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

From Ashburn, it will be visible in the dawn sky, rising at 01:03 (EDT) and reaching an altitude of 45° above the south-eastern horizon before fading from view as dawn breaks around 05:25.

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 day-by-day progress is charted below, with all times are given in Ashburn local time.

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.

The last quarter moon appears high 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 June 19, the ecliptic is inclined at 45° to the eastern dawn horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion the Moon is 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 374,000 km. Its celestial coordinates will be as follows:

The coordinates above are given in J2000.0.

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.

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Image credit

Simulated image courtesy of Tom Ruen.

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