Welcome a new thing I’m going to try and do on my site! I’m going to try and point out some interesting things happening in the night sky over the month. I’m a little late on September, so we’re gonna ignore events that happened before I posted this.
September 9: Mars Stands Still
On Wednesday, September 9 all night Mars will appear to be motionless in the sky compared with other nights. The best way to see this is to compare the position of Mars with the background stars of Pisces in previous nights and watch as Mars slows to a halt on this night. It is ending is eastward motion, and will start to move towards the west in its retrograde loop. It will be on this loop through October and end in mid-November. Retrograde loops are caused by the Earth passing a superior planet (Mars through Neptune) in its orbit and these happen all the time.
September 10: The Last Quarter Moon
The Moon will rise around midnight local time and remain visible in the southern sky all morning. At 4:26 AM Eastern, the Moon will be half illuminated, marking the beginning of its transition to a New Moon later in the month. In this phase, the Moon is illuminated on its western side, towards the pre-dawn Sun. If you were to look above the Earth at its orbit around the Sun and the Moon’s orbit around the Earth, you would see that the Moon is positioned ahead of the Earth. In about 3.5 hours, the Earth will have moved to occupy the same spatial position that the Moon once held. Thankfully the Moon will have moved in its orbit by then!
September 11: The Moon Near M35 and Neptune at Opposition
Shortly after the Moon rises around midnight local time on Friday, September 11, it will be positioned several degrees to the celestial west of a star cluster in Gemini: Messier 35, also known as the Shoe-Buckle Cluster. This cluster is an open cluster, a group of stars that were formed together and are roughly the same age. The stars in the cluster are loosely gravitationally bound to each other and will become disrupted by close encounters with other stars and clusters. Messier 35 is about 3,870 light years distant and likely contains around 400 stars. To see the cluster’s stars more easily, make sure to hide the Moon just beyond the edge of your telescope or binocular’s field of view.
Also happening on Friday, the most distant planet Neptune (sorry Pluto!) will be directly opposite the Sun in the sky. This is called opposition. Let me clarify that this does not mean that Neptune is behind the Sun opposite from Earth; it just means that Neptune is on the other side of the sky from the Sun. Neptune at opposition also coincides with its closest approach to the Earth. Don’t worry though, it is still 28.9 astronomical units distant! You can find Neptune in your telescope or binoculars (it is invisible to the naked eye) by looking for the star Phi Aquarii; Neptune will appear about two degrees to the left.
September 12: Jupiter Goes Prograde
Just as Mars is entering its retrograde loop, Jupiter is exiting its loop. On this day, Jupiter will appear to become motionless in the sky as it exits its retrograde loop, becoming prograde once again. It had been in retrograde motion since mid-May.
September 13: The Demon Star Dims
Algol (Beta Persei), also known as the Demon Star since it marks the head of Medusa that Perseus holds, is a variable star. This means that its brightness visibly changes over time. For Algol, it will dim and brighten repeatedly for about 10 hours every 2 days, 20 hours, and 49 minutes. Sometimes variable stars change brightness because of an intrinsic property of the star, but Algol changes brightness because of a dim companion star that orbits nearly edge-on as viewed from Earth. As the companion passes in front of Algol, the light we receive is reduced, causing the star to appear to dim. On Sunday, September 13 at 9:08 PM Eastern, Algol will reach its minimum brightness above the northeastern horizon. Five hours later, Algol will have brightened again high in the eastern sky.
September 14: The Moon and Venus Meet Up with M44
The Moon, now a waning crescent, will rise at about 3:15 AM local time on Monday, September 14. Sitting nearby about four degrees to the left is the bright planet Venus. The pair will remain visible in the east until sunrise, fitting nicely into the field of view of a good pair of binoculars or telescope. Sitting near the two local objects is a much further deep sky object, Messier 44, the Beehive Cluster. The Beehive Cluster is one of the nearest clusters to the Earth, only about 610 light years away. The cluster has been known since antiquity, having been described by Ptolemy as the “nebulous mass in the breast of Cancer.” It contains more stars than Messier 35, about 1000 stars within ~13 light years. If you’d like to see Messier 44 more easily, make sure to hide Venus and the Moon from view as their light can drown out the light from the Beehive.
September 17: The New Moon
Finally, the Moon has reached its New phase, traveling between the Earth and the Sun. This means that it certainly won’t be visible in the night sky, and just barely visible in the day. But please don’t stare directly at the Sun!
September 18: The Moon Visits Mercury
Just after sunset on Friday, September 18, under exceptional viewing conditions, sharp-eyed observers might spot the slim crescent Moon sitting above the western horizon. Just below the Moon, about five degrees, sits Mercury, the innermost planet. Both objects should be visible in the same field of view of a good pair of binoculars or a telescope, but please make sure the Sun has set before you stare through your telescope!
September 21: Hermes Visits the Virgin
Low in the southwestern sky after sunset on Monday, September 21, Mercury will have moved to get close to Spica (Alpha Virginis), the brightest star in Virgo the Virgin. It will be very hard to see, given that the Sun just set below the horizon, but once again, those keen-eyed observers might be able to make the pair out. If you can’t see the pair this evening, the following evening, Mercury will sit just above Spica. Those observers further south will be able to see the duo more easily, not so much me in Cleveland.
Unobservable to the naked eye and to many telescopes is the true nature of Spica: it is actually two stars that orbit each other every four days. They can only be resolved by looking at the spectrum of the system. The pair are orbiting each other so closely that the primary star has been extended, no longer resembling a sphere and instead appearing as an egg-shape, called an ellipsoid.
September 22: Venus, Vesta, and the First Day of Fall
In the eastern pre-dawn sky on the morning of Tuesday, September 22, very bright Venus will overtake and pass the asteroid Vesta. Venus will be dropping sunward while Vesta climbs in the opposite direction. Although Venus is clearly visible to the naked eye, to see Vesta you’re gonna need a telescope; Venus is 8300 times brighter than Vesta!
Later in the day, at approximately 3:31 PM Eastern, the Sun will cross the celestial equator moving southward. This event marks the autumnal equinox in the Northern Hemisphere and the beginning of fall. On this day, the day and night are of equal length and the Sun will rise exactly east and set exactly west.
September 23: First Quarter Moon
The Moon has now moved sufficiently far enough in its orbit around the Earth to appear half illuminated again. This time the eastern side is lit, marking the First Quarter Moon. The Moon will rise around noon and set around midnight, so it is best seen in the afternoon daytime sky. Now is a great time to get out your telescope and look at the lunar landscape as it is illuminated by sunlight.
September 24: The Moon and Jupiter
Every month as the Moon travels along the ecliptic (the line across the sky that marks the orbital plane of the planets) it will visit the gas giants. This month those visits will be back-to-back, starting with Jupiter. The gibbous Moon will be about 4.5 degrees southwest of the most massive planet, Jupiter. Nearby is Saturn awaiting its turn with the Moon. Through the night, the Moon will slide closer to Jupiter while Jupiter moves higher int he sky. The two will be very close together, called a conjunction. This would be a great time for some astrophotography!
September 25: The Moon and Saturn
Now that Jupiter has had its fun with the Moon, Saturn gets a turn! This time the Moon will sit ~3.5 degrees to the lower left of Saturn in the southern sky after dusk. Jupiter will still be nearby sitting to their right. Once again, Saturn’s lunar visitor will get close to form a conjunction. Another excellent night for some astrophotography!
September 29: Saturn Goes Prograde
Just like Jupiter before it earlier in the month, on Tuesday, September 29, Earth’s faster orbit will cause Saturn to appear to stop moving with respect to the more distant stars. This temporary pause in motion marks the end of Saturn’s retrograde motion that began in early May.