September always marks the beginning of fall for us in the northern hemisphere. This year that will happen at 10:21 a.m. on Sept. 22. This is a critical point in helping to understand 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 always on as we all orbit the sun.

The autumnal equinox and the vernal equinox are the only two days each year that the sun actually 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 of east, on the way to its lowest point, which will be the winter solstice in three months.

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 is a proof that the Earth orbits in ellipses and not perfect circles 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 half a year later. These are imaginary extended planes, but they are important to visualize if you want to 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 to enjoy and appreciate all it has to offer because we tend to have more clear nights this month and it is less humid and there will be fewer bugs. There are several interesting highlights this month, but it is always well worth any effort you make just to get out under a clear sky, keep learning, be surprised and share the same sky that is above all of us with others.

A perfect example of this was the Perseid meteor shower last 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 just 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 for us 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 last month. Look low in the southwestern evening sky half an hour after sunset and you will see Venus getting a little higher in the sky each evening even as Jupiter slowly sinks out of sight. A very slender waxing crescent moon joins the pair on the second and third of this month.

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, ever-changing triangle with the red supergiant star Antares in Scorpius. A first-quarter moon will join the trio on the 8th and 9th of the month. Last month, it appeared like Mars was being sling shotted through the sky by an imaginary slingshot that you can create by connecting Saturn and Antares. All three lined up perfectly on the 23rd, and then Mars slowly proceeded eastward each night after that, as it still continues all of 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 along with the Earth.

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. Based on Kepler’s laws of planetary motion, you would expect that as well as Saturn traveling much slower since it is about 10 times farther away than Mars. Saturn travels at 6 miles per second, and takes nearly 30 years to orbit the sun once.

The real surprise comes with Antares. It is traveling at nearly 500,000 miles per hour around the center of our galaxy, just like our sun is doing. It is about 600 light years away, instead of just over one light hour away like Saturn. So 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 much better appreciation of how intrinsically 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 than our sun, 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 last month, there will be another little shower emanating from Perseus this month. They are called the Epsilon Perseid shower and they will peak after midnight Sept. 9. However, they will not be nearly as prolific as the Perseids, since they will only produce 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 first of the month, but it will not produce much more than the Epsilon Perseids. It is good to know about these showers and use them to get out under the sky and experience other wonders as well.

If you have access to a good telescope, you can see 12th magnitude comet 43P/Wolf- Harrington glide through Cancer the Crab this month. A close approach to Jupiter in three years will alter the path of this comet, so that 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 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.

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

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

Sept. 3: Viking 2 lands on Mars on this day 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 is 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. for us in the Northern Hemisphere.

Sept. 23: J. Galle discovered Neptune on this day in 1846. It has made just over one orbit since.

Sept. 28: Mercury is at 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.