Skylights: September 2024

Sun

Nights get noticeably longer through September. We will experience our last sunset in the 7:00pm hour on the 11th; it will not be above the horizon this late again until March 22.

The Sun crosses the celestial equator into a southerly declination, marking the equinox, at 8:43am on the 22nd. A few days later, on the 25th, the amount of darkness becomes greater than the daylight hours.

After traversing Leo for the past 37 days, the Sun crosses into Virgo on the 16th.

Moon

September begins with the waning crescent Moon joining Mercury in the eastern sky at dawn. The 27.3-day, 2.5% illuminated crescent is 4.2° to the north of the planet. Take note of the angle of the Moon, as the cusps of the crescent will be oriented nearly horizontally, due to the Moon’s position well north of the ecliptic, and the steep angle of the ecliptic on the eastern horizon in early September.

The Moon is new at 9:56pm on the 2nd, marking the beginning of Lunation 1258.

As it progresses through its waxing crescent phase, it can be found near Venus, 4.5° west of the planet on the 4th, and 6.3° southeast of it on the 5th. On the 6th, the Moon is 2.6° southeast of Spica, in Virgo.

On the 9th, at 8:08pm EDT, the waxing crescent Moon occults magnitude 2.8 Fang (pi Scorpii). Egress from the bright limb of the Moon occurs 42 minutes later.

First quarter Moon occurs at 2:06am EDT on the 11th, in Ophiuchus.

The Moon is full, in Aquarius, at 10:34pm EDT on the 17th, As this is the closest full Moon to equinox, this is the Harvest Moon. This is no ordinary full Moon, however, as there are two events to watch for. The first is a partial lunar eclipse.

The second show put on by this month’s Harvest Moon will be an occultation of Neptune, which begins at 4:13am and ends at 4:33am on the 18th. Neptune shines at a relatively bright magnitude 7.7; and seeing it beyond the bright limb of the full Moon will require higher magnification with a medium-sized telescope to not miss it.

The Moon passes Saturn 1.3° west-southwest of the planet, on the morning of the 17th. On the evening of the 21st, it is 3.3° north-northwest of Uranus.

The waning gibbous Moon is 0.2° south of the Pleiades cluster in Taurus on the 22nd, and occults Electra (17 Tauri), the southwesternmost of the bright members of the cluster, during civil twilight. Ingress occurs at 6:24am, during civil twilight, and egress is at 7:19am, during daylight.

The waning gibbous Moon is 5.7° northeast of Jupiter before midnight on the 23rd.

The Moon is last quarter, in Gemini, at 2:50pm EDT on the 24th

During the Moon’s waning crescent phase, it passes 4.5° north of Mars on the 25th, 1.6° south of Pollux, in Gemini, on the 26th, and 3.1° north-northwest of the open cluster M44, the Beehive, in Cancer, on the 27th.

Of note is the orientation of the crescent Moon as it wanes late in the month. Due to its position north of the ecliptic, and the high angle of the ecliptic from the eastern horizon, the cusps of the crescent are nearly horizontal, with the Earthshined globe sitting neatly atop. This view is particularly stunning when the Moon is just 2.2° north-northeast of Regulus on the morning of the 29th, and on the 30th, when the 0.00% illuminated crescent rises from the darkened horizon at 4:30am.

Mercury

Mercury undergoes its best morning apparition of 2024 during September, rising as early as 4:43am EDT, over 90 minutes before sunrise, on the 4th, when it reaches its greatest elongation,18.0° west of the Sun.

The 27.3-day, 2.5% illuminated crescent Moon is 4.2° north of Mercury on the 1st.

Mercury passes 0.5° north of Regulus, in Leo, on the 9th.

Mercury remains above the eastern horizon for at least an hour before sunrise through the 17th, then rapidly sinks into twilight and out of view as it approaches superior conjunction on the 30th.

Through a telescope, Mercury shows a crescent phase until the 5th, when it shines at 50% illumination, then progresses through its waxing gibbous phases. 

Venus

Venus is visible low in the west, setting about an hour past sunset throughout September.

The waxing crescent Moon joins Venus on the 4th and 5th, when it will be 4.5° west and 6.3° southeast of the planet, respectively.

Venus joins Spica on the 17th, when the brilliant planet is 2.5° north of the magnitude 1.0 star in Virgo.

Mars

Mars becomes an evening planet on September 15, when it rises before midnight for the first time. The Red Planet continues to move eastward at a faster pace than Jupiter, extending its separation from the giant planet from 8.8° in early September to 23.4° at the end of the month.

With both Mars and Jupiter traversing the Winter Hexagon, it will be interesting to watch how the two bright planets redraw the pattern of the familiar asterism over the next several months.

On the 9th, Mars lies 0.8° south of the open cluster M35 in Gemini.

Jupiter

Jupiter is the most prominent object in September’s evening sky, after the Moon. Rising before midnight, it continues to move eastward through Taurus, and reaches its point of quadrature, 90° west of the Sun, on the 12th.

At the end of the month, Jupiter rises by 10:00pm.

A peculiar arrangement of Jupiter’s Galilean satellites occurs on the morning of the 4th, when all four moons form a flattened diamond to the east of the planet. Clockwise from Jupiter, they are arranged Io, Europa, Ganymede, and Callisto. When the moons are in a tight arrangement such as this, it is easy to perceive their motion in a short amount of time, perhaps as brief as five to ten minutes.

A similar tight diamond appears on the 11th, with the moons arranged in order: Io, Callisto, Europa, Ganymede.

Saturn

Saturn is in Aquarius, and reaches opposition on the 8th. At 8.658 au from Earth, Saturn appears larger and brighter than it will all year. The ring tilt is now at 4.5° and will appear to gradually increase until mid-november, when the rings appear to be tilted 6.4°, due to the geometry of Earth’s orbit with respect to Saturn as we circle the Sun at a faster rate than the outer planet.

Keep watching Saturn’s moons through a telescope. Its largest moon, Titan, is visible in small telescopes at magnitude 8.4. Early on the 9th, Titan appears just over Saturn’s north pole.

The Moon joins Saturn on the 16th-17th, getting as close as 1.3° west-southwest of the planet just before Moonset.

Uranus

Uranus is stationary in Taurus on the 1st, and begins its apparent retrograde motion, which will continue through January 2025. Although the seventh planet is still two months from opposition, it is still in prime position for viewing. Its position near the northernmost section of the ecliptic puts it in optimal position for observers in the northern hemisphere, as it attains a high elevation in our sky, although in September we have to wait until well after midnight for this to occur.

At magnitude 5.7, Uranus is well within reach of binoculars, even in a bright moonlit sky, and from a dark sky, may even be visible without optical aid.

Located just 5° southeast of the Pleiades cluster, Uranus is quite easy to locate. From the westernmost bright stars of the Pleiades, move directly south 4.5° to find a pair of 6th magnitude stars, 14 and 13 Tauri, which are separated by 1/3° and lie on an east-west line. The nearest object of similar brightness to this pair of stars, just over 1° to the west-southwest, will be the blue-green glow of Uranus.

Neptune

Our outermost planet reaches opposition, in Pisces, on the 20th, and shines at magnitude 7.7. Throughout September it is located about 13° east-northeast of Saturn, and 5.0° southeast of lambda Piscium, the southeasternmost star of the Circlet asterism.

Another way to find Neptune is to look for a quadrilateral of 4th and 5th magnitude stars located between the Circlet and magnitude 3.60 iota Ceti, to the southeast. The quadrilateral consists of two parallel lines just under 1.0° apart, 2.5° tall, and aligned in a roughly south-southeast to north-northwest orientation, with the easternmost pair extending slightly longer to the north. Drawing a line through the westernmost pair and extending north by 1.8° will lead to directly Neptune on the 12th. The planet wanders westward of the line as the month progresses, but will remain well within the same binocular field of view as the quadrilateral.

This month, there is a third way to locate Neptune, as the just-past-full Moon occults it between 4:13am and ends at 4:33am on the 18th.

Neptune’s opposition brings the planet and its system of 16 moons to within 28.893 au of Earth. All but one of these moons is practically invisible to backyard telescopes, Triton. When Neptune is closest to Earth, Triton, which is 77% the size of our Moon, shines at a dim yet accessible 13.4 magnitude, and appears as far as 16 arcseconds from Naptune. This should, under ideal conditions, be achievable with a 12-inch telescope. Triton has a retrograde orbit and completes one revolution around Neptune every 5.8 days.

Minor Planets

1 Ceres ends its retrograde motion and begins moving eastward through Sagittarius in September. It remains located within the center of the Teapot asterism, and dims slightly from magnitude 8.5 to 8.8. It begins the month about 1.5° north-northeast of globular cluster M69, and continues moving almost due east in the direction of globular cluster M54, coming within 1.5° of the latter by month’s end.

Distant Pluto is well-positioned during early evening throughout December. At magnitude 14.4, it can be found 1.7° south-southwest of the globular cluster M75 on the border of Sagittarius and Capricornus.

Stars & Constellations

Beyond our solar system, the stars of September begin to indicate the changing of seasons. Arcturus, the prominent star marking the southern tip of Bootes, takes up position over the western horizon in evening twilight. The star will have departed our evening sky by midnight early in the month, and at 9:30pm at the end of the month.

The Big Dipper tips “upright” as it rotates into its horizontal position low over the north-northeastern horizon.

We’re still waiting for recurrent nova T Coronae Borealis to flare. Its position within the Northern Crown remains high enough above the horizon such that we won’t miss it if it does flare this month, as it is not yet visible in the morning sky.

The Milky Way arcs high overhead during September evenings. The Summer Triangle, still well-placed for exploration, gives way to the stars of autumn as the hours pass. By midnight, we’re looking up at the Great Square of Pegasus and the Andromeda Galaxy.

In mid-September, the Big Dipper’s pointer stars attain their 6 o'clock position directly below Polaris at midnight, and Fomalhaut (alpha Piscis Austrini) rides the meridian low in the south.

Finally, a preview of winter comes in the early morning during September, as just after 3:00am mid-month, the entire Winter Hexagon is visible above the eastern horizon.