Comet cuts the Coathanger
Second to the total eclipse of the Moon, the most interesting sky event for Northern Hemisphere observers this month is arguably the passage of Comet 2P/Encke through the Coathanger asterism on Saturday night, November 22.
What's an asterism? It's not a constellation, but rather a group of stars that forms an eye-catching pattern (like the Big Dipper) or resembles a loose cluster. For more than a century, astronomers thought the Coathanger was an organized group and cataloged it as Collinder 399. Guidebooks popularized it as both Brocchi's Cluster and the Coathanger.
In the mid 1990s, however, the Hipparcos spacecraft took precise positional measurements of more than 100,000 stars, including many in the Coathanger. The data proved that the stars are unrelated. But the poets at heart still see a thing of beauty, thanks to its recognizable shape and perfect size for binoculars. Most backyard telescopes have a too narrow field of view, allowing observers to see only four or five stars at once.
Comet Encke comes within binocular range of the Coathanger on November 20. European observers get to see the comet's core pass right through the asterism on the 22nd, while North Americans are treated to the sight of the comet's tail sliding among the asterism's bright stars.
Earlier in the month, Encke passes near the Blue Snowball Nebula and the Veil Nebula. See page 65 for a detailed chart showing the comet's path past these deep-sky wonders.
A real cluster
Just off the eastern edge of the Coathanger lies a real star cluster, NGC 6802. It forms a small box aligned north-south and about 11/49 the apparent size of the Moon. On a dark, moonless night away from the city, a 4.5-inch reflector catches the soft glow at 32x. Pushing the power to 150x starts to bring out individual stars, although the overall view proves more satisfying at 90x because you're not straining to see the faint specks. The cluster exudes a definite charm in large apertures thanks to its sparkling stars and striking shape.
Piazzi's "Flying Star"
On the evening of November 12, Encke cruises near the beautiful binary star 61 Cygni. Any backyard telescope at low power will separate the pair easily. From our earthly perspective, only five stars appear to move faster across the sky than 61 Cygni's 5.3" per year.
Italian astronomer Giuseppe Piazzi first called the world's attention to 61 Cygni in 1792. Nearly fifty years later, German astronomer and mathematician Friedrich Bessel accurately measured 61 Cygni's much smaller side-to-side shift. Caused by Earth's movement around the Sun, this so-called parallax was only 0.29". Modern visual observers are happy under conditions that let them resolve 0.5". As a comparison, the gap between the stars is 30".
Martin Ratcliffe is former president of the International Planetarium Society. Alister Ling is a meteorologist with Environment Canada.