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Moon at Last Quarter

Sun, 16 Jul 2017 at15:27 EDT(124 days ago)
19:27 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 dawn sky, rising at 00:48 (EST) – 5 hours and 10 minutes before the Sun – and reach an altitude of 49° above the south-eastern horizon before fading from view as dawn breaks at around 05:38.

The Moon's path in coming days

Over coming days, the Moon will rise later each day, so that it is visible for less time before sunrise and it 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 below in Ashburn local time. Over the next few days, the distance between the Moon and the Sun will decrease and it will rise later each day. By the time it disappears into the Sun's glare as it approaches new moon, it will only be visible very shortly before sunrise.

All times given below in Ashburn local time.

Date Sun
sets at
Moon
sets at
Altitude of Moon
at sunset
Direction of Moon
at sunset
11 Jul 201720:3208:0020°south-west
12 Jul 201720:3208:5728°south-west
13 Jul 201720:3109:5636°south-west
14 Jul 201720:3110:5843°south
15 Jul 201720:3011:5949°south
16 Jul 201720:3013:0252°south
17 Jul 201720:2914:0751°south-east
18 Jul 201720:2915:1647°south-east
19 Jul 201720:2816:2440°east
20 Jul 201720:2717:3232°east
21 Jul 201720:2718:3822°east
22 Jul 201720:2619:3910°east

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 July 16, the ecliptic is inclined at 57° 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 375,000 km. Its exact position will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 01h36m10s +05°10' Pisces 31'49"

The coordinates above are given in J2000.0.

The sky on 16 July 2017
Sunrise 05:56
Sunset 20:33
Twilight ends
22:27
Twilight begins
04:02

22-day old moon
Age of Moon
22 days

All times shown in EDT.
Rise Culm. Set
Mercury 07:53 14:52 21:50
Venus 03:05 10:17 17:29
Moon 00:36 06:49 13:02
Mars 06:10 13:29 20:48
Jupiter 12:39 18:27 00:18
Saturn 18:08 22:55 03:46

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

16 Jul 2017, 15:27 EDT  –  Moon at Last Quarter
23 Jul 2017, 05:47 EDT  –  New Moon
30 Jul 2017, 11:24 EDT  –  Moon at First Quarter
07 Aug 2017, 14:12 EDT  –  Full Moon

Image credit

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Ashburn

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Longitude:
Timezone:

39.04°N
77.49°W
EST

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