 201720:1701:1936°south
02 Aug 201720:1601:5533°south
03 Aug 201720:1502:3529°south-east
04 Aug 201720:1403:1824°south-east
05 Aug 201720:1304:0618°south-east

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 July 30, the ecliptic is inclined at 33° 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 399,000 km. Its exact position will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 14h26m50s -09°12' Libra 29'54"

The coordinates above are given in J2000.0.

The sky on 30 July 2017
Sunrise 06:07
Sunset 20:22
Twilight ends
22:09
Twilight begins
04:21

7-day old moon
Age of Moon
7 days

All times shown in EDT.
Rise Culm. Set
Mercury 08:28 14:59 21:30
Venus 03:11 10:30 17:49
Moon 13:46 19:17 00:14
Mars 06:00 13:11 20:22
Jupiter 11:52 17:38 23:23
Saturn 17:10 21:57 02:48

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

30 Jul 2017, 11:24 EDT  –  Moon at First Quarter
07 Aug 2017, 14:12 EDT  –  Full Moon
14 Aug 2017, 21:16 EDT  –  Moon at Last Quarter
21 Aug 2017, 14:31 EDT  –  New Moon

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Ashburn

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