I still recall from childhood how the Harvest Moon would appear in the eastern sky night after night at almost the same time. The repetitious nature of astronomy has filled this life with more full moons than I can count. Like a Zoom meeting I try to show up on time to each moonrise and mute my internal microphone to better enjoy the experience.

The full moon always lies opposite the sun in the sky. When the sun sets, the moon rises. This will be the scene Monday night, September 20th, facing east-southeast shortly after sunset. The Harvest Moon will shine from the constellation Pisces the Fish. Contributed / Stellarium
The full moon always lies opposite the sun in the sky. When the sun sets, the moon rises. This will be the scene Monday night, September 20th, facing east-southeast shortly after sunset. The Harvest Moon will shine from the constellation Pisces the Fish. Contributed / Stellarium

The Harvest Moon is the full moon that occurs closest to the autumnal equinox, better known as the first day of fall. Since full phase occurs on Monday, Sept. 20, this year's Harvest Moon is summer's last full moon. Fall begins two days later on Wednesday, Sept. 22, at 2:20 p.m. Central time. Astronomically speaking, we have a big week ahead.

The moon orbits around the Earth at an average speed of 2,300 miles an hour (1 km/sec). This causes it to move eastward across the sky at the rate of one full-moon diameter per hour or about 12°-13° each day. Contributed  / Bob King
The moon orbits around the Earth at an average speed of 2,300 miles an hour (1 km/sec). This causes it to move eastward across the sky at the rate of one full-moon diameter per hour or about 12°-13° each day. Contributed / Bob King

Each night the moon moves about 12°-13° to the east, roughly the width of a horizontally-held fist held to the sky. Normally, this motion delays successive moonrises by about 50 minutes a night. But in September and October, the full moon's path at the eastern horizon lies at a very shallow angle. When it slides 13° to the east, much of that distance is to the left (north as you face east) with only a small portion in the downward direction below the horizon.

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In late summer and early fall the full moon's path lies at a shallow angle to the horizon. For several nights in a row it never dips very far below the horizon, so moonrise times are just 20 minutes apart. The opposite occurs in spring when the path slants at a steep angle, delaying successive moonrises by more than an hour. Contributed / Bob King
In late summer and early fall the full moon's path lies at a shallow angle to the horizon. For several nights in a row it never dips very far below the horizon, so moonrise times are just 20 minutes apart. The opposite occurs in spring when the path slants at a steep angle, delaying successive moonrises by more than an hour. Contributed / Bob King

For several nights in a row the nearly full moon dips only about 5° below the horizon. That's not much. The Earth only needs to spin a little extra each night to bring the moon back into view, with successive moonrises spaced only about 20 minutes apart. No wonder the full moon seems to stagnate in the eastern sky this time of year.

This is part of a larger painting titled "The Harvest Moon" by George Mason that was exhibited in 1872. It depicts an agrarian scene by moonlight. Contributed / George Mason, public domain
This is part of a larger painting titled "The Harvest Moon" by George Mason that was exhibited in 1872. It depicts an agrarian scene by moonlight. Contributed / George Mason, public domain

Now, imagine yourself 150 years ago before the invention of electric lighting. The moon provided useful illumination, helping to light the way at night. It was also handy at harvest time because you could continue to work by moonlight long into the evening. The same way modern humans use electricity to accomplish chores at night, our ancestors employed the bountiful light from the Harvest Moon to bring in the harvest. In the past, life was more closely tied to the land and sky. It's less so today and with mixed results. Convenience has its price.

This panel compares the full moon's path at the eastern horizon in September (left) and in March (right). Contributed / Stellarium
This panel compares the full moon's path at the eastern horizon in September (left) and in March (right). Contributed / Stellarium

The best way to demonstrate the changing angle of the full moon's path is to compare fall and spring. In March and April, its track slants at a steep angle to the eastern horizon. Even though the moon still slides 13° eastward per day, most of that movement takes it down below the horizon, with only a small amount of sideways movement. In spring, the full moon is moving southward along its path, so its rising point shifts to the right a little each night.

In spring, the full or nearly-full moon dips about 12° below the horizon each night — more than twice what it does in September. The Earth must rotate considerably longer to "carry" the moon into view at the horizon. Successive moonrises can be delayed by as much as 75 minutes, making the spring full moon of limited use for nighttime activities.

The farther north you go the more pronounced the Harvest Moon Effect. This diagram shows the moon's changing position and rise times for Nome, Alaska a few years back. Notice that the moon's path is nearly parallel to the horizon, with a difference of just 5-6 minutes between successive moonrises. Contributed / Stellarium
The farther north you go the more pronounced the Harvest Moon Effect. This diagram shows the moon's changing position and rise times for Nome, Alaska a few years back. Notice that the moon's path is nearly parallel to the horizon, with a difference of just 5-6 minutes between successive moonrises. Contributed / Stellarium

The path the moon follows is called the ecliptic, and it's the same path the planets and sun take. Since the moon's orbit is tipped about 5° in relation to Earth's orbit, it "weaves" around the ecliptic a bit, but still follows the same basic "highway" in the sky. The angle that highway meets the horizon varies during the course of the year from steep to shallow to steep again. Two things make this happen: the 23.5° tilt of the Earth's axis combined with the planet's year-long journey around the sun.

The tilt of Earth's axis and yearly orbital revolution are responsible for both the changing seasons and the shifting rising points of the sun and moon along the eastern horizon. Contributed / Sonoma University
The tilt of Earth's axis and yearly orbital revolution are responsible for both the changing seasons and the shifting rising points of the sun and moon along the eastern horizon. Contributed / Sonoma University

The tilt causes the sunrise point to vary from southeast in winter to northeast in summer. If there were no tilt, the sun would rise due east and set due west every day of the year. Earth's slant, paired with it yearly revolution, makes the rising direction continuously shift back and forth along the horizon — southeast in winter, east in spring, northeast in summer, east in fall and back to southeast in the winter. As you'd expect, the same principle applies to the setting sun and moon.

Since the full moon lies opposite the sun, it follows that its rising point must also shift back and forth across the year but in the opposite sense: when the sun rises in the northeast in summer, the moon rises in the southeast. The only times they rise and set in the same direction are on the first days of fall and spring.

Two simple facts — Earth's tipped axis and yearly revolution — conspire every fall to provide us with a Harvest Moon and its welcome light.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.