The month of September always marks the beginning of fall for us in the northern hemisphere. This month, that will happen at exactly 9:04 p.m. on the 22nd. The autumnal equinox, along with the vernal equinox, are the only two days each year when the sun will rise due east and set due west for everyone on Earth except at the poles. The days and nights will also be 12 hours for everyone on Earth – except at the poles – within a few days of the equinoxes. That is because we are tilted on our axis and orbit in an ellipse, otherwise it would happen on the same day.

We are now tilted at 23.5 degrees. You can prove that for yourself by simply measuring the angle of the sun above your horizon at noon on the winter solstice and again at noon on the summer solstice, subtract your latitude, and divide by two. For us at this latitude near Portland at 43.6 degrees, the sun reaches about 67 degrees high on the summer solstice and only 24 degrees on the winter solstice at noon.

Our obliquity actually changes slowly from 24.5 degrees down to 22.1 degrees and back again on a 41,000-year cycle. We will reach our minimum tilt in about 10,000 years. The eccentricity of our elliptical orbit around the sun is also changing a little over a 100,000-year period.

The third cycle is the 26,000-year cycle called the precession of the equinoxes. Our north star is continually changing. It will be Gamma Cephei in 2,000 years and Vega in about 13,000 years. Our axis traces a circle with a roughly 45 degree diameter in the sky every 26,000 years because the whole earth is wobbling like a top. We will be able to see the famous southern cross here in Maine when Vega becomes our north star, but I recommend you travel south to see it way before then.

Those three cycles are called the Milankovitch cycles after the Serbian engineer who discovered them in the 1930s. These play a major role in the formation and dissolution of our ice ages. We have been through eight of them in the last million years. We are very lucky that our tilt is not more extreme like on some planets and that we have a moon to further stabilize our seasons and tides.

As the nights are getting longer and cooler now, there are several good highlights to enjoy this month. These include the opposition of Jupiter on the 26th and the opposition of Neptune on the 16th. Since Saturn reached opposition last month, it is now rising before sunset and becomes visible low in the east as soon as it gets dark enough, which will be around 8:30 p.m. The second largest asteroid, Vesta, is visible near Saturn, and comet PanSTARRS is still visible in Scorpius. Then there are the usual close conjunctions of the moon with the planets.

Jupiter spends one year in each of our 12 zodiac constellations. You can find it in southern Pisces now, and it will rise exactly at sunset on the 26th. It will then be at its closest and brightest for the year, revealing many of its cloud bands, its Great Red Spot, and shadows of its moons in transit across its disk even in a small telescope.

You can see Neptune in Aquarius just to the right of Jupiter and a quarter of the way toward Saturn with just a pair of binoculars, since it will reach its brightest for the year, 7.7 magnitude, on the 16th. It glows with a beautiful faint bluish light. Mars now rises around midnight to begin this month and it will rise by 10 p.m. at the end of the month. It will reach its own opposition on Dec. 8. Notice that the red planet becomes a new “star” in Taurus this month, since it can be seen right above Aldebaran. Then look just to the left to see orange Betelgeuse in Orion, now at magnitude 0.6, having returned to its more normal brightness after reaching its dimmest magnitude in modern times in late 2019. Aldebaran is a first magnitude star, the faintest of the three neighboring orange objects. Compare their color and brightness all month long as Mars gets brighter each night while Aldebaran and Betelgeuse remain the same.

Mercury will make a poor appearance low in the western evening sky for the first two weeks this month. Then Venus is still the brilliant morning star low in the eastern sky in Leo. Through a telescope, you would see that it is nearly fully illuminated now, near superior conjunction with the sun. Look for a thin waning crescent moon with earthshine just above Venus on the morning of the 24th, just after fall will have started.

Look for the second largest asteroid, Vesta, at 330 miles across or about the size of Arizona, 10 degrees below and to the left of Saturn in Capricorn. It will reach sixth magnitude, so it should be easy to see in a pair of binoculars if the moon is not too bright. Many of our meteorites found on Earth come from Vesta, so we have a good idea of its composition even though we have never landed on it.

Comet C/2017 K2(PanSTARRS) is still about seventh magnitude and visible in binoculars low in the western sky between Scorpius and Libra. Discovered in May of 2017, this comet is four times larger than Halley’s, but it does not have much of a tail because it remains too far from the sun for the solar wind to sublimate its surface into gas and dust that would always stream away from the sun no matter which direction the comet is traveling through space.

SEPTEMBER HIGHLIGHTS

Sept. 3: First quarter moon is at 2:08 p.m. In 1976, Viking 2 landed on Mars.

Sept. 4: Venus passes just north of Regulus.

Sept. 7: James Van Allen was born in 1914. He discovered the Van Allen radiation belts that surround the earth.

Sept. 8: The moon passes 4 degrees south of Saturn this evening. Mars passes 4 degrees north of Aldebaran this morning.

Sept. 10: Full moon is at 5:59 a.m. This is the famous harvest moon, since it is the closest full moon to the fall equinox. It will rise only about half an hour later each night instead of the average time of 55 minutes because of the shallow angle of the ecliptic with our eastern horizon now. The moon passes 3 degrees south of Neptune.

Sept. 11: The moon passes 1.8 degrees south of Jupiter.

Sept. 12: Anousheh Ansari was born in 1966. She is Iranian-American and is part of the Ansari X-prize to encourage innovation that benefits humanity. She was also the first woman space tourist, as she spent 11 days in space aboard the ISS in Sept. of 2006.

Sept. 14: John Dobson was born in 1915. He invented the Dobsonian telescope that is easy to make and he was a key contributor to Stellafane for many years.

Sept. 15: James Christy was born in 1938. He discovered and named Charon the largest moon of Pluto’s five moons, which is about half the size of Pluto at 750 miles.

Sept. 16: The moon passes four degrees north of Mars. Neptune is at opposition.

Sept. 17: Last quarter moon is at 5:52 p.m.

Sept. 22: The autumnal equinox is at 9:04 p.m.

Sept. 25: New moon is at 5:55 p.m.

Sept. 26: Jupiter is at opposition at 4 p.m.


Bernie Reim of Wells is co-director of the Astronomical Society of Northern New England.

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