September astronomy: Jupiter is as close to Earth as it’s been in decades—and our planet is closing in on Mars

Jupiter is closer to Earth this month than it has been in 71 years—and it won’t be this close again for 107 years!

On Sept. 1, Jupiter rises in the east a bit more than an hour after sunset, some 46 degrees to the lower left of Saturn in the southeast. Jupiter rises just more than 4 minutes earlier per day, and first appears on our evening twilight chart during the second week. On Sept. 26, Earth passes between the sun and Jupiter. The giant planet, shining at magnitude -2.9, then rises at sunset and appears at opposition, 180 degrees from sun, and is up all night: low in the eastern sky at dusk, highest in the south in the middle of the night, and low in the west at dawn—setting at sunrise.

On the night before, Sept. 25, Jupiter makes its closest approach to Earth since October 1951, at a distance of 367 million miles. Not until October 2129 will Jupiter come closer!

Jupiter, with its dark cloud belts and four bright moons discovered by Galileo, and Saturn, with its rings currently tipped nearly 15 degrees from edge-on, are attractive for telescopic viewing, and now conveniently visible in the evening! Even binoculars readily detect Jupiter as a disk, 50 arcseconds across, and reveal as many as all four Galilean moons, especially when near greatest elongation in their nearly edgewise orbits—as far to the side of Jupiter as they can appear. As it happens, on night of Sept. 25, brightest Ganymede appears farthest east of Jupiter, and outermost Callisto appears even farther to the planet’s west. Innermost Io appears outbound to its greatest distance west, while inbound Europa passes very close to Io just before midnight.

Mars now rises in the east-northeast late in the evening, by 11:30 p.m. on Sept. 1, and around 10:15 p.m. on the 30th. Watch for it rising far to the lower left of Jupiter, by 60 degrees on the 1st, and by 77 degrees on the 30th.

Best seen in the morning, before dawn, Mars flares up from magnitude -0.1 to -0.6 this month while Earth closes in, from 89 to 73 million miles. Mars gleams against the spectacular background of Taurus, the Bull, and passes within 5 degrees of Aldebaran, the Bull’s eye, from Sept. 2-12. The minimum distance apart (4.3 degrees) occurs on Sept. 7, slightly closer than the 4.5 degrees separating Pollux and Castor, in the neighboring zodiac constellation to the east, Gemini, the Twins. Mars remains in Taurus through late March 2023, a total of 7 1/2 months! Mars lingers here, because our planet Earth will overtake Mars in early December, causing the red planet to appear to retrograde, or go backward, in Taurus, for about 11 weeks. Mars goes past the Pleiades only once, but will pass Aldebaran, the Hyades star cluster and the horns of Taurus three times each—all triple conjunctions!

As Earth closes in on Mars, try to spot Syrtis Major, the first surface feature of another planet ever recorded (in 1659). Using a 6-inch telescope at 150-200x, observers can note it as a dark triangular marking near the center of the tiny Martian disk, on Sept. 20 at 1:22 a.m., and about 39 minutes later on each successive morning for the next week, through Sept. 27, at 5:55 a.m.

Mars’ south pole is now tipped about 20 degrees into the sunlit side, but only slightly toward Earth, so the remnant mid-summer cap may not be visible. Mars’ disk will grow in apparent size to 17 arcseconds by its closest approach to Earth, within 51 million miles, on Nov. 30.

The five outer planets in September span an arc of 107 to 121 degrees, and are arranged in five consecutive zodiac constellations: Saturn in Capricornus, Neptune in Aquarius, Jupiter in Pisces, Uranus in Aries, and Mars in Taurus. On the night of Sept. 27-28—best from 11 p.m. until 2:30 a.m.—the five planets can be viewed spanning 120 degrees, or one-third of the way around the circle of the zodiac. Detailed finder charts including faint stars are needed to identify 5.7-magnitude Uranus and 7.8-magnitude Neptune.

In evening twilight in September, the southern half of the belt of zodiac constellations is on display, from Virgo setting in west, through Libra, Scorpius with Antares, Ophiuchus, Sagittarius low in the south, Capricornus with Saturn, Aquarius with Neptune, and Pisces about to rise or rising in the east, containing Jupiter. When the moon is near first quarter phase in Ophiuchus on Sept. 3 and Oct. 2, it’s close to half full and about 90 degrees, or one-quarter of a circle, east of the sun’s location. The moon’s location one-quarter of the way around the zodiac from the sun previews where the sun will be in a quarter of a year, or three months later. Those dates are close to the winter solstice, Dec. 21, the date of the lowest midday sun of the year. So expect the evening half-moons of Sept. 3 and Oct. 2 to be low in the southern sky at dusk. Also expect the waxing crescent moons in the western sky on the evenings leading up to Sept. 3 and Oct. 2 to be tipped over, like a bowl spilling its contents.

In September’s morning twilight sky, the belt of the zodiac is steeply inclined to the horizon. The northern half of the belt is on display, from Pisces and Jupiter setting in the west; through Aries with Uranus; Taurus, with Mars and Aldebaran high in the southern sky; Gemini with Pollux and Castor; Cancer with the Beehive cluster; and Leo with Venus and Regulus low in the east. Regulus in the first week of September is just emerging from the far side of the sun. Using binoculars, watch for a few days around Sept. 5, when Venus passes 0.8 degrees north of the star. When you see the moon near last quarter phase (about half full) high in the southern sky in morning twilight on Sept. 17 and 18, it will be in the northernmost part of the zodiac, in Taurus and Gemini, among the same stars the sun appeared near three months earlier, in June, not far from the date of the summer solstice, when the midday sun was highest of the year. Expect the waning crescent moons in the eastern sky in morning twilight Sept. 20-24 to be oriented like an upright bowl, holding its contents.

Follow the moon: Evenings an hour after sunset, find the moon near Antares Sept. 2, 3 and 30; and near Saturn on Sept. 7 and 8. The Full Harvest moon occurs on the night of Sept. 9-10, rises in twilight the next three evenings, and appears near Jupiter nearly all night long on Sept. 10-11 and Sept. 11-12.

Mornings an hour before sunup, find the moon 5 to 6 degrees south of the Pleiades star cluster on Sept. 16. That morning, the waning gibbous moon forms an attractive triangle with Mars and Aldebaran, 6 to 7.5 degrees on a side. On Sept. 19 and 20, the waning crescent moon forms attractive patterns with Pollux and Castor. On Sept. 23, find a thin crescent moon 5 degrees to the lower left of Regulus. Look a half-hour closer to sunrise to catch Venus rising within 20 degrees below the moon. On Sept. 24, a half-hour before sunrise, the 2 percent old crescent moon will be low in the east, with Venus rising 8 degrees below.

Venus rises a full hour before the sun on Sept. 1, shrinking to less than a half-hour by month’s end. Although Venus shines at magnitude -3.9, it doesn’t seem impressive because of bright twilight.

The lineup of planets Venus-Mars-Jupiter marks well the ecliptic, or plane, of Earth’s orbit—the centerline of the zodiac—through our morning sky. Note the brightest star, Sirius, the Dog Star, is well to the south of that plane, and Capella, the Mother Goat Star, is well to the north. These stars mark the southern and northern vertices of the Winter Hexagon. In clockwise order, its stars are Sirius, Procyon, Pollux and Castor, Capella, Aldebaran, Rigel, and back to Sirius. Betelgeuse, Orion’s shoulder, is inside the polygon. On Sept. 2, Spaceship Earth, in its motion around the sun, is heading in a direction 5 degrees north of Aldebaran.

Autumn begins in Earth’s northern hemisphere on Sept. 22 at 6:04 p.m., four days before our home planet overtakes Jupiter.

Also on Sept. 2, Earth is moving away from a point 5 degrees north of Antares in the evening sky. A line from Jupiter to Saturn extended westward across the sky passes not far north of Antares and Spica and closely marks the plane of Earth’s orbit. Note the Summer Triangle of Vega, Altair and Deneb nearly overhead, and the star Arcturus in the west, are well north of the Earth’s orbital plane.

Sky Calendar includes illustrations of many of the events described in this article. To subscribe or to view a sample issue, visit www.abramsplanetarium.org/skycalendar.

Robert Victor originated the Abrams Planetarium monthly Sky Calendar in October 1968, and still produces issues occasionally, including the October and December 2022 editions. He enjoys being outdoors sharing the wonders of the night sky.

Robert Miller, who provided the twilight charts, did graduate work in planetarium science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.