SKY GUIDE: This chart represents the 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. Mars, Saturn and Jupiter are shown in 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 George Ayers.

The month of September always marks the beginning of autumn for us in the northern hemisphere and will happen on Tuesday, Sept. 22 at 9:31 a.m. There are only two days each year that the sun will rise due east and set due west and this is one. The other is the vernal equinox half a year later.

Within a couple of days of the equinox the days and nights are exactly 12 hours long for everyone on Earth except for the poles. That is because we travel around the sun in a slightly elliptical orbit and we are tilted at 23.5 degrees now. However, that tilt is constantly and slowly changing over a 41,000-year time period. The maximum obliquity we reach is 24.5 degrees and the minimum is 22.1 degrees. It is currently diminishing toward the 22.1 degrees, which we will reach again in about 20,000 years. Without that tilt there would be no seasons and life on earth would probably never have evolved much past hunter-gatherer tribes because our climate would become stratified. If we did not have a moon no higher life forms may have evolved on earth at all since the gravitational forces between the earth and moon stabilize all those cycles along with creating stronger tides and good circulation in our oceans to make them suitable for life without getting too extreme.

The other two continually and slow-changing cycles that contribute to the formation and dissolution of ice ages are the changing eccentricity of our orbit around the sun over 100,000 years, and the 26,000-year precession of the equinoxes. Those three together are called the Milankovitch cycles, named after the Serbian engineer who discovered them in the 1930s. Right now our north star is Polaris but it will be Vega, the brightest star in the summer triangle, in 13,000 years.

We will soon get cooler and crisper nights that will make for great viewing of the night sky this month. There are many good highlights this month that include the four brightest planets all near their best for the year, Mars doubling in brightness to outshine Jupiter, a very close conjunction of the moon and Mars, and another comet close to comet Neowise.

Jupiter and Saturn are already up before sunset and then Mars rises by 9:30 as the month begins and by 8:30 at the end of the month. Then Venus still rises about 3 am in Gemini in the winter hexagon each morning this month as it is getting farther ahead of us in our respective orbits. Mercury is the only one that can only manage a poor appearance toward the end of the month in the evening sky. Look for our first planet to pass within just one degree of Spica in Virgo, near where comet Neowise was a couple of weeks ago.

Both Jupiter and Saturn end their retrograde motions this month and go back to their normal eastward treks across our skies. That will happen on Sept. 12 for Jupiter and Sept. 29 for Saturn. After that notice that Jupiter will be slowly closing the 8 degree gap currently between them. That will continue for the entire season of autumn and on the winter solstice they will reach their closest conjunction in about 400 years at less than one tenth of a degree apart, easily visible in the same field of view in a telescope. Usually each planet is quite a treat and learning experience by itself, but just one season from now, they will look like a great double planet.

I recently showed these two showcase planets, along with several other celestial favorites, to a small group of people through my telescope. The moon was a slender crescent with earthshine and we saw about 10 Kappa Cygnid meteors over the course of a couple of hours. This is a minor shower and I did not expect to see any of them, so that was a nice surprise. We also saw several satellites, which was expected because they are now launching 120 new satellites every month, 60 at a time. Then we watched the Cygnus arm leading us right into the center of our Milky Way galaxy below Sagittarius and Scorpius slowly become more defined as the night got darker and remained crystal clear after the moon set soon after sunset.

Jupiter is about half as far away as Saturn and is 15 times brighter than the ringed planet. Jupiter is about half a billion miles away, or 42 minutes at the speed of light. They are both getting slightly fainter and farther away now, but they are still brighter and closer than usual since they are not too far past their oppositions.

Mars is a different story. The red planet will double in brightness this month to outshine Jupiter as it gets considerably closer and brighter as we rapidly catch up with it in our orbits. That will happen on Oct. 6 of next month when Mars will reach a close opposition and rise right at sunset. You can already see some detail on the Martian surface in an average telescope and that will improve dramatically throughout this month and next as long as no planet-wide dust storms obscure its surface from view as often happens near its opposition because it is also getting closer to the sun. You can expect to see some dark markings on its surface, one or both of its polar icecaps, and even some of its thin atmosphere.

Watch carefully on the night of Sept. 5 around 11 p.m. as the waning gibbous moon will pass just half a degree below Mars. It will even occult the planet in Central America and parts of South America. The other nice lunar conjunctions this month are Venus and the moon on the morning of the 14th, Jupiter and the moon on the evening of Sept. 24, and Saturn and the moon the next evening.

There is another comet now that will be visible in our evening sky in Libra and Scorpius this month and next. It will not be as bright as NEOWISE was, but it should reach 9th magnitude by the 26th, when it will be at perihelion or closest to the sun for this orbit. It is called 88P/Howell and was discovered back in August of 1981 by Ellen Howell. It orbits the sun every 5.5 years, so it is nothing like NEOWISE which will not return again for nearly 7,000 years

Comet Howell was the target of a proposed NASA mission in 2017, but it did not make the cut. That was called CORSAIR, which stands for comet rendezvous, sample acquisition, investigation and return. We did successfully land a spacecraft named Philae, part of the Rosetta Mission, on Comet Churymov-Gerasimenko in November of 2014. Then we also smashed a probe named Deep Impact into Comet Tempel 1 on July 4, 2005. It created a crater 150 meters in diameter on this comet and released a large dust cloud that was carefully analyzed to learn more about the nature of comets. It is ironic that comets have been hitting the earth for billions of years, but this is the first time that humans could actually hit a comet.

SEPTEMBER HIGHLIGHTS

Sept 2: Full moon is at 1:23 a.m. This would normally be the famous Harvest moon if it occurred within two weeks of the equinox, but this one is too early and is therefore simply called the corn moon, similar to last month’s moon, which was green corn or grain.

Sept. 3: On this day in 1976 Viking 2 landed on Mars. It was preceded by Viking 1 in July of that year. We had landed some earlier missions on Mars in the early 1970s, but they both failed. Only about half of the Mars missions were successful.

Sept. 5: The moon and Mars will be less than half a degree apart in Pisces this evening by 11 p.m. They will rise together and then get even closer. Mars will be occulted by the moon in some parts of the world.

Sept. 9: Mars starts its retrograde, or westward motion.

Sept. 10: Last quarter moon is at 5:27 a.m.

Sept. 11: Neptune will reach opposition in Aquarius at 7.8 magnitude. It will be four hours away at the speed of light, which is nearly 3 billion miles.

Sept. 14: The moon and Venus will be 5 degrees apart in the morning sky.

Sept. 17: New moon is at 7:01 a.m.

Sept. 23: First quarter moon is at 9:56 p.m. On this day in 1846 J. Galle discovered Neptune. It has only completed one orbit since then because it takes 165 years for Neptune to orbit the sun.

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