The Moon will pass last quarter phase, rising in the middle of the night and appearing prominent in the pre-dawn sky.
From Cambridge, it will be visible in the dawn sky, rising at 23:25 (EDT) and reaching an altitude of 64° above the southern horizon before fading from view as dawn breaks around 06:01.
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 last quarter, it rises in the middle of the night and appears high in the sky by dawn. It sets at around lunchtime. More information about the Moon's phases is available here.
A thin crescent
The months around the autumn equinox – September and October in the northern hemisphere – present the best opportunity to see a very thin crescent Moon immediately before sunrise in the days running up to new moon.
This comes about because the Moon appears higher in the dawn sky sooner after new moon in the autumn months as compared to other times of year.
At all times, the Moon lies close to a line across the sky called the ecliptic, which is shown in yellow in the planetarium above. This line traces the path that the Sun takes through the zodiacal constellations every year, and shows the plane of the Earth's orbit around the Sun. Since all the planets circle the Sun in almost exactly the same plane, they too move across the sky following the same line.
When the Moon is widely separated from the Sun, it is separated from it along the line of the ecliptic. But, at different times of year, the ecliptic meets the horizon at different angles at sunrise. This means that a certain number of days after new moon, the Moon appears at different altitudes above the horizon at different times of year.
At sunrise, the ecliptic makes its steepest angle to the horizon at the autumn equinox – in September in the northern hemisphere, and in March in the southern hemisphere. Conversely, it meets the horizon at its shallowest angle at the spring equinox. And so, very thin crescent moons are most visible in the dawn sky in the autumn.
The inclination of the ecliptic plane to the horizon at Cambridge varies between 71° (sunrise at the autumn equinox) and 24° (sunrise at the spring equinox). On September 12, the ecliptic is inclined at 70° to the eastern dawn horizon.
The exact moment of last quarter
The exact moment of last 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 last quarter moon can be observed at any time in the pre-dawn sky.
At the moment it reaches last quarter, the Moon's distance from the Earth will be 374,000 km. Its celestial coordinates will be as follows:
| Object | Right Ascension | Declination | Constellation | Angular Size |
| The Moon | 05h18m30s | 18°05'N | Taurus | 31'50" |
The coordinates above are given in J2000.0.
| The sky on 25 October 2021 | ||||||||||||||||||||||||||||||||||
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73% 19 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
| 12 Sep 1998 | – Moon at Last Quarter |
| 20 Sep 1998 | – New Moon |
| 28 Sep 1998 | – Moon at First Quarter |
| 05 Oct 1998 | – Full Moon |
Image credit
Simulated image courtesy of Tom Ruen.