Our morning twilight all-sky chart for September, viewable below, shows the changes in positions of the naked-eye planets and the stars of first-magnitude or brighter, less than an hour before sunrise.
Changes are caused by the motion of the Earth and the other planets in their orbits. Stars will appear to drift from east to west across the sky as weeks pass, thanks to the revolution of Earth around the sun. Venus now dominates the predawn sky, but is getting a little lower each morning, because it is heading toward the far side of the sun, where it will arrive in January 2018.
Annually in September and October, the huge Winter Hexagon—in clockwise order from its brightest member, Sirius, Procyon, Pollux and Castor, Capella, Aldebaran and Rigel, with Betelgeuse inside—is well-placed high in the sky before dawn. The flashing blue-white Dog Star Sirius is its most prominent and southernmost member. Sirius is the brightest star in Earth’s night skies, but it does not appear as bright as Venus.
Morning planet gatherings: Low in the eastern morning sky in early September, Mars, Mercury and Regulus emerge to the lower left of Venus. Regulus, at magnitude +1.4, is the faintest of the first-magnitude stars. Mars is now even fainter, at magnitude +1.8, as dim as it gets—but Earth will catch up to it and close the gap. In July 2018, Mars will gleam at magnitude -2.8, visible all night, in the southwest at dawn (and southeast at dusk), and will be closer to Earth and brighter than at any time since the very close approach of August 2003.
On Sept. 4, Mercury, at magnitude +1.7, is a close match in brightness to faint Mars; they rise nearly at the same time, with Mars 3 degrees to the left of Mercury, and Regulus below them. Binoculars will show them in twilight about 17 degrees to the lower left of Venus, with the contrasting colors of reddish Mars and blue-white Regulus. All rise earlier each morning and get easier to see, especially Mercury, which brightens sharply, to magnitude +1.0 by Sept. 6; magnitude 0 on Sept. 9 and 10; and magnitude -1.0 on Sept. 18. Watch for these close pairs, all less than one degree apart: Mercury-Regulus on Sept. 10; Mercury-Mars on Sept. 16; Venus-Regulus on Sept. 18 and 19; Venus-Mars on Oct. 5; and Venus-Jupiter on Nov. 13. Binoculars will give a wonderful view of all these gatherings.
On Tuesday, Sept. 12, Mercury reaches a favorable greatest elongation, 18 degrees from the sun and 11 degrees to the lower left of Venus. Earlier on the same morning, telescopes show the moon covering and uncovering the first-magnitude star Aldebaran, eye of Taurus, the Bull. From Palm Springs, the star’s disappearance at the moon’s leading bright edge occurs just before 4:39 a.m., with reappearance at the trailing dark edge just after 5:53 a.m. Still earlier that morning, the dark edge of the moon uncovers a close pair of stars in the Hyades star cluster, at 1:31 and 1:38 a.m.
A few days later, on the mornings of Sept. 17 and 18, a waning crescent moon will appear near the Venus-Regulus pair, with the Mercury-Mars pairing below.
Follow the moon in evening sky: Careful viewers might spot a thin, young crescent moon very low in the west to west-southwest early on Thursday evening, Sept. 21, about 30 minutes after sunset, to the right of Jupiter. Find a spot without mountains blocking the view, and hope for very clear skies. It’ll be much easier to spot the crescent on the next evening, to the upper left of Jupiter. A much fatter crescent moon will pass the other giant planet, Saturn, on Tuesday evening, Sept. 26, with the twinkling reddish star Antares to their lower right. On the next evening, Sept. 27, the moon will reach first-quarter phase, 90 degrees or a quarter-circle east of the sun, and will appear half full. On Oct. 5, the moon will be full and will rise shortly after sunset, a little north of east, almost opposite to the sun’s direction in the sky.
Jupiter and Saturn are the slowest of the bright naked-eye planets. Jupiter takes nearly 12 years to make one trip around the sun, and Saturn takes about 29.5 years. Jupiter will pass 3.4 degrees north of Spica, the spike of grain in Virgo’s hand, on Sept. 11, and will return for a triple encounter with that star in 2029. Saturn appeared near Antares, heart of the Scorpion, in 2016, and will return to that star’s vicinity in 2045. Watch Jupiter creep slowly past Spica for several evenings around Sept. 11. Binoculars will help you pick up Spica to lower left of Jupiter as both sink into the twilight glow.
When the moon makes it around to Jupiter and Saturn from September 20-26, the planets will appear lower in the sky than they were early in the month, as shown on our all-sky evening twilight chart for September. So, if you’d like to enjoy telescopic views of both Jupiter and Saturn during an evening session, do it soon! (See a list of Astronomical Society of the Desert star parties at astrorx.org.) On these charts, note also the seasonal westward motions of Arcturus and the Summer Triangle of Vega, Altair and Deneb. The slow-moving outer planets seem to get dragged along with the stars’ seasonal motions.
To help you plan evening or morning planet-viewing sessions during the coming school year, see Jeffrey Hunt’s two-page graphic summary of moon and planet setting and rising times relative to times of sunset and sunrise—exact for Palm Springs—below.
Wishing you clear skies!
Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing sky watching opportunities for school children in and around Palm Springs. Robert D. Miller 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. Dr. Jeffrey L. Hunt, a retired planetarium director now living in the Chicago area, has taught astronomy and sky watching to all ages. He studied astronomy education at Abrams Planetarium at Michigan State University. Jeff writes an astronomy blog at jeffreylhunt.wordpress.com and can be followed on Twitter at @jeff_hunt.