Natural History Magazine

Skylog

September 2008

By Joe Rao

The Teapot (red lines) and the Milk Dipper (yellow lines) are two simple patterns that connect stars in the constellation Sagittarius, the Archer, whose traditional figure is suggested by an illustration from a seventeenth-century star atlas. The blue lines define the portion of the sky that modern astronomers assign to the constellation.

Star map created by Brian Abbott/Digital Universe/American Museum of Natural History; engraving from the U.S. Naval Observatory Library, Uranographicum by J. Hevelius

Jupiter shines high above the southern horizon at dusk, making it September’s most prominent planet. It is enthroned in the constellation Sagittarius, the Archer, just to the upper left of the eight stars that form a pattern known as the Teapot. What, you might ask, does a teapot have to do with a mythological centaur drawing a bow?

Forty-seven constellations, including Sagittarius, bear the names of mythological figures that the ancients saw in certain bright patterns of stars. Other constellations were defined in the seventeenth and eighteenth centuries, with names such as “microscopium” and “telescopium.” Modern star atlases, following the precedent set by the International Astronomical Union in 1930, recognize eighty-eight constellations in total, and draw boundaries between them so that every bit of sky is unambiguously assigned to one of them. The Teapot falls within Sagittarius, but it has no official status as a constellation. It is one of the many striking patterns of stars called asterisms. (In fact, some of the Teapot’s stars are shared with another asterism called the Milk Dipper, a ladle that appears to be dipping into the Milky Way.)

Asterisms are often more familiar than their host constellations. They also can include stars from more than one constellation. For instance, the Summer Triangle comprises the brightest stars in three different constellations: Vega (in Lyra, the Lyre), Deneb (in Cygnus, the Swan), and Altair (in Aquila, the Eagle). That asterism lies nearly overhead soon after darkness falls on September evenings.

Joe Rao is a broadcast meteorologist and an associate and lecturer at the Hayden Planetar- ium in New York City.

SEPTEMBER NIGHTS OUT

1 Mercury, Venus, and Mars spend much of September clustered close together—unfortunately they are hard to see against the bright evening twilight. Scanning the western horizon through binoculars, about fifteen to thirty minutes after sunset this evening, you might pick out the three planets forming a right triangle, with Mercury at the bottom, Venus to Mercury’s upper right, and Mars off to Mercury’s upper left. Below and left of the triangle is the narrow crescent Moon, approaching three days past new.

7 The Moon waxes to first quarter at 10:04 a.m. eastern daylight time (EDT).

11 Venus and Mars are separated by only 0.3 degree.

12 Venus is within 3.6 degrees of Mercury.

15 The Moon becomes full at 5:13 a.m. EDT. As the full Moon closest to the autumnal equinox (for the Northern Hemisphere), it is called the Harvest Moon.

22 The Moon wanes to last quarter at 1:04 a.m. EDT. At 11:44 a.m., the equinox takes place, as the Sun crosses the celestial equator (Earth’s equator projected against the sky) from north to south. Autumn begins in the Northern Hemisphere and spring begins in the Southern Hemisphere.

29 The Moon is new at 4:12 a.m. EDT.