BY BERNIE REIM

Correspondent

This month always marks the beginning of autumn for us in the northern hemisphere. That will happen at 4:44 p.m. on Sunday, Sept. 22. That moment is further defined by the sun on its ecliptic path crossing over the celestial equator on a downward path. The vernal and autumnal equinoxes are also the only two days each year that the sun rises due east and sets due west everywhere on Earth except at the poles.

Within a few days of those dates the length of the days are exactly 12 hours for everyone on Earth except at the poles. The reason is that we orbit the sun in ellipses and not perfect circles.

There will be many interesting new highlights this month as summer gradually fades into fall. This will include a great conjunction of Venus and Saturn, a minor meteor shower, keeping track of the exciting nova in Delphinius just below the Summer Triangle, and your first chance to get a look at Comet ISON right above Mars in the morning sky.

Venus continues to get closer and closer to Saturn in the evening sky at the rate of about 1 degree per day. On Sept. 8, the celestial pair will be just 10 degrees apart, which can be measured by one fist at arm’s length. Look low in the west-southwestern sky 45 minutes after sunset and you will also see a slender waxing crescent moon gliding less than 1 degree below Venus with Spica, the brightest star in Virgo, just below the pair. The next evening the moon will already be 12 degrees farther east and closer to Saturn.

Brilliant Venus will pass just 4 degrees below the ringed jewel of our solar system during the evening of Sept. 18. After that the pair will drift farther apart again, but they will still form a nice, ever-changing conjunction along with Spica throughout the month. Notice that Venus is almost 100 times fainter than Saturn.

The minor meteor shower is called the Aurigids and it will peak on Sept. 1. Usually it only produces about six meteors per hour, which is just above the background rate of four stray meteors that you could see any night, depending on the darkness of your viewing location. However, as with most minor meteor showers, the Aurigids could suddenly produce a much greater number of meteors. This last happened in 2007, when it reached 130 meteors per hour. The moon will not interfere this year, so it will be worth checking it out and spending some quality time under the night sky around 4 a.m. on Sunday.

You will be looking into a winter sky at that time, seeing Orion and the Winter Hexagon again in the eastern sky. You will also see Jupiter just above and to the right of Mars along with a waning crescent moon. Notice that Jupiter is about 30 times brighter than the red planet. After the meteor shower, keep watching as Mars drifts into the next constellation, Cancer the Crab. Mars will pass right through M44, also known as the Beehive cluster in Cancer on Sept. 8 and Sept. 9.

Try to get an early view of the next great comet, named ISON. It is following a very similar path to Mars all month long, only about 3 degrees above. Look for the comet early this month directly above the Beehive cluster before the moon gets too bright. You will need a good telescope to see it, since it will only be about 12th magnitude. It is expected to brighten to ninth or 10th magnitude by the end of the month. If it survives its close encounter with the sun in December, this comet could become really spectacular and possibly even visible in the daytime.

Nova Delphini is already fading again, but it will be very interesting to keep watching it to see how rapidly it will fade. I was lucky to first see this nova just hours after it first exploded. It was a faint, 17th magnitude star in Delphinius the Dolphin that suddenly became 100,000 times brighter. It reached its brightest magnitude of 4.2 on Aug. 16. This is called a recurrent nova and the mechanism that causes its sudden outburst is fascinating. It consists of a white dwarf, which is what our sun will turn into in 5 billion years, along with another star in that binary system that has hydrogen gas spiraling onto the surface of the white dwarf. As this gas gets thicker and denser on the surface of the white dwarf star, it will suddenly explode in a runaway hydrogen fusion reaction. It will then expel this thin, earth-sized shell of a hydrogen bomb, leaving the white dwarf intact. This could happen many times with a period of tens of years up to tens of thousands of years.

In the extreme case, if a red giant is spiraling vast amounts of hydrogen gas onto the very dense surface of a white dwarf, it will completely blow apart both stars as it reaches a critical mass of 1.44 solar masses, also called the Chandrasekhar limit after the astronomer from India who first discovered the laws governing this phenomenon. This is called a Type 1A supernova. It always explodes at exactly that mass, so it can be used to accurately determine distances to far away galaxies in our universe. Thousands of Type 1A supernovae were recently used to figure out that our entire universe is expanding at an ever-increasing rate.

I recently attended the Stellafane astronomy convention in Springfield, Vt. This is an annual pilgrimage for thousands of amateur astronomers. They always have great practical workshops and excellent speakers to further inspire everyone’s interest in astronomy, which is really obtaining that greater cosmic perspective that we all need to better understand where we are in space and even to understand what is actually happening right here on Earth.

The Milky Way was just incredible over Breezy Hill at 1,300 feet above sea level. I could easily see thousands more stars than I can usually see even from a good location in southern Maine. It seemed to engulf the entire scene in its great beauty and energy. Everyone needs to really experience a wonderful dark sky like this more often to better appreciate our connection to it and how all these stars directly provide for our lives.

We also saw a dozen or so good Perseid meteors from that fantastic location. Around 2 a.m., one brilliant and memorable meteor created a twisted trail that seemed to pulsate in the sky for a few seconds before it faded out and the night once again returned to its native blackness.

September highlights

* Sept. 3. On this day in 1976, Viking 2 landed on Mars.

* Sept. 5. New moon is at 7:36 a.m. Voyager 1 was launched on this day in 1977.

* Sept. 8. The moon passes less than half a degree below Venus this evening.

* Sept. 9. The moon passes just south of Saturn this evening.

* Sept. 12. First quarter moon is at 1:08 p.m.

* Sept. 15. The moon is at perigee — its closest to Earth — at 228,286 miles today.

* Sept. 18. On this day in 1977, Voyager 1 took a photo of Earth and the moon from deep space.

* Sept. 19. Full moon is at 7:13 a.m. This is also called the Harvest Moon. It will rise only about half an hour later each night instead of the usual 55 minutes later, due to the shallow angle of the ecliptic to our horizon at this time of year in the evening sky in this hemisphere.

* Sept. 22. Pioneer 10 left the solar system on this day in 1990. Fall starts at 4:44 p.m.

* Sept. 23. On this day in 1846, J.G. Galle discovered Neptune.

* Sept. 26. Last quarter moon is at 11:55 p.m.

* Sept. 27. The moon is at apogee — its farthest from Earth — today at 251,225 miles.

* Sept. 28. The moon passes 5 degrees south of Jupiter this morning at 5 a.m.

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