Skylights: September 2023

September brings a notable change of season in the sky. Besides the autumnal equinox, when the Sun lies directly on the celestial equator, 0° declination, giving equal times of day and night, the visible star patterns in the night sky are also indicating some changes.

Arcturus, the bright beacon of spring that has been a fixture in the evening sky for the past six months, is getting lower in the west with each passing night. Along with it, the big bear, Ursa Major, is beginning to stand upright on the northwestern horizon. A sweep with binoculars beneath the Big Dipper reveals the asterism the Three Leaps of the Gazelle darting along the horizon, to the right of Coma Berenices and towards the north.

Spica, another springtime star, disappears from view during September, and just as twilight fades, the heart of the Milky Way lies due south, indicating that the summer star patterns of Scorpius and Sagittarius will also be leaving the sky soon.

The Summer Triangle, made up of the stars Vega, in Lyra, Deneb in Cygnus, and Altair in Aquila, are high overhead and will be with us for quite a while longer.

Low in the southeast, almost directly below Saturn, we welcome the return of Fomalhaut, the 18th brightest star in the sky, and the most southerly first magnitude star visible at our latitude. 

Towards the east and northeast, the familiar star patterns of autumn are now fully visible. Pegasus, Andromeda, Cassiopeia and Cepheus will be prominently positioned in our evening sky for the next several months.

Early morning observers will welcome the entirety of the Winter Hexagon and its associated constellations high in the southeast before dawn. If you enjoy exploring this part of the sky but can’t bear the cold temperatures associated with viewing it when it is visible in the early evening, September mornings are your best time to experience the winter sky under late summer climate conditions.

The first sunset that occurs before 7:00pm happens on September 13. The Sun will continue to set earlier than 7:00pm until March 22.

After having spent the past 37 days traversing Leo, the Sun enters Virgo on September 17.

The Sun crosses the celestial equator, moving southward, at 2:50am EDT on September 23, marking the autumnal equinox.  

The Moon begins the month in its waning gibbous phase, passing Neptune on the 1st. Last quarter occurs in Taurus on the 6th.

On its way to its new phase on the 14th, it passes 3.0° north of open cluster M35 in Gemini on the 8th, 2.8° southeast of Pollux (beta Geminorum) on the 10th, 3.0° north of the Beehive Cluster (M44) in Cancer on the 11th, and 3.9° northeast of Regulus, in Leo, on the 13th.

On the waxing side, the 1.7-day crescent appears just 1.0° southeast of Mars on the 16th, and passes 2.6° east of Spica on the 17th. On the 20th, the 5.5-day Moon is 4.8° west of Antares.

First quarter Moon occurs in Sagittarius on the 22nd.

Late in the evening of the 26th, the waxing gibbous Moon is 2.7° south of Saturn.

The Full Harvest Moon occurs at 5:58am EDIT on September 29. The Moon rises at 6:25pm on the 28th, seven minutes before sunset. It sets at 6:43am the following morning.

Mercury reaches inferior conjunction on the 6th, and then appears in the morning sky for a very favorable apparition throughout the remainder of the month. It reaches greatest elongation on the 22nd, when it will be 18° east of the Sun, and at a near vertical angle with respect to the horizon, and will rise 90 minutes ahead of the Sun.

This is a great time to observe the rapidly changing phases of Mercury with a telescope, morphing from an 8% illuminated crescent on the 12th, to a “quarter Moon” phase just 10 days later. . 

Venus, likewise, is also in a favorable position in the morning sky, and is going through its crescent phase throughout September, though it takes nearly two months to undergo the same progression of phases as Mercury does in under two weeks.

During the first week of September, Venus lies within 2° of the open cluster M67 in Cancer. It shines as bright as magnitude -4.8 on the 19th, when it reaches its maximum brilliancy, and moves into Leo on the 24th. In early September, Venus rises at 3:15am EDT, and by the end of the month it is visible just after 3:00am.

Although it doesn’t reach superior conjunction until mid-November, Mars is becoming difficult to view in evening twilight. At the beginning of September, it sets about an hour after the Sun, and by month’s end, it remains in the sky for just 30 minutes after sunset. On the 4th, Mars lies 0.8° south of Zaniah (eta Virginia), a magnitude 3.8 star that is 265 light years away. Mars is just about as far from Earth as it gets, at 2.5 AU, or 21 light minutes away.

If there is one last time you should try to observe Mars this season, it is on the 16th, when the 1.7-day, 3.2% illuminated crescent Moon appears just 1.1° to its southeast.

Jupiter is still a few weeks from opposition, but it is becoming a prominent object in the evening sky in September. Rising just before 10:00pm EDT in early September, it reaches its stationary point and begins its retrograde loop through Aries on the 4th.

With your small telescope, watching Jupiter’s Galilean moons change positions each night, and even over the hours, can be a fascinating experience. One peculiar alignment occurs just before 3:00am EDT on September 26, when Io, Europa, and Callisto form a nearly perfect north-south line just to the west of Jupiter. Each passing minute reveals motion of the moons, as the line quickly changes.

Saturn, having passed opposition late last month, is well-placed for observing all night throughout September. As the Sun-Saturn-Earth angle increases in the weeks following opposition, the shadow of Saturn on the backside of its rings becomes more visible, giving prominence to the three-dimensional appearance of the planet in the eyepiece of a telescope.

While Saturn is at its closest, this is a good time to explore its moons. Its largest, Titan, is visible in small telescopes, at magnitude 8.5. It orbits Saturn once every 16 days, and at maximum elongation, can appear as far as 3 arcminutes from the planet.

A slightly larger telescope will reveal some of Saturn’s medium-sized moons, Tethys, Dione, and Rhea. These moons are all 10th magnitude, with Rhea being about ½ magnitude brighter than the others, and orbit Saturn in 1.9, 2.7, and 4.5 days, respectively, so their positions change dramatically on successive nights, and can even be seen changing by the hour, just as Jupiter’s inner Galilean moons do. They orbit considerably closer to Saturn than Titan, so a bit more magnification is needed to watch them. A favorable elongation of both Dione and Rhea occurs on the evening of the 13th, when the two moons are near their maximum elongation west of the planet, at about 34 and 57 arcseconds from the ring edge.

A larger telescope will reveal two of Saturn’s smaller, inner moons, Mimas and Enceladus. These moons, although not exceptionally dim at 12th and 11th magnitude, respectively, are challenging to see due to their proximity to the brightness of Saturn and its rings. The moons orbit in just 0.9 and 1.4 days, and are elongated only up to 7 and 15 arcseconds from the ring edge.

Finally, there is Iapetus, Saturn’s bizarre outer moon that has dark and light hemispheres. The tidally locked moon is covered in darker material on its leading hemisphere, leaving its trailing hemisphere lighter, by up to two magnitudes as seen from Earth, when it is on the western side of Saturn, moving away from us and toward the back side of the Saturnian system. This moon orbits in 79 days and undergoes one of its western elongations during the first week of September. Iapetus can be as far as 10 arcminutes from the planet at its greatest elongations.

Uranus is near the border of Aries and Taurus, and rises just a few minutes after Jupiter. At a distance of 19.0 AU, Uranus’s greenish globe shines at magnitude 5.7, putting it within unaided eye visibility from a dark site, and easy to locate with binoculars even under bright sky conditions. With binoculars or a small telescope, aim for the patch of sky midway between Jupiter and the Pleiades cluster in Taurus. Look for a quadrilateral of 4th and 5th magnitude stars resembling a radio telescope, with the open end of the dish pointed south. The stars are Botein (delta Arietis), zeta, tau1 and tau2 Arietis. Uranus (magnitude 5.8) forms an equilateral triangle with the southern two stars (the wide end of the radio dish).

Early in the morning of September 5, the waning gibbous Moon is 1.5° north-northwest of Uranus.

Our most distant planet, Neptune, reaches opposition on the 19th, making September the best time to observe it. It can be found just a fraction of a degree from the 5.5 magnitude star 20 Piscium all month, coming within 220 arcseconds north of the star on the 12th. This location can be found rather easily by drawing a line through the eastern segment of the Circlet asterism in Pisces, from magnitude 4.1 iota Piscium through magnitude 4.5 lambda Piscium, and continuing along this line about the same distance. At the end, move slightly east to find 20 Piscium and Neptune.

At opposition, the distant ice giant is 28.9 AU away, where its light takes almost exactly four hours to reach us. Its 2.4 arcsecond bluish globe shines at magnitude 7.7, well within range of binoculars on a dark, moonless night. 

After midnight on September 1st, the just-past-full Moon will pass 1.4° south of Neptune, making the planet easier to locate. Neptune receives a second visit from the Moon at the end of the month when the full Harvest Moon appears 4° to its east.

At over 34 AU, distant Pluto dimly shines at magnitude 14.4 in eastern Sagittarius, 1.7° southwest of globular cluster M75. Early September is still a good time to hunt for Pluto, when it is at its highest in the south in the early evening, and bright moonlight isn’t interfering.

Magnitude 8.9 Ceres remains visible in Virgo, but it is rapidly falling out of view, and will be difficult to see by the end of the month, when it will be 3.5 AU away. During the first week of September, it is located about 8° above Spica, just over the western horizon.

Vesta becomes brighter than magnitude 8 in September, and enters northern Orion on September 6. Throughout the month, it traces a path 1° south of and parallel to the line connecting Chi1 and Chi2 Orionis, the two 4th magnitude stars that mark the top of the hunter's club.

Asteroid 3 Juno is fairly unremarkable, at 10th magnitude, but its position in early September is notable as it shares the same area of sky as Venus. Around the time that Venus is making its closest pass to the open cluster M67 in Cancer, Juno appears just 1.2° southwest of the brilliant planet. Juno then “chases” Venus, continuing to appear nearly due south of it at a distance of no greater than 4° as the pair move eastward into Leo during the latter days of the month.