Simulated image courtesy of Tom Ruen.

Moon at Last Quarter

Dominic Ford, Editor
From the Moon feed

Objects: The Moon
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The Moon will pass last quarter phase, rising in the middle of the night and appearing prominent in the pre-dawn sky.

From Fairfield, it will be visible in the dawn sky, rising at 01:14 (EDT) and reaching an altitude of 36° above the south-eastern horizon before fading from view as dawn breaks around 04:57.

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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.

Observing the Moon at last quarter

Over coming days, the Moon will rise later each day, so that it is visible for less time before sunrise and rises 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 daily progress is charted below, with all times are given in Fairfield local time.

Date Sun
rises at
Moon
rises at
Altitude of Moon
at sunrise
Direction of Moon
at sunrise
16 Jun 202205:1921:12south-west
17 Jun 202205:2022:1914°south-west
18 Jun 202205:2023:1422°south
19 Jun 202205:2023:5429°south
20 Jun 202205:2100:2734°south
21 Jun 202205:2100:5537°south
22 Jun 202205:2101:1639°south-east
23 Jun 202205:2201:3939°south-east
24 Jun 202205:2202:0136°south-east
25 Jun 202205:2202:2332°east
26 Jun 202205:2302:4927°east
27 Jun 202205:2303:1821°east

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.

The inclination of the ecliptic to the horizon changes over the course of the year, affecting how high objects appear in the sky.

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 Fairfield varies between 72° (sunrise at the autumn equinox) and 25° (sunrise at the spring equinox). On June 20, the ecliptic is inclined at 44° 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 382,000 km. Its celestial coordinates will be as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 00h04m30s 3°59'S Pisces 31'11"

The coordinates above are given in J2000.0.

The sky on 20 June 2022
Sunrise
05:21
Sunset
20:29
Twilight ends
22:41
Twilight begins
03:09

21-day old moon
Waning Crescent

44%

21 days old

Planets
Rise Culm. Set
Mercury 04:08 11:19 18:30
Venus 03:29 10:38 17:46
Moon 00:55 06:31 12:17
Mars 01:50 08:14 14:37
Jupiter 01:15 07:23 13:30
Saturn 23:38 04:49 10:01
All times shown in EDT.

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

20 Jun 2022  –  Moon at Last Quarter
28 Jun 2022  –  New Moon
06 Jul 2022  –  Moon at First Quarter
13 Jul 2022  –  Full Moon

Image credit

Simulated image courtesy of Tom Ruen.

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