Harvest and Hunters' Moons

During the months of September and October in the northern hemisphere, the rising of the Moon on successive days following full moon gives rise to what used to be called Harvest and Hunter's Moons. I say "used to" because the relevance of the Moon in the night sky as an aid to work outside after sunset has lost much of its significance. In an age where tractors and other machinery are equipped with powerful headlamps and so forth (to say nothing of floodlit golf courses, football and cricket pitches) the light provided by even a full moon can seem meager if not irrelevant.

Despite this the old terms should be understood. For example, many people appear to be under the impression that the Harvest Moon only applies to the full moon itself at or around the autumnal equinox. The real significance of the term has more to do with the rising of the Moon at or very nearly the same hour on successive evenings and nights from about the time of full moon. This event is particularly noticeable from high northern latitudes.

Consider the motion of the Moon about the Earth which makes it appear to move from west to east against the star background at the rate of approximately 13 arc degrees in a 24 hour period. (Do not confuse this movement with the diurnal motion which the Moon partakes in common with the planets, Sun, stars etc.) Were the Moon to follow a path close to the celestial equator (or parallel to it), then the Moon's rising would be retarded by a little under one hour every 24 hours. However, in common with most of the planets and the Sun, the Moon keeps fairly close to the path of the ecliptic (the plane of the Earth's orbit projected onto the celestial sphere).

From the northern hemisphere in autumn the ecliptic makes a shallow angle with the eastern horizon. As a result, the Moon's increasing declination following full moon offsets its easterly motion in Right Ascension causing it to rise progressively north of east for about a week.

Thus, although the Moon's phase decreases from night to night following full moon (and therefore its contribution to lighting the night sky also diminishes), its presence does add some light to the sky before midnight on a number of consecutive nights at the season associated with harvest (and in some localities, hunting).

Dia. 1: The Moon from October 8 to 15 showing its position relative to the eastern horizon at 18 30 GMT as seen from Kirkwall. Click the image for an enlargement

On October 10 2003, for example, the full moon will rise as seen from Kirkwall at 17 40 GMT. On the 11th it will rise just three minutes later and, on the 12th, a further four minutes later. On the 15th moonrise is at 18 23 GMT, still less than an hour after the rising of full moon on the 10th.

From an observational point of view autumn gives the best opportunity to see the Moon in its third and last quarters at good elevations above the horizon.

Dia. 1 illustrates the Moon's progression along the eastern horizon from night to night between October 8th and October 15th.

Mercury in the morning sky early October

Mercury reaches greatest western elongation on September 27th. The planet continues to brighten and should be visible to the naked eye up to around October 8th. [See archive for previous article.]

JV 25/09