SKY GUIDE: This map represents the night sky as it appears over Maine during September. The stars are shown as they appear at 10:30 p.m. early in the month, at 9:30 p.m. at midmonth, and at 8:30 p.m. at month’s end. Jupiter and Saturn are shown at their midmonth positions. To use the map, hold it vertically and turn it so that the direction you are facing is at the bottom.  Sky Chart prepared by Seth Lockman

The month of September marks the beginning of fall for us in the northern hemisphere. This year that will happen at exactly 2:50 a.m. on Saturday, Sept. 23. The autumnal and the vernal equinoxes are the only two days each year that the sun will rise due east and set due west for everyone on Earth except for the poles. Within a few days of those dates the days will also be exactly 12 hours long for everyone on earth except for the poles. They are off by a few days since we orbit in ellipses and not perfect circles and we are tilted at 23.5 degrees on our axis.

Autumn tends to be our best season for viewing the heavens since there is less humidity, more clear days and it is not too cold yet to enjoy the ever-lengthening nights. There are several good highlights to see this month including Saturn still near its best for the year, Jupiter getting a little brighter and closer each night, Venus at its greatest brilliancy in the morning sky, the best morning apparition of Mercury for the year, and the beginning of the zodiacal light becoming visible in the morning sky for several months.

Saturn has been at its best and brightest and closest for the year toward the end of August. In September, it rises just a few minutes earlier each night but is still visible almost all night long, reaching its highest point in the sky around midnight. The ringed planet is getting slightly dimmer and farther away each evening, but you won’t really notice that until near the end of September.

Look for its soft golden glow in Aquarius, the water bearer. Through a good telescope you may even see some of the white spots in its atmosphere which signal the beginning of another season of mega storms that occur every 25-30 years, or about the time it take for Saturn to orbit the sun once. They are caused by higher concentrations of ammonia falling as rain or hail from its upper atmosphere into its lower atmosphere.

A bright waxing gibbous moon will be near Saturn on Sept. 26. Notice that its rings are fairly narrow now at only 9 or 10 degrees. They can be tilted at a maximum angle of up to 27 degrees. They are on their way down to zero now, which it will reach in 2025. Then they will reach their maximum angle again by 2039, halfway back to its next 29-year cycle around the sun. Humans will most likely be walking around on Mars by the time Saturn’s rings reach their maximum angle again.

Jupiter is not far behind since it now rises around 10 p.m. early in September and it will rise by 8 p.m. by the end of the month. The king of the planets begins its retrograde or westward motion against the fixed background of stars on Sept. 4 in the constellation of Aries the Ram, two constellations to the east of Saturn in Aquarius. Jupiter will reach its own opposition on Nov. 3.

Venus will be at its greatest brilliancy for the year on Sept. 19 at minus 4.8 magnitude, or almost 100 times brighter than Saturn and still seven times brighter than Jupiter. Through a telescope you will notice that it is a thin crescent only 11% lit by the sun at the beginning of the month and it will grow all the way to 36% lit by the end of the month. It will slowly be getting dimmer again after Sept. 19, but it will continue to get more illuminated even as it is getting smaller and farther away from us. Venus rises around 5 a.m. and it will be just below a slender waning crescent moon on Sept. 11 in Cancer the Crab.

Watch for Mercury to pop back into the morning sky one week into September after its inferior conjunction with the sun. Look for our first planet below Regulus in Leo the Lion by the middle of the month below and to the left of Venus and then it reaches its greatest western elongation from the sun at 18 degrees. It will be at its brightest on Sept. 29 when it will reach 9 degrees high above the eastern horizon half an hour before sunrise.

There should be many comets brighter than 10th magnitude visible to us over the coming year. The one for this month should reach 8th magnitude and will be closest to us on Sept 25 and 26 in the constellation of Auriga, which marks the top of the winter hexagon. It is named 103P/Hartley 2. It orbits the sun every 6.5 years and was discovered by Malcolm Hartley in 1986. It will be just 5 degrees above the colorful California Nebula in Perseus during the middle of this month, which would be a great time to get a picture of this ancient and primordial cosmic interloper if you have the equipment. Otherwise, just look for it in a pair of binoculars around the middle of the month since it will be new moon on Sept. 14.

The zodiacal light will become visible again in the morning sky starting late in September into November. This faintly glowing pyramid of ghostly light is caused by sunlight bouncing off trillions of tiny dust particles in the ecliptic plane of our solar system. This dust ring is always there, but is best visible to us when the angle of the ecliptic to our horizon is at its steepest, which is about one hour before sunrise in the fall and one hour after sunset in the spring.

SEPTEMBER HIGHLIGHTS

Sept.1: The moon passes 1.4 degrees south of Neptune at 3 in the morning.

Sept. 2: Venus is stationary at midnight. It started its retrograde or westward motion against the fixed background of stars on July 22 this summer. Venus retrogrades every 18 months, spending about 40 days seeming to move backwards.

Sept. 3: In 1976 Viking 2 landed on Mars. Viking 1 landed there a few weeks earlier.

Sept. 4: The moon passes 3 degrees north of Jupiter this afternoon. Jupiter is stationary and then begins its retrograde motion at 5 p.m.

Sept. 6: Mercury is in inferior conjunction with the sun today at 7 am.

Sept. 7: James van Allen was born in 1914. He discovered the van Allen radiation belts in 1958, a zone of energetic charged particles caused by the solar wind and captured by the earth’s magnetic field. I interviewed his son and grandson for “Scientifically Speaking,” a radio show aired Friday mornings from 11:30 to noon WMPG (90.9 FM) that I co-host with Sarah Chang.

Sept. 11: The moon passes 11 degrees north of Venus this morning.

Sept. 14: The new moon is at 9:40 p.m. The new moon next month will create an annular solar eclipse visible from Oregon to Texas on Oct. 14. It will be a partial solar eclipse for the rest of the country and we will only see about 5% here in Maine.

Sept. 16: The moon passes 0.7 degrees north of Mars this afternoon.

Sept. 19: Venus is at greatest brilliancy at magnitude minus 4.8.

Sept. 21: The moon passes near Antares in Scorpius this morning.

Sept. 22: Mercury is at greatest western elongation from the sun at 18 degrees this morning. First quarter moon is at 3:32 p.m.

Sept. 23: The autumnal equinox is at 2:50 a.m.

Sept. 26: The moon passes 3 degrees south of Saturn tonight.

Sept. 29: The full moon is at 5:58 a.m. This is the famous Harvest moon since it is close to the equinox. The Harvest moon only rises about half an hour later each night instead of the usual 55 minutes later because the angle of the ecliptic to the horizon is very shallow now.

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