September marks the beginning of fall in the northern hemisphere. This year, that will happen at 10:21 a.m. on Thursday, Sept. 22. This is a critical point in understanding our orbit around the sun and the relative positions of the earth and the sun on the ecliptic, which is the path that the sun, moon and all of the planets are on as we orbit the sun.

The autumnal equinox and the vernal equinox are the only days each year when the sun rises due east and sets due west for everyone on earth. The very next day after fall starts, the sun will already rise a little farther south, on the way to its lowest point, which will be the winter solstice in three months.

This chart shows the sky as it appears over Maine in September. The stars are 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. Mars and Saturn are shown in their midmonth positions. Hold the map vertically and turn it so that the direction you are facing is at the bottom. Sky chart prepared by George Ayers

The word equinox means “equal night.” That is always true on the equator, but you will see that the days will not be exactly 12 hours long until about three days after the fall equinox and they are already 12 hours long about three days before the spring equinox. That helps show that the earth orbits in an ellipse and not a perfect circle around the sun.

The sun on the ecliptic crosses the celestial equator on a downward path as fall starts and on an upward path as spring starts. These are imaginary extended planes, but they are important to visualize and better understand how the dynamic inner solar system works and how mathematical it really is.

This is a great month to get out under the night sky, because we tend to have more clear nights and it is less humid with fewer bugs. There are several highlights this month, but it is always well worth it just to get out under a clear sky to keep learning and be surprised, and share the sky that is above all of us with others. A perfect example was the Perseid meteor shower earlier this month. We saw several hundred meteors at its peak after the moon set after midnight. Along with attaining a better sense of the vastness and dynamic nature of the sky above us, we also heard elemental terrestrial sounds like the plaintive hoots of two owls communicating in the woods and the exciting yelps and howls of a pack of coyotes racing through the field.

We will lose Mercury and Jupiter this month, but all of the other planets will still be visible in the evening sky. There will be some nice conjunctions of planets with the moon; another meteor shower from Perseus called the Epsilon Perseids; the Aurigid meteor shower; a faint comet in Cancer the Crab; the third largest asteroid, Pallas, traveling through Equuleus the Little Horse; and even a lunar and a solar eclipse – but those two will not be visible in this country.

As the terrestrial seasons are changing this month on the earth, so are the celestial seasons above us. You will see the Pleiades in Taurus and the top of the Winter Hexagon emerge once more over the eastern horizon only a few hours after sunset as fall begins. You can already see this now if you stay out late, like we did for the Perseids.

Then keep watching Venus and Jupiter after their great conjunction at the end of August. Look low in the southwestern sky half an hour after sunset and you will see Venus getting a little higher each evening, even as Jupiter slowly sinks out of sight. A very slender waxing moon joins the pair on Sept. 2 and 3.

Then travel three constellations to the east along the ecliptic and you will see orange Mars extending its distance from golden Saturn a little more each evening. They still form a nice, changing triangle with the red supergiant Antares in Scorpius. A first-quarter moon will join the trio on Sept. 8 and 9. Last month, it looked like Mars was being sent through the sky by an imaginary slingshot that you could create by connecting Saturn and Antares. All three lined up perfectly on Aug. 23, and then Mars slowly proceeded eastward after that – as it continues to this month. That is a great way to look at this part of the sky to get a better sense of true motions and speeds of those three objects.

Mars takes nearly two years to orbit the sun, and it lines up with the earth at opposition every 26 months. Its average speed around the sun is 15 miles per second, or just a little slower than our own speed of 18.6 miles per second. Saturn travels at 6 miles per second and takes nearly 30 years to orbit the sun once.

The surprise comes with Antares. It is traveling at nearly 500,000 miles per hour around the center of our galaxy, just like our sun. It is about 600 light years away, whereas Saturn is just over one light hour. That places Antares about 5 million times farther away. Shining at first magnitude, its distinct orange color looks only a little less bright than Mars, which is now about zero magnitude, so you gain a better appreciation of how bright and large Antares has to be. It is 700 times larger than our sun, which means that if you could place Antares where our sun is in the sky, the earth and Mars and even the asteroid belt would all be orbiting inside this massive star. Antares is 15 times as massive as our sun, which makes it 50,000 times more luminous, since a star’s luminosity is the fourth power of solar mass. If a star is twice as massive as our sun, it would be only 16 times as bright.

If you missed the Perseid meteor shower, there will be another little shower emanating from Perseus in September. They are called the Epsilon Perseids and they will peak after midnight on Sept. 9. However, they will not be nearly as prolific as the Perseids, producing only about five meteors per hour, which is just above the background rate of three to four meteors per hour that you could see on any clear night.

The Aurigid meteor shower peaks on the 1st, but it will not produce much more activity than the Epsilon Perseids.

If you have access to a good telescope, you can see 12th magnitude comet 43P/Wolf-Harrington glide through Cancer this month. A close approach to Jupiter in three years will alter the path of this comet, so next time around it will probably be twice as far away and four times fainter than it is now.

Since we are in an eclipse season again, the two eclipses this month will be an annular eclipse over Central Africa and Madagascar during the new moon on Sept. 1 and a penumbral lunar eclipse visible in the Eastern Hemisphere over Europe, Africa and Asia on Sept. 16 during the full moon. There will be a brilliant ring of sunshine visible around the moon during the annular eclipse because the moon will be a little too far away to completely cover the sun, and the moon will not even go into the deepest part of the earth’s shadow, called the umbra, during the penumbral lunar eclipse on Sept. 16.

SEPTEMBER HIGHLIGHTS

Sept. 1: New moon is at 5:03 a.m.

Sept. 2: Neptune is at opposition in Aquarius. The moon will be very close to Jupiter tonight.

Sept. 3: Viking 2 landed on Mars in 1976. The moon is near Venus tonight.

Sept. 8: The moon is near Saturn tonight and Mars the next night.

Sept. 9: First quarter moon is at 7:49 a.m. Viking 2 was launched in 1975.

Sept. 12: Mercury is at inferior conjunction, or closest to earth, tonight. The transit of Mercury happened on May 9 of this year during the last time our first planet was at inferior conjunction.

Sept. 16: Full moon is at 3:05 p.m. This is the famous Harvest moon near the fall equinox.

Sept. 22: The autumnal equinox is at 10:21 a.m. in the Northern Hemisphere.

Sept. 23: Johann Gottfried Galle discovered Neptune in 1846. It has made just over one orbit since.

Sept. 28: Mercury is at its greatest western elongation and makes its best appearance for the year in our morning sky.

Sept. 30: The second new moon of this month happens at 8:11 p.m.

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