On Sept. 2, as Earth traces its nearly circular orbit around the sun, our home planet is heading toward a point 5° above Aldebaran, eye of Taurus, in the morning sky, and receding from a point 5° above Antares, heart of Scorpius, in the evening sky.
This orbital motion causes the sun to appear to move through the constellations of the zodiac. In just three months, in early December, the Earth will have traveled a quarter of the way around its orbit. Antares will then be hidden from view on the far side of the sun, while Aldebaran will be at opposition, nearly 180° from the sun, and will appear low in the east-northeast at dusk; high in the south in the middle of the night; and low in the west-northwest at dawn. The motion of the Earth around the sun is the cause of the seasonal drift of stars from east to west.
While the visibility of stars repeats on an annual cycle, the planets have periods of revolution around the sun differing from Earth’s, so their positions in the sky do not repeat at one-year intervals. September 2023 is an especially good month for rising before dawn to enjoy the planets. Venus recently passed between Earth and the sun, and is now ascending quickly into the eastern predawn sky. On Sept. 1, it rises nearly two hours before the sun, and by the 30th, nearly 3.5 hours before. On the near side of its orbit this month, Venus is backlighted by the sun, and shows crescent phases, easily visible through binoculars—if you look not long before sunrise, or even after, to avoid the contrast of brilliant Venus against a darker sky. On Sept. 1, the Venus crescent is 12 percent full and 50 arcseconds across, larger than any other planet ever appears. During September, Venus recedes from 31 million to 48 million miles from Earth. By Sept. 30, the crescent fills out to 36 percent—but has shrunk to 33 arcseconds across. In mid-September, Venus reaches its greatest brilliancy, at magnitude -4.8. That’s bright enough to spot with the unaided eye in daylight, 27° to 44° to the upper right of rising sun.
Jupiter, shining at magnitude -2.6 to -2.8, next-brightest after Venus, also is well seen these mornings. Look high in the southwest quadrant of the sky as twilight brightens. On Sept. 4, Venus ends retrograde (westward) against the starry background, while Jupiter begins its nearly four months of backward apparent motion as Earth prepares to overtake it in early November. On Sept. 4, these two brightest planets are at their least separation during their current morning apparition, 86.5° apart. The separation widens to 90° on Sept. 17. On Sept. 25, Spaceship Earth will aim its motion just above the star Eta in Gemini, marking the toe of Castor, one of the Twins—midway between Venus and Jupiter that morning. The bright planets will appear 120° apart by Oct. 22. On Dec. 10, Venus and Jupiter will be nearly 180° apart, barely above opposite horizons, more than three hours before sunrise. What will be the last date you can see Venus and Jupiter simultaneously before then?
Sirius, at magnitude -1.4 in the southeastern sky in morning twilight, ranks next in brightness. Between Venus and Jupiter, look for the huge “Winter Hexagon,” in clockwise order Sirius, Procyon, the “Twins” Pollux and Castor, Capella, Aldebaran, Rigel and back to Sirius.
Two objects follow Venus into the eastern sky during September. Binoculars will help you spot Regulus, heart of Leo, emerging 19° to the lower left of Venus in morning twilight Sept. 4-9. By Sept. 16, Regulus is much easier to spot, 16° to the lower left of Venus. Mercury, which passed inferior conjunction nearly between Earth and the sun on Sept. 6, brightens to magnitude +0.8 by Sept. 16 and is 8° below Regulus. During Sept. 16-20, Mercury lingers 23° to Venus’ lower left and brightens to magnitude zero. On Sept. 22, Mercury appears highest in twilight, in its best morning apparition of the year. By Sept. 30, Mercury shines at magnitude -1 and is nearly 30° to the lower left of Venus. Mercury is 3° lower in twilight than on Sept. 22, but is still easy to see.
Visible through binoculars in dark skies, Uranus, at magnitude 5.7, is about halfway between Jupiter and the Pleiades star cluster in September, about 8° to 9° from each. Look for 4.4-magnitude star Delta in Aries in that approximate location, and then find Uranus 2.9° to 2.6° southeast of it this month. We’re planning to post finder charts for Uranus and Neptune at www.abramsplanetarium.org/msta/ by mid-September.
Neptune is at opposition on the night of Sept. 18-19. It’s very faint, magnitude 7.7, so it’s best seen within about two hours of the time when it’s highest, in the middle of the night, which occurs around 1 a.m. at mid-month. With binoculars, look for the six-star asterism we’re calling Neptune’s Dipper, comprising the stars 20, 24, 27, 29, 33 and 30 in Pisces, of magnitude 4.4 to 5.9, located south-southeast of the Circlet of Pisces, and north-northwest of Iota in Cetus. The entire asterism fits into a binocular field 5 1/4° across. Its northernmost star, 5.5-magnitude 20 Piscium, marks the end of the dipper’s handle. Neptune is retrograding, moving from east-northeast to west-southwest past the star. On the morning of Sept. 9, Neptune is 6’ (arcminutes) or 0.1° northeast of 20 Psc. On the night of Sept. 11-12, including the morning of Sept. 12, Neptune passes only 3.7 arcminutes (about 0.06°) north-northwest of the star. By the morning of Sept. 23, Neptune has moved 0.3° west-southwest of 20 Psc. Don’t confuse Neptune with the brighter 7.3-magnitude star HIP 117112, 57 arcminutes, or nearly one degree west-southwest of 20 Psc.
Evening sky: Saturn is the only planet now visible to the unaided eye at dusk. Look for it in the east-southeast to southeast. Telescopically, its ring system now appears almost as wide as the disk of Jupiter, and the rings are tipped 9° to 10° from edgewise. Saturn, at magnitude 0.5 to 0.6, ranks next after golden Arcturus in the west, and blue-white Vega nearly overhead, both stars shining near magnitude zero. Other bright stars at dusk include Altair and Deneb, completing the Summer Triangle with Vega; reddish Antaresin the south-southwest to southwest; and Spica in the west-southwest, 33° to the lower left of Arcturus.
The moon, full near Saturn on Aug. 30, rises soon after twilight ends on Sept. 2. In the morning sky, watch the moon hover near Jupiter on Sept. 4; pass the Pleiades, Aldebaran and Elnath in Taurus Sept. 5-7; Pollux and Castor in Gemini on Sept. 9 and 10; widely north of Venus on Sept. 11; and skip past Regulus on Sept. 12 and 13. On Sept. 16, Mars, to the lower right of a thin crescent moon very low in the west in bright twilight, will be a serious challenge for binoculars. Spica, to lower right of the moon on the next evening, will be easier. Catch Antares near the moon on Sept. 20, and Saturn near the moon on Sept. 26.
Autumn begins on Sept. 22 at 11:50 p.m., making the full moon of the night of Sept. 28-29 the harvest moon. For the next few evenings, watch the moon rise not much later each night, and farther north each time.
Robert Victor originated the Abrams Planetarium monthly Sky Calendar in October 1968 and still helps produce an occasional issue. He enjoys being outdoors sharing the beauty of the night sky and other wonders of nature. Robert Miller, who provided the evening and morning 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.