The term 'Milky Way' is a translation of the Latin 'via lactea', from the Greek 'γαλαξίας κύκλος', 'galaxías kýklos', which translates as 'milky circle'. From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within.
The Milky Way is a barred spiral galaxy that has a diameter usually considered to be roughly 100,000–120,000 light-years but may be 150,000–180,000 light-years. The Milky Way is estimated to contain 100–400 billion stars, although this number may be as high as one trillion. The Solar System is located within the disk, about 27,000 light-years from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm (yes, named for the Orion constellation). The Milky Way was just one of eleven 'circles' the ancient Hellenes identified in the sky, others being the zodiac, the meridian, the horizon, the equator, the tropics of Cancer and Capricorn, Arctic and Antarctic circles, and two colure circles passing through both poles.
The ancient Hellenes were aware of the Milky Way, but were very unsure of what it actually was. Among many others, Aristotle (384–322 BC) wrote about it that the Hellenic philosophers Anaxagoras (c. 500–428 BC) and Democritus (460–370 BC) proposed that the Milky Way might consist of distant stars. However, Aristotle himself believed the Milky Way to be caused by 'the ignition of the fiery exhalation of some stars which were large, numerous and close together' and that the 'ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions'. From the 'Meteorologica':
"Let us now explain the origin, cause, and nature of the milky way. And here too let us begin by discussing the statements of others on the subject. Of the so-called Pythagoreans some say that this is the path of one of the stars that fell from heaven at the time of Phaethon's downfall. Others say that the sun used once to move in this circle and that this region was scorched or met with some other affection of this kind, because of the sun and its motion.
But it is absurd not to see that if this were the reason the circle of the Zodiac ought to be affected in the same way, and indeed more so than that of the milky way, since not the sun only but all the planets move in it. We can see the whole of this circle (half of it being visible at any time of the night), but it shows no signs of any such affection except where a part of it touches the circle of the milky way.
Anaxagoras, Democritus, and their schools say that the milky way is the light of certain stars. For, they say, when the sun passes below the earth some of the stars are hidden from it. Now the light of those on which the sun shines is invisible, being obscured by the of the sun. But the milky way is the peculiar light of those stars which are shaded by the earth from the sun's rays.
This, too, is obviously impossible. The milky way is always unchanged and among the same constellations (for it is clearly a greatest circle), whereas, since the sun does not remain in the same place, what is hidden from it differs at different times. Consequently with the change of the sun's position the milky way ought to change its position too: but we find that this does not happen. Besides, if astronomical demonstrations are correct and the size of the sun is greater than that of the earth and the distance of the stars from the earth many times greater than that of the sun (just as the sun is further from the earth than the moon), then the cone made by the rays of the sun would terminate at no great distance from the earth, and the shadow of the earth (what we call night) would not reach the stars. On the contrary, the sun shines on all the stars and the earth screens none of them.
There is a third theory about the milky way. Some say that it is a reflection of our sight to the sun, just as they say that the comet is. But this too is impossible. For if the eye and the mirror and the whole of the object were severally at rest, then the same part of the image would appear at the same point in the mirror. But if the mirror and the object move, keeping the same distance from the eye which is at rest, but at different rates of speed and so not always at the same interval from one another, then it is impossible for the same image always to appear in the same part of the mirror. Now the constellations included in the circle of the milky way move; and so does the sun, the object to which our sight is reflected; but we stand still. And the distance of those two from us is constant and uniform, but their distance from one another varies. For the Dolphin sometimes rises at midnight, sometimes in the morning. But in each case the same parts of the milky way are found near it. But if it were a reflection and not a genuine affection of these this ought not to be the case.
Again, we can see the milky way reflected at night in water and similar mirrors. But under these circumstances it is impossible for our sight to be reflected to the sun. These considerations show that the milky way is not the path of one of the planets, nor the light of imperceptible stars, nor a reflection. And those are the chief theories handed down by others hitherto." [1.8]
But it is absurd not to see that if this were the reason the circle of the Zodiac ought to be affected in the same way, and indeed more so than that of the milky way, since not the sun only but all the planets move in it. We can see the whole of this circle (half of it being visible at any time of the night), but it shows no signs of any such affection except where a part of it touches the circle of the milky way.
Anaxagoras, Democritus, and their schools say that the milky way is the light of certain stars. For, they say, when the sun passes below the earth some of the stars are hidden from it. Now the light of those on which the sun shines is invisible, being obscured by the of the sun. But the milky way is the peculiar light of those stars which are shaded by the earth from the sun's rays.
This, too, is obviously impossible. The milky way is always unchanged and among the same constellations (for it is clearly a greatest circle), whereas, since the sun does not remain in the same place, what is hidden from it differs at different times. Consequently with the change of the sun's position the milky way ought to change its position too: but we find that this does not happen. Besides, if astronomical demonstrations are correct and the size of the sun is greater than that of the earth and the distance of the stars from the earth many times greater than that of the sun (just as the sun is further from the earth than the moon), then the cone made by the rays of the sun would terminate at no great distance from the earth, and the shadow of the earth (what we call night) would not reach the stars. On the contrary, the sun shines on all the stars and the earth screens none of them.
There is a third theory about the milky way. Some say that it is a reflection of our sight to the sun, just as they say that the comet is. But this too is impossible. For if the eye and the mirror and the whole of the object were severally at rest, then the same part of the image would appear at the same point in the mirror. But if the mirror and the object move, keeping the same distance from the eye which is at rest, but at different rates of speed and so not always at the same interval from one another, then it is impossible for the same image always to appear in the same part of the mirror. Now the constellations included in the circle of the milky way move; and so does the sun, the object to which our sight is reflected; but we stand still. And the distance of those two from us is constant and uniform, but their distance from one another varies. For the Dolphin sometimes rises at midnight, sometimes in the morning. But in each case the same parts of the milky way are found near it. But if it were a reflection and not a genuine affection of these this ought not to be the case.
Again, we can see the milky way reflected at night in water and similar mirrors. But under these circumstances it is impossible for our sight to be reflected to the sun. These considerations show that the milky way is not the path of one of the planets, nor the light of imperceptible stars, nor a reflection. And those are the chief theories handed down by others hitherto." [1.8]
Mythologically speaking, Hyginus is our sole surviving account of the mythology behind it. He believed it to be either the breastmilk of Rhea or Hera. From the 'Astronomica':
"There is a certain circular figure among the constellations, white in color, which some have called the Milky Way. Eratosthenes says that Juno [Hera], without realizing it, gave milk to the infant Mercury [Hermes], but when she learned that he was the son of Maia, she thrust him away, and the whiteness of the flowing milk appears among the constellations.
Others have said that Hercules [Hēraklēs] was given to Juno to nurse when she slept. When she awoke, it happened as described above. Others, again, say that Hercules was so greedy that he couldn’t hold in his mouth all the milk he had sucked, and the Milky Way spilled over from his mouth.
Still others say that at the time Ops [Rhea] brought to Saturn [Kronos] the stone, pretending it was a child, he bade her offer milk to it; when she pressed her breast, the milk that was caused to flow formed the circle which we mentioned above." [II.43]
As viewed from Earth, the visible region of the Milky Way's Galactic plane occupies an area of the sky that includes 30 constellations. The center of the Milky Way lies in the direction of the constellation Sagittarius; it is here that the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass westward to the Galactic anticenter in Auriga. The band then continues westward the rest of the way around the sky, back to Sagittarius. The band divides the night sky into two roughly equal hemispheres. The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light such as light pollution or stray light from the Moon. For observers from approximately 65 degrees north to 65 degrees south on Earth's surface, the Milky Way passes directly overhead twice a day.
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