In Our Skies: Shine on, Shine on Harvest Moon

When we define terms in science we often have to specify our reference points.

One distinct example of this involves the amount of time it takes the moon to orbit around Earth. Strictly speaking, the moon does not orbit around Earth; rather, both the Earth and the moon orbit around their common center of gravity, however this point lies approximately one thousand miles underneath Earth’s surface, so the distinction is somewhat moot.

It takes the moon slightly over 27 days to appear in front of the same background stars; this is the moon’s sidereal orbital period. However, during this time the Earth has traveled approximately one-twelfth of the way on its orbit around the sun, and it takes the moon an extra two days before it will exhibit the same phase as before. This 29-day interval is known as the moon’s synodic orbital period.

On average, the moon rises about 24/29 of a day, or 50 minutes, later each night. This would be the actual difference in moonrise times throughout the year if the moon were orbiting in the plane of the Earth’s Equator, however the moon orbits more in the plane of the Earth’s orbit around the sun, which is tilted some 23 degrees with respect to the Earth’s Equator. Because of gravitational pulls by the other planets, the difference between the moon’s orbit and the Equator can vary between 18 degrees and 29 degrees.

Thus, in addition to the general west-to-east motion of the moon against the background stars, there is also a distinct north-to-south, and vice versa, component to the moon’s motion.

From the northern hemisphere, if two objects are at the same celestial longitude but one is northward of the other, the more northerly of the two objects will rise before the other one. The opposite is true from the southern hemisphere.

Thus, from the northern hemisphere, when the moon is in the part of its orbit where it is traveling northward, this will partially offset the 50-minute average in moonrise times, and the difference in moonrise time shrinks to about 20 minutes.

At the opposite end of the moon’s orbit, where it is traveling southward, the difference in moonrise times increases to about 80 minutes. Again, the opposite situation occurs in the southern hemisphere.

It so happens that, around the time of the Autumn Equinox in the northern hemisphere, the moon is going through its steepest south-to-north climb while it is at its full moon phase. We accordingly experience a succession of several bright moonlit nights where the moon rises only a little bit later each night.

Since this occurs near the traditional time of harvest in agricultural societies, the full moon that takes place nearest to the Autumn Equinox is traditionally called the Harvest Moon.

This year, the Autumn Equinox, when the sun passes through the celestial Equator from north to south, occurs just before 8 p.m. on this coming Saturday evening, Sept. 22. Meanwhile, the full moon occurs on Monday evening, Sept. 24. Thus, for the next several nights we will be experiencing the bright moonlit evenings of this year’s Harvest Moon.

Our friends in the Southern Hemisphere, on the other hand, are experiencing the opposite phenomenon, with the moon rising about 80 minutes later each night, and thus the full moon quickly escapes the early evening sky.

They won’t be experiencing their Harvest Moon until the full moon on March 20, 2019, at which time we northerners will be experiencing the opposite effect.

The phrase Harvest Moon brings to mind the classic early 20th century song “Shine On, Harvest Moon,” the writing of which is generally credited to the husband-and-wife team of Jack Norworth and Nora Bayes, although there is evidence that it was actually written by songwriter David Stamper, who among other things was Bayes’ pianist.

The song was premiered during the 1908 edition of the Ziegfeld Follies, a vaudevillian theatrical review that ran for several decades on Broadway and which featured many of the well-known performers of that era, including – in the 1918 edition exactly one century ago – the legendary actor and comedian W.C. Fields.

“Shine On, Harvest Moon” has been recorded by many artists in the decades since then and has been performed in a large number of movies and can legitimately be considered one of the best-known American songs of the entire 20th century.

With the bright moonlit nights, full moon periods, and especially the Harvest Moon, are often when astronomers take a break from observational activities, although in this era of digital imaging technology where moonlight can be filtered out, this is less true nowadays than it was in the past.

This author certainly intends to take advantage of this break period, and while the Ziegfield Follies are unlikely to be available, there are other forms of entertainment to be had. And before we know it, the moonlit nights of the Harvest Moon will soon be over, and dark nights will be here once again...

Alan Hale is a professional astronomer who resides in Cloudcroft. He is involved in various space-related research and educational activities throughout New Mexico and elsewhere.