Jupiter dazzles all night
After months with few if any evening planets, September offers several solar system worlds at a decent hour. You’ll first want to target Mercury, which is on the back end of its finest display this year. On September 1, the world stands 16° high in the west 45 minutes after sunset. Glowing at magnitude 0.4, it appears conspicuous in the twilight sky.
Although the inner planet dims and sinks closer to the horizon each day, it remains a fine sight for the next two weeks. On September 14, the magnitude 1.5 world hangs 10° high a half-hour after the Sun goes down. Be sure to follow Mercury’s changing appearance through your telescope. On the 1st, the planet measures 8" across and is just under half-lit. By the 14th, it spans 10" and the Sun illuminates 16 percent of its Earth-facing hemisphere.
Saturn reached opposition in mid-August and now lies high in the east once darkness falls. The ringed planet shines at magnitude 0.4 and stands out against the much fainter background stars of eastern Capricornus.
September evenings find Saturn perfectly placed to view through a telescope. Even a small instrument reveals the planet’s 18"-diameter disk surrounded by a stunning ring system that spans 42" and tilts 15° to our line of sight. The 8thmagnitude moon Titan also appears obvious. In moments of good seeing, the dark Cassini Division separating the outer A ring from the brighter B ring stands out. If you increase the aperture to 10 centimeters, a trio of 10th-magnitude satellites — Tethys, Dione, and Rhea — comes into view.
The solar system’s largest planet, Jupiter, lies 45° east of Saturn along the ecliptic and thus trails about three hours behind its cousin. The giant world crosses from Cetus into Pisces on September 1 and continues to swim with the Fish the rest of the month. The planet gleams at magnitude –2.9 throughout September, far outpacing its starry surroundings.
Jupiter shines so brightly in part because it reaches opposition September 26. It then lies opposite the Sun in our sky and remains visible all night. It also approaches closest to Earth at opposition and thus shines brightest and looms largest when viewed with a telescope. The gas giant’s equatorial diameter swells to 50" on the 26th, larger than any other planet can appear except for Venus when the inner planet is near inferior conjunction.
Plan to spend some time observing Jupiter once it climbs higher in the sky around midevening. Look for two parallel dark belts sandwiching a bright zone that coincides with the planet’s equator. Moments of good seeing should reveal a whole series of alternating belts and zones. Also be sure to track the movements of Jupiter’s four bright moons, which can change relative positions in as little as an hour.
Even Mars manages to rise by midnight local time in late September, though it comes up an hour later when the month begins. The Red Planet resides in Taurus, with its eastward trek starting between the magnificent Hyades and Pleiades star clusters and ending roughly two-thirds of the way from the Bull’s head to the tips of the horns. Mars also brightens significantly during September, from magnitude –0.1 to magnitude –0.6.
This brightening coincides with the ruddy world coming closer to Earth and growing larger when viewed through a telescope. Mars’ apparent diameter swells from 10" to 12" during September — big enough to show surface features under good seeing conditions. The best time to look is when the planet climbs highest in the sky as dawn starts to break.
Venus rises barely 30 minutes before the Sun in early September and is essentially lost from view. It will return to the evening sky in December.
The starry sky
Sagittarius the Archer lies nearly overhead as darkness falls in September, making this a great month to ponder the odd pattern of its star designations. Most backyard observers think the Greek letter designations German astronomer Johann Bayer developed in the early 1600s reflect the order of a star’s brightness within a given constellation, with Alpha (α) the brightest, Beta (β) second, and so on. This is not strictly the case, however. Bayer grouped the stars in brightness classes and then assigned Greek letters within each class.
But Sagittarius doesn’t even come close to this ideal: Alpha, Beta, Gamma (γ), Delta (δ), and Epsilon (ε) Sagittarii actually run in order of increasing brightness! It pays to remember that Bayer observed from Augsburg, Germany, at a latitude of 48° north, where Sagittarius hangs low in the sky. In fact, Beta never rose above his horizon, and Alpha barely did.
Star names have always fascinated me. Perhaps the most intriguing one in Sagittarius is magnitude 2.1 Nunki (Sigma [σ] Sgr), the Archer’s secondbrightest star. The name comes from the Sumerian Tablet of Thirty Stars, in which it is star number 29.
Yet Nunki wasn’t originally applied to Sigma Sgr. In the late 1800s, author Robert Brown identified Nunki with Altair (Alpha Aquilae); a century later, Ian Ridpath suggested the name actually applied to a group of stars. Whatever its origin, Nun-ki was the cuneiform representation of the ancient Sumerian city of Eridu, which today is a relatively small archaeological site in Iraq called Tell Abu Shahrain, not far from the larger site of Ur.