Throughout September, all five outer planets of our solar system—those more distant from the sun than the 8.3 minutes of light-travel time from the sun to our home planet—form a long lineup across the predawn sky.
Three are clearly visible as bright “stars” to the unaided eye. The waning moon fits within the lineup during Sept. 1-8, and again during Sept. 27-Oct. 5. The order of the moon and planets within the lineup, from west to east one hour before sunrise on Sept. 1, are Saturn, in Pisces in the southwest; the moon, in waning gibbous phase, 80 percent full, in Aries and 20° above Saturn; Mars, in central Gemini in the east, forming a nearly isosceles triangle with the “Twin” stars, Pollux and Castor, 12° to its lower left; and Jupiter, the brightest morning “star” in Cancer, low, between east and east-northeast, 30° to Mars’ lower left.
Because of the Earth’s faster revolution around the sun, all of these distant planets, along with the entire field of stars, will drift from east to west across the sky as weeks pass. The motion is depicted on our morning twilight sky map accompanying the online edition of this column.
If you look closely, you can see these planetary bodies have their own motions against the background. The moon is close to us, completing one circuit around the Earth with respect to background stars in 27.3 days, so its change in position is obvious from one day to the next. On Sept. 3, the 59 percent moon will appear very close to the Pleiades star cluster. Use binoculars to see some of its members close to the moon’s lower right. Close, but not really—light from the Pleiades takes about 440 years to reach us, but light reflected from the moon takes only 1.2 seconds to get here. Bright Aldebaran, southern eye of Taurus, is 14° to the moon’s lower left. Just 3° to the upper right of Aldebaran lies 3.5-magnitude Ain, or Epsilon Tauri, or Oculus Borealis, the northern eye of the Bull. The V-shaped asterism forming the head of Taurus includes both these stars, but Aldebaran, 67 light years away, is not really a member of the physical group called the Hyades, the rest of the “V,” out at a distance of about 150 light years. Visualize the sky in three dimensions as you enjoy these scenes.
An hour before sunup on Sept. 4, the moon, 48 percent full and just past last quarter phase, within 90° west of the sun, will be farther along in Taurus, within 8° to the west (upper right) of 1.7-magnitude Elnath, tip of the Bull’s northern horn. This morning, Jupiter is nearly 60° east of the moon, and Saturn is nearly 62° to the moon’s west, so the moon appears about midway between Saturn and Jupiter. The lineup—Saturn-moon-Mars-Jupiter—is 121° long, and Mars is spotted nearly midway between the moon and Jupiter, close to 30° from each. The actual light-travel times from the planets to Earth this morning, in minutes, are Saturn, 71; Mars, 15; and Jupiter, 51. Compare to the closest and brightest star visible in the predawn sky, Sirius, the Dog Star, in the southeast, is 8.6 light years away.
If you want to build a scale model of our solar system in which one inch represents the distance between Earth and the sun, and place stars in the same model, then one mile would represent one light year, and Sirius would be 8.6 miles away—in the model! Neptune, the most distant known planet of our solar system, would be 2 1/2 feet away from the sun in the scale model. The Pleiades cluster would be some 440 miles away, and it’s a near neighbor within our Milky Way Galaxy.
On Sept. 6, the 26 percent moon will have company: Mars is 4° to its lower right, with the Twin stars of Gemini, 9-10° to its lower left. On Sept. 7, the 16 percent moon will lie within the triangle formed by Mars, Pollux and Jupiter. Can you spot the star Regulus, heart of Leo, the Lion, 15° to the lower left of Jupiter this morning? Binoculars may help you spot this star emerging from the glare of twilight. It appeared on the far side of the sun just 15 days ago, on Aug. 23, and owing to the Earth’s revolution around the sun, it is now visible!
Stars rise two hours earlier each month, so by late February, Regulus will rise at sunset and be visible all night. Mars and Jupiter will remain nearby through mid-2027, forming interesting gatherings with each other and with Regulus. Mars will pass Regulus three times in an event called a triple conjunction. There’ll be some quasi-conjunctions where a body makes an approach, but doesn’t follow through. We’ll keep you posted. Check the illustrations on the Sky Calendar, available by subscription for $12 per year, from abramsplanetarium.natsci.msu.edu/sky-calendar.
On Sept. 8, Jupiter appears closely below the 9 percent crescent moon. Use binoculars before twilight brightens to spot the Beehive cluster 5° to the moon’s upper right.
On Sept. 9, find the 3 percent crescent 11° to the lower left of Jupiter, and then try for Regulus, just 3°-4° to the moon’s lower left. This will likely be your last chance to spot the old moon in the morning until the closing days of the month, when the moon will reappear in the west. In the interim, watch Mars, moving east about 0.6° per day, pass within 6° to the south of Pollux on Sept. 17. Crossing into Cancer on Sept. 25, it will form a 13° long straight line with the Twin stars. (Pollux and Castor are 4.5° apart.) Jupiter, as it crosses from Cancer into Leo on Sept. 24, is 12° from Regulus and closing in by less than 0.2° per day, and slowing down. It’ll be interesting to see what happens!
Saturn is retrograding this month, moving west against the stars. Although it crossed into Cetus on Sept. 5, it is now just south of a pair of fourth-magnitude stars, Delta and Epsilon in Pisces. Watch for changes in the shape of the triangle Saturn forms with those stars as Saturn retrogrades. A telescope shows the planet’s rings temporarily getting closer to edgewise until early December, before opening again.
In dark skies, Uranus is now an easy target for binoculars. It begins retrograde on Sept. 10, and barely moves all month. Using binoculars, note five objects in a nearly straight line, 10 3/4° long, joining 2.9-magnitude Alcyone, or Eta Tauri, brightest member of the Pleiades, to 3.5-magnitude Ain, or Epsilon Tauri, Bull’s northern eye in the Hyades, described above. In order from west-northwest to east-southeast, counting the endpoints of the line, they are: Eta; go 4.3° east-southeast to 4.4-magnitude 37 Tauri; go 2.5° east-southeast to 5.8-magnitude Uranus; go 0.95° east-southeast to 4.9-magnitude Omega-2 or 50 Tauri; and finally, go 3° east-southeast to Epsilon.
Evening sky: Venus reaches greatest brilliancy at magnitude -4.8 in late September, but gets quite low as the month progresses. Find it as soon as you can right after sunset, or even before, and that’ll be a good time to examine it with optical aid, even binoculars, to resolve the ever-thinner, ever-larger-in-apparent-diameter crescent, as the planet approaches Earth before next month’s swing between Earth and the sun. During September, Venus changes from a 38 percent crescent, 31” (arcseconds) across, to a 15 percent crescent, 49” across. The conjunction of Venus and Spica, 1.5° apart on Sept. 1, is the first of a triple conjunction between the two bodies—but the next two, in the morning sky in November, will have only one minimum separation associated with them.
While Venus and Spica are still in the evening sky, zero-magnitude Mercury passes within 1° north (to the upper right) of first-magnitude Spica on Sept. 25. But by then, Venus will be lower, and Spica will be mired very low in bright twilight, 14° to Venus’ lower right. Mercury might then be seen through binoculars, but Spica likely not. Venus will reach its maximum distance of 15° east of invisible Spica on Oct. 3, when Mercury will be 7° to Venus’ upper right.
The moon in the evening sky: On Sept. 12, the 5 percent crescent moon sets in bright twilight, 17° to the lower right of Venus. On Sept. 13, catch a 10 percent waxing crescent moon low in the west-southwest at dusk, nearly between Venus and Spica, 4°-5° from each. On the next evening, Sept. 14, find the 17 percent crescent moon 7° to the upper left of Venus. In mid-September, one hour after sunset, ranking next after Venus in brightness are two zero-magnitude stars, golden Arcturus in the west, and blue-white Vega overhead. On Sept. 16 and 17, the moon will be a fat crescent in the south-southwest, near the first-magnitude red supergiant star Antares, heart of the Scorpion. Other bright stars at dusk are Altair and Deneb, completing the Summer Triangle with Vega; and Fomalhaut, mouth of the Southern Fish, rising in the southeast before the end of twilight. The moon is full in the daytime on Sept. 26, yet will still rise just before sunset. As the sky darkens that Saturday evening, look for Saturn 6 below the moon. This outermost of the bright planets remains about the same angular distance from the moon through the rest of that night, until dawn on Sunday, Sept. 27.
An hour before sunrise in the closing days of September, Orion stands high in the south. Note his three-star belt—midway between the constellation’s brightest stars, red Betelgeuse and blue-white Rigel—points to the lower left to Sirius, and to the upper right past Aldebaran and the Hyades to the Pleiades. Procyon completes the almost equilateral Winter Triangle with Sirius and Betelgeuse.
Capella, northernmost of the stars of first-magnitude or brighter, passes north of overhead at nearly the same time that Rigel, Orion’s foot, reaches its high point in the south.
In the morning sky from Sept. 30-Oct. 7, the moon will slide past, in order from west to east, the Pleiades, the Hyades, Aldebaran, and Elnath, all in Taurus; Castor and Pollux in Gemini; Mars and the Beehive cluster in Cancer; and Jupiter and Regulus in Leo.
Robert Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys promoting sky watching to folks of all ages. 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.
