Along the path of the zodiac, just east and a little north of Sagittarius, lies the smile-shaped constellation Capricornus. After the gaudy splendors of Sagittarius and other northern summer constellations, Capricornus isn’t much to look at. It’s the smallest constellation of the zodiac and the second-faintest after Cancer. It has just two stars brighter than 4th magnitude, so it’s a challenge to see this constellation in the city. But Capricornus offers several splendid alignments of stars that make for excellent viewing with a pair of binoculars on nights in August through October. Let’s have a look…
Like all zodiacal constellations, Capricornus is one of Ptolemy’s original 48 from his celestial maps of nearly two millennia ago, and its origins go back thousands of years. In Greek mythology, Capricornus represents a “Sea Goat”. We don’t come across many sea goats these days, so a few words of explanation are in order. As the Greek myth goes, the gods of Olympus were feasting by a river when the fierce and horrifying monster Typhon approached. The beast was too much even for the gods to defeat, so they disguised themselves as animals to escape. The god Pan fled into the river so quickly (in a “panic”), he misused his Olympian powers and turned his lower half into a fish and his upper half into a goat. Another legend comes from the much earlier Sumerian civilization some 5,000 years ago. The Earth god Enki came to be symbolized by a goat-fish and took a place in Sumerian star patterns. The Greeks certainly knew of this legend and concocted their own story to place the sea goat in the sky.
The northwestern tip of Capricornus is marked by the lovely optical double star α Capricorni, also called Algedi (Arabic for “goat”). At magnitude 3.6 it is not the brightest star in the constellation, but it is a lovely sight in binoculars. The star consists of two 4th magnitude components separated by nearly 1/10th of a degree. The westernmost of the two stars is a yellow supergiant some 690 light years away. The other star, physically unrelated to the first, is a yellow giant star 109 light years away. Sharper-eyed observers can resolve the pair without optical aid. In antiquity, the stars were not resolvable by most observers. Only since the 18th century has their motion through the galaxy moved them far enough apart to resolve without optics. In binoculars or a finder scope, the pair is also set against a fine background of 5th and 6th-magnitude stars.
Move down a little to see β Capricorni, another fine double star for binoculars. This is Dabih, which is Arabic for “butcher”. This pair is also widely spaced, though a little tighter than Algedi, but the contrast in brightness between the two components makes this pair much more challenging to resolve without optics. Binoculars show them easily. The 3rd-magnitude primary star in this pair is yellow-orange while the 6th-magnitude secondary is light blue. The two stars of Dabih form a true binary pair, but they are so far away from each other their orbital period is of the order of a million years. Each of these stars is itself a very closely-spaced double star, and there may be a fifth star in this system as well.
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The brightest star in Capricorn is δ Capricorni, also known as Deneb Algedi (the “tail of the goat”). This whitish star, which lies in the easternmost horn of the celestial goat-fish, is just 39 light years away.
There are more lovely star arrangements in Capricornus. If you have binoculars, put Dabih in the upper right of your field of view. In the center of the field you will see a triangle of lovely stars, the brightest of which is 5th-magnitude ρ Capricorni. Put rho on the right side of the field and you’ll see at the left edge the 6th-magnitude star 19 Capricorni below which lies a cascade of half a dozen stars aligned by chance. Very pretty.
Over on the other side of the constellation, a lovely curving arrangement of stars some 3o long lies northward of zeta (ζ) Capricorni. About two degrees southeast of zeta lies the 7th-magnitude globular cluster M30, a dim smudge in binoculars. At a declination of -23o, the cluster is visible from northern Europe to the deep southern hemisphere, but southern-hemisphere observers have the advantage here. The cluster is easily visible in binoculars or a finderscope.
A 4-inch scope at 100x resolves perhaps a dozen stars in the halo of M30 and shows brightening towards the core. Averted vision brings out more detail in this compact cluster, which is only 1/10 of a degree across. In a larger telescope, especially from locations where the cluster is well above the horizon, M30 appears clearly asymmetric. The halo stars look to be arranged in four short appendages, two on either side, that give the cluster the appearance of a cartoon character with spread-out legs and arms thrust aside.
The Tropic of Capricorn is the latitude on Earth at which the Sun appears overhead at noon on the winter solstice around December 21. This latitude is 23.5o south. When this was first formalized by ancient Greek astronomers, the Sun was in Capricornus on this date. Now, as a result of the wobble of the Earth’s axis, the Sun’s in Sagittarius at the winter solstice. The Tropic of Cancer is similarly defined. At 23.5o north latitude, the Sun is directly overhead on or about June 21. In ancient times, the Sun was in Cancer on this date. It now lies in Taurus.
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