It's occasionally mistaken for a high, thin cloud when seen for the first time by city dwellers who don't often get a chance to see stars in a lightfree sky. And in a sense, it is a cloud, a great cloud of stars vaulting across the sky like some ethereal, luminous bridge. More accurately, though, it is a galaxy - our galaxy - seen from the inside out.
We call it the Milky Way, a name derived from Roman mythology, which explained the whitish glow as a stream of milk from the breast of the goddess Hera, wife of Zeus.
According to the legend, Zeus was known for frequent philandering with mortal women. One marathon three-day liaison with the exceed ingly comely young earth woman Alcmene (whom Zeus deceived into thinking he was her husband), resulted in the conception of Heracles, better known as Hercules the Hero. Zeus, like any immortal father, wanted his son to be immortal, too. But for that to happen, the baby Hercules first had to suckle the breast of Hera. Upon learning of Zeus's infidelity, however, Hera was understandably in no mood to cooperate with such a request. Nevertheless, Zeus lay Hercules upon the breast of the goddess as she slept. She soon awoke with a start and brushed the child from her. But the milk had already begun flowing and spurted across the heavens, forming the Milky Way.
Other legends about the Milky Way abound. American Indians believed it to be the celestial path taken by the souls of the dead on their arduous journey to Heaven, and that the brighter stars along the Milky Way were the campfires where they briefly stopped to rest. In China and Japan, the band of light was viewed as a river inhabited by spirits swimming toward the 'Land of Peaches.' The river metaphor appears again in Egypt, where the Milky Way was analogous to the Nile River, and in India where it represented the venerable Ganges. The Greeks viewed the Milky Way as the river Styx, across which the souls of the dead were ferried to the underworld.
Ancient astronomers thought the Milky Way was a kind of vapor emitted by the Earth. But Galileo put this notion to rest the first time he pointed his telescope at it and discovered that, in fact, the Milky Way consists of countless stars.
Today, you can see what Galileo saw using a small telescope or, better still, a pair of modest binoculars. The best conditions in which to see the Milky Way are under a dark sky. But even from the outskirts of a city, the cloudy band can be detected, especially when it is high overhead, which it is this week around 1:30 a.m. It can be seen earlier in the evening in a dark, moonless sky, arching above the eastern and southeastern horizons around 10:30 p.m.
Looking above the eastern horizon, you can easily see Cygnus the Swan, or the Northern Cross, lying on its side with the bright star Deneb shining at the head of the cross. This constellation runs down the middle of the Milky Way, the long axis of the cross pointing toward Aquila, Ophiuchus, and the star-rich southern realms of Sagittarius and Scorpius.
South of Cygnus, if your sky is dark enough, you may notice a dark cleft dividing the Milky Way, and that, indeed, just south of the tail of Aquila the Eagle, part of the Milky Way veers away to the southwest and dies out entirely. We see a major dark region in the southern sky, starting just past the tail of Scorpius and ending near Alpha Centauri. These dark 'rifts' are caused by concentrations of dense, interstellar dust lying along our line of sight. The dust is so thick that it curtains the stars behind it. If these dark clouds were not present, we'd see many more brilliant stars.
When you turn your binoculars on Sagittarius, which will be higher in the sky two weeks hence, you will be able to see many dark voids interspersed among the bright star clouds of that constellation (July 12 - 18). The most well-known 'hole' in the Milky Way appears in the Southern Hemisphere. The irregular patch of dark dust is so distinct to the unaided eye that it is called the Coal Sack.
As the night wears on, the Milky Way hoops up into the sky, revealing itself in all its glory. If your sky is dark enough, you may feel a little vertigo, for no other view gives you the sense of standing on the edge of the Earth and looking into a bottomless universe.
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