September always marks the beginning of fall for us in the Northern Hemisphere. This year the autumnal equinox will occur at 10:49 a.m. Saturday, Sept. 22. Fall is always a good time to get reacquainted with the sky since the air is crisper and cooler.

That moment is further defined as the minute that the sun along the ecliptic is crossing over the celestial equator on a downward path. We will notice the days getting shorter after that as we head into winter, just three months away.

Both of the equinoxes are special times for the Earth because those are the only two days each year when the sun rises due east and sets due west for every place on Earth except at the poles, where the sun will take all day to slowly circle the horizon. Within a few days of the two equinoxes, the day and the night are also of equal length everywhere on Earth except for the poles.

A comet will be visible in Bootes the Herdsman in September. You will need a telescope to see it, because it will only get to 10th or 11th magnitude. Named Comet C/2011 F1, it was discovered by the LINEAR (Lincoln Near Earth Asteroid Research) telescopes. Operated by Lincoln Labs at the Massachusetts Institute of Technology, it consists of two automated telescopes at the White Sands Missile Range in Socorro, N.M. Observers there started looking for dangerous asteroids that may hit Earth back in 1997. In just the past year, these telescopes have found more than 2,400 near-Earth asteroids and 279 comets.

Venus rises about 3 a.m. Our sister planet is now over half full. Its dramatic transit across the face of the sun happened just three months ago, on June 5. It will begin September near Pollux in Gemini, but it shines about 100 times brighter than the star. Then Venus will cross into Cancer the Crab and pass just below the Beehive star cluster on the morning of Sept. 12. A waning crescent moon will be just to the southwest of the pair, which will make a stunning sight in the sky about 6 a.m.

Similar to the Pleiades, there are about 50 stars in the Beehive that are visible using binoculars. There are actually about 1,000 stars in the Beehive. They are located about 600 light-years away and are quite young at 700 million years.

Jupiter now rises before midnight in Taurus the Bull. The king of the planets will be about 10 degrees, which is one fist at arm’s length, northeast of the Hyades star cluster all month long, radically changing the look of this familiar constellation. You can see the planet’s four bright Galilean moons with just a pair of good binoculars.

Marking the face of Taurus, the Hyades star cluster is about 150 light-years away and about 600 million years old, which is just over one-eighth the age of our sun and Earth. In mythology, the Hyades are the five daughters of Atlas and they are half sisters to the Pleiades. All of these stars have a common motion toward a point in the sky just east of the red supergiant star Betelgeuse in Orion. What is even more interesting is that they can be traced back to a common origin in the sky with the Beehive star cluster.

Mars and Saturn are the evening planets. By the end of September, Saturn will set just one hour after sunset. Mars still shines with an orange hue and is a little fainter than golden Saturn. They met in close conjunction in August along with the star Spica in Virgo. Now Mars is rapidly moving eastward and will cross into Libra by the middle of the month.

Uranus will reach opposition, when it is closest to Earth, on Sept. 29. It will be in the constellation of Pisces and you can see it with a pair of binoculars. Discovered on March 13, 1781, by William Herschel, our seventh planet was first named George in honor of King George of England. Our eighth and last full-fledged planet, Neptune, was discovered on Sept. 23, 1846. It was the first and only planet in our solar system to be predicted mathematically and then actually discovered very close to where it was predicted to be. You can see our last planet through a telescope in Aquarius. It reached its opposition in August. 

Sept. 1 — Venus passes 9 degrees south of Pollux in Gemini this morning.

Sept. 2 — The moon passes 5 degrees north of Uranus this evening.

Sept. 7 — The moon is at apogee, or farthest from the Earth, at 251,217 miles this morning.

Sept. 8 — The moon passes less than 1 degree south of Jupiter this morning. Last-quarter moon is at 9:15 a.m.

Sept. 9 — The moon passes less than 1 degree north of our largest asteroid, Ceres, this morning.

Sept. 12 — The waning crescent moon passes just south of Venus this morning.

Sept. 14 — Giovanni Cassini, the astronomer and mathematician, died on this day in 1712. He was born on June 8, 1625. He was the first person to see four of Saturn’s moons. The largest gap in Saturn’s rings is named after Cassini. NASA also launched a Saturn mission named after Cassini. The probe took many terrific pictures and made many amazing discoveries, including the fact that Saturn’s rings are extremely thin, perhaps only 20 to 30 feet thick.

Sept. 15 — New moon is at 10:11 p.m.

Sept. 18 — The moon passes just south of Saturn and Spica in Virgo this evening.

Sept. 19 — The moon passes just south of Mars this evening.

Sept. 22 — The autumnal equinox is at 10:49 a.m. First-quarter moon is at 3:41 p.m.

Sept. 24 — Our second-largest asteroid, Pallas, about 330 miles in diameter, is at opposition tonight. Our largest asteroid, Ceres, is about 600 miles in diameter. Our four largest asteroids, Ceres, Pallas, Vesta and Juno, make up more than half of the mass of all the millions of asteroids orbiting in the asteroid belt between Mars and Jupiter.

Sept. 27 — The moon passes 6 degrees north of Neptune this morning.

Sept. 29 — Uranus is at opposition this morning. Full moon is at 11:19 p.m. This is the famous harvest moon. It is special because it only rises about half an hour later each evening instead of the usual hour or so later. This way it provides more consistent light for several nights for the farmers to harvest their crops. As you go farther north, the difference in rising times is even less. This is because the angle of the ecliptic along which the sun, moon and all the planets always travel is most shallow in relation to our horizon at this time of year.

Sept. 30 — Mercury passes just north of Spica this evening.

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