When does the Moon rise? Understanding moonrise and moonset

Why the Moon does not keep the Sun’s timetable

Our twenty-four-hour day follows the Sun approximately. During that interval, the Moon moves further along its orbit. Relative to the Sun’s direction, its average eastward advance is about 12.2° per day. Earth therefore needs some additional rotation before the Moon returns to the local meridian.

The average interval between successive meridian passages is about twenty-four hours and fifty minutes, often called a lunar day. That explains the familiar average delay, but the interval between rises is not fixed. Moonrise also depends on the changing lunar declination and the angle at which its track crosses your horizon.

The monthly cycle provides another perspective. From one new moon to the next, the Moon gains a complete turn relative to the Sun. It consequently does not fit the solar day’s sequence exactly. Adding fifty minutes to today’s rise time is a useful intuition, but not a reliable prediction for tomorrow.

Some civil calendar dates contain no moonrise. A rise late at night can be followed by the next after midnight on the following date, leaving an intervening date without one. Moonset can show a similar gap. The Moon has not vanished; its horizon crossings simply do not line up with calendar boundaries.

The harvest moon effect

The Moon travels close to the ecliptic, and that band meets the eastern horizon at a changing angle. If the track is shallow, much of the daily displacement is sideways rather than downwards relative to the horizon. The next rise is then delayed by less than usual.

In middle northern latitudes, full moons near the September equinox show this particularly well. For several evenings, a bright Moon can rise with relatively little extra delay, extending natural light after sunset. That is the astronomical background to the North American name harvest moon.

The size of the effect depends on latitude. New York and the Grand Canyon therefore have their own patterns of changing moonrise times. In northern spring, the geometry can instead produce a much larger delay between rises. In the southern hemisphere, the seasonal relationship is reversed.

The cultural name and the geometry are different things. A farming tradition helps explain the name without making one region’s harvest calendar universal.

What counts as moonrise

The usual convention refers to the upper edge of the lunar disc reaching an ideal mathematical horizon. Two important effects enter the apparent position:

• Refraction. Earth’s atmosphere bends light and raises the apparent Moon near the horizon. Part of the disc can be visible while its geometrical position is still below the horizontal plane.
• **Parallax.** The Moon is close enough that its direction from the surface differs markedly from the direction from Earth’s centre. Near the horizon, this difference can approach a degree and acts in the opposite sense to refraction.

The direction of rise changes too. Lunar declination swings north and south, moving the rise point along the horizon. In middle northern latitudes, winter full moons generally follow higher paths than summer full moons, because a full moon occupies approximately the opposite part of the sky from the Sun.

Mountains, trees and buildings can delay the first actual glimpse. A time for an ideal horizon does not automatically model the cliff opposite a Grand Canyon viewpoint or the buildings outside a New York window.

Watching and photographing moonrise

A rising full moon often looks huge and orange. The colour has a physical atmospheric explanation: its light passes through a long path of air that scatters and absorbs part of the spectrum. Much of the exaggerated size is the Moon illusion, a perceptual effect rather than an enlarged disc.

For a planned photograph, consider the time, rise azimuth and actual horizon profile together. Azimuth helps choose a position where the Moon appears beside a tower or above a ridge. The height of the obstruction determines how long you wait after the ideal rise.

Allow time for haze: the disc may not become clear until it has gained some altitude. Near full moon, a rise during twilight can give a useful balance between a bright landscape and the Moon. The exact phase instant does not guarantee that every location gets its best photographic rise on the same evening.