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Ceiling Frescoe, Villa Farnese - MAP OF THE HEAVENS - Giovanni Antonio da Varese - 1575
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Outside our solar system stretch the limitless reaches of interstellar space. When we look up at the sky, we are looking at nothing less than the entire Universe. It is a very big place, and very easy to get lost in. Grouping stars into constellations provides a framework for the otherwise overwhelming spectacle of a star filled sky. The constellations give us a road map for the sky, and for the Universe, so we can find our way around.
There are 88 constellations in the sky. (There are also 88 keys on a piano; God works in mysterious ways.) Each constellation occupies a designated area of space with defined boundaries, so that the sky is divided into 88 oddly shaped sections.
But the constellations do a lot more than provide a map of the heavens. They bring the sky alive, with images of kings and queens, heroes and princesses, and beasts foul and fair, telling ancient tales of adventure, and romance.
The first thing to understand about the constellations is that they do not move, at least not in a way discernible to Earth bound eyes. Earth, on the other hand, does move. As it turns, great and ponderous on its axis, the constellations appear to move across the sky. Like the Sun and the Moon, constellations rise in the east, and set in the west. And as Earth travels around the Sun, our view of the heavens gradually shifts, and the constellations slowly change positions, rising and setting almost exactly four minutes earlier each night, until they ultimately become lost in the light of dawn. The good news is that there are always new constellations gradually becoming visible in the evening dusk to take their place. As the seasons change, so do the constellations. The links at the top of the page group the constellations visible from the northern hemisphere into the seasons in which they are visible in the night sky. This is also the order in which they are presented in this tutorial. Alphabetical links to the constellations are at the bottom of each constellation page.
When we look up at the night sky, all the thousands of stars we see are but a tiny fraction of the hundreds of billions of stars that compose our Milky Way galaxy. The spiral galaxy in which we reside is shaped roughly like a disk, averaging only 8,000 light years thick, but stretching over 100,000 light years across. In order to take a photo of the galaxy we live in, we would have to somehow send a camera at least a million light years out into space, and we don't have the technology to do that. We can't even come close. The Voyager I spacecraft has been speeding away from Earth at over 38,000 miles per hour for more than three decades now, and has reached a distance of 11.2 billion miles (17.9 billion kilometers). Since one light year is over 6 trillion miles, Voyager has traveled just over one thousandth of one light year, or just over 15 light hours. Obviously it will be a very long time - if ever - before we can send a camera far enough away to see exactly what our Milky Way galaxy looks like.
Fortunately, NASA scientists are nothing if not innovative, and in 2009 they were able to analyze infrared images from the Spitzer Space Telescope, and construct a reasonably accurate artist's concept of the structure of our home galaxy. It turns out the Milky Way is a barred spiral galaxy with two major spiral arms extending from the ends of the bars. They are the Scutum-Centaurus Arm and the Perseus Arm. Smaller spiral arm segments include the Sagittarius Arm, Norma Arm, and Outer Arm. We reside in a small spiral segment called the Orion Spur, about 30,000 light years from the galactic center.
When we look up at the sky, we are usually looking up through the thin part of the galactic disk, and therefore looking through a relatively thin layer of stars, perhaps only a few thousand, out into the vast blackness of intergalactic space. But on a clear, moonless night you can sometimes see a hazy band of light stretching across the sky, like a trail of smoke from some far off celestial campfire. And what looks like smoke is in fact an unimaginable number of stars, stretching thousands of light years down through the length of the galactic disk. In the winter the Milky Way is relatively faint, as our view is looking outwards towards the edge of the disk. In the summer it is much brighter, since we are now looking towards the centre of the disk, and seeing stars, behind stars, behind stars. Billions and billions of stars. So many stars, the heart of our galaxy is completely hidden behind them. To the ancient Greeks, the band of hazy light that stretched across the sky looked like a trail of spilt milk, from the breast of Hera, queen of the gods, as she suckled the infant Hercules, depicted below by artists Jacopo Tintoreeto in 1575, and Paul Rubens in 1636. Hence, The Milky Way galaxy.
Any study of the cosmos has to start with the constellations, and although this may seem like a daunting task at first, the truth is that nothing could be easier. You don't need a telescope, or even binoculars, just a star chart and a willingness to get up off the couch and go outside after dark. Take it one constellation at a time. Identify it on the chart, then go outside, wait a few minutes for your pupils to dilate, and look up. It may take a while at first, but when you finally recognize it, I guarantee you'll be very pleased with yourself. Before you know it you'll be familiar with the dozen or so major landmark constellations, and you'll never look at the night sky (or your place within it) the same way ever again.
You can jump to any constellation you like with the alphabetical links at the bottom of each page, or follow the sequential links which will take you on a journey through all the constellations visible from the northern hemisphere, as well as some of the more notable constellations of the southern hemisphere, starting at the beginning of the terrestrial year, in January, with the magnificent winter constellation of Orion.
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