The Moon will pass first quarter phase, appearing prominent in the evening sky and setting in the middle of the night.
From Cambridge , it will be visible from soon after it rises, at 12:57, until soon before it sets at 00:18.
At this time in its monthly cycle of phases, it appears almost exactly half illuminated.
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 first quarter, it appears high in the sky at sunset before sinking towards the horizon and setting in the middle of the night. More information about the Moon's phases is available here.
Observing the Moon at first quarter
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 daily progress is charted below, with all times are given in Cambridge local time.
| Date | Sun sets at |
Moon sets at |
Altitude of Moon at sunset |
Direction of Moon at sunset |
| 12 Jul 2021 | 20:21 | 22:22 | 21° | west |
| 13 Jul 2021 | 20:20 | 22:51 | 27° | west |
| 14 Jul 2021 | 20:20 | 23:16 | 31° | west |
| 15 Jul 2021 | 20:19 | 23:41 | 34° | south-west |
| 16 Jul 2021 | 20:18 | 00:05 | 35° | south-west |
| 17 Jul 2021 | 20:18 | 00:31 | 34° | south |
| 18 Jul 2021 | 20:17 | 01:00 | 31° | south |
| 19 Jul 2021 | 20:16 | 01:36 | 26° | south |
| 20 Jul 2021 | 20:15 | 02:20 | 20° | south-east |
| 21 Jul 2021 | 20:14 | 03:14 | 12° | south-east |
| 22 Jul 2021 | 20:14 | 04:19 | 5° | south-east |
| 23 Jul 2021 | 20:13 | 05:31 | -2° | south-east |
The exact moment of first quarter
The exact moment of first quarter is defined as the time when the Moon's ecliptic longitude is exactly 90° away from the Sun's ecliptic longitude, as observed from the center of the Earth. However, the Moon does not appear in any way special at this instant in time, and a first quarter moon can be observed at any time in the evening sky.
At the moment it reaches first quarter, the Moon's distance from the Earth will be 372,000 km. Its celestial coordinates will be as follows:
| Object | Right Ascension | Declination | Constellation | Angular Size |
| The Moon | 13h37m00s | 6°12'S | Virgo | 32'04" |
The coordinates above are given in J2000.0.
The sky on 17 Jul 2021
| The sky on 17 July 2021 | ||||||||||||||||||||||||||||||||||
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55% 7 days old |
All times shown in EDT.
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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
| 17 Jul 2021 | – Moon at First Quarter |
| 23 Jul 2021 | – Full Moon |
| 31 Jul 2021 | – Moon at Last Quarter |
| 08 Aug 2021 | – New Moon |
Image credit
Simulated image courtesy of Tom Ruen.