When we define scientific terms, we often need to specify our reference points.

A distinct example of this involves the amount of time it takes the moon to orbit around the Earth. Strictly speaking, the moon does not turn around the Earth; the Earth and Moon gravitate around their common center of gravity, but this point is about a thousand miles below the surface of the Earth. The distinction is therefore somewhat debatable.

It takes a little more than 27 days for the moon to appear in front of the same background stars; it's the sidereal orbital period of the moon. However, during this time, Earth has traveled about a twelfth of its orbit around the sun, and it takes two more days for the moon to display the same phase as before. This 29-day interval is known as the synodal orbital period of the moon.

On average, the moon rises about 24/29 days, 50 minutes later each night. This would be the actual difference in moonrise times throughout the year if the moon was in orbit in the plane of the Earth's equator, but the moon orbits more in the plane of the orbit around the sun, tilted 23 degrees from the equator of the earth. Because of the gravitational forces of other planets, the difference between the orbit of the moon and the equator can vary between 18 and 29 degrees.

Thus, besides the general west-east movement of the Moon against the background stars, there is also a distinct component of the north-south movement and vice versa.

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

So, from the northern hemisphere, when the moon is in the part of its orbit where it moves north, this will partially offset the 50-minute average at the moonrise times and the difference in time from Moonrise is reduced to about 20 minutes.

At the other end of the moon's orbit, where it moves to the south, the difference in moonrise time increases to about 80 minutes. Again, the opposite situation occurs in the southern hemisphere.

It turns out that at the time of the autumnal equinox in the northern hemisphere, the moon crosses its steepest south-north ascent when it is in its full moon phase. As a result, we live a succession of bright moonlit nights where the moon does not rise until a little later each night.

As happens near the traditional harvest period in agricultural societies, the full moon that takes place closest to the autumnal equinox is traditionally called the moon of the harvest.

This year, the autumn equinox, when the sun crosses the celestial equator from north to south, occurs just before 8 pm. this saturday night september 22nd. In the meantime, the full moon will take place on the evening of Monday, 24 September. Thus, for the next nights, we will attend the moonlight evenings of this year's Harvest Moon.

Our friends from the southern hemisphere, on the other hand, know the opposite phenomenon, the moon rising about 80 minutes later each night and the full moon so quickly escapes to the sky in the early evening.

They will not know their Harvest Moon before the full moon of March 20, 2019, when we will have the opposite effect.

The phrase Harvest Moon evokes the classic song "Shine On, Harvest Moon" of the early twentieth century, whose writing is generally attributed to the husband and wife team of Jack Norworth and Nora Bayes, although everything suggests in fact written by singer-songwriter David Stamper, who was among others the pianist of Bayes.

The song was created during the 1908 edition of the Ziegfeld Follies, a vaudeville-style theatrical revue that lasted for several decades on Broadway and which featured many famous performers from that era, including legendary actor and comedian WC Des Champs.

"Shine On, Harvest Moon" has been recorded by many artists for decades and has been featured in a number of films and can legitimately be considered one of the most famous American songs of the twentieth century.

With bright nights, the periods of full moon, and especially the moon of the harvest, often occur when astronomers take a break from observation activities, although in this era of digital imaging technology where the clarity of moon can be filtered in the past.

This author certainly intends to take advantage of this break, and although the Ziegfield Follies are probably not available, there are other forms of entertainment. And before we know it, the moonlit nights of the Harvest Moon will be over soon and the dark nights will be here again …

Alan Hale is a professional astronomer residing in Cloudcroft. He is involved in various space research and education activities in New Mexico and elsewhere.

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