From the sea or a flat prairie, it is easy to see the horizon as a circle around you, but from most places where people live today, the horizon is at least partially hidden by mountains, trees, buildings, or smog. Figure 1: The Sky around Us. If you lie back in an open field and observe the night sky for hours, as ancient shepherds and travelers regularly did, you will see stars rising on the eastern horizon just as the Sun and Moon do , moving across the dome of the sky in the course of the night, and setting on the western horizon.
Watching the sky turn like this night after night, you might eventually get the idea that the dome of the sky is really part of a great sphere that is turning around you, bringing different stars into view as it turns.
The early Greeks regarded the sky as just such a celestial sphere Figure 2. Some thought of it as an actual sphere of transparent crystalline material, with the stars embedded in it like tiny jewels. Figure 2: Circles on the Celestial Sphere. Here we show the imaginary celestial sphere around Earth, on which objects are fixed, and which rotates around Earth on an axis.
In reality, it is Earth that turns around this axis, creating the illusion that the sky revolves around us. Note that Earth in this picture has been tilted so that your location is at the top and the North Pole is where the N is. The apparent motion of celestial objects in the sky around the pole is shown by the circular arrow.
Today, we know that it is not the celestial sphere that turns as night and day proceed, but rather the planet on which we live. It is because Earth turns on this axis every 24 hours that we see the Sun, Moon, and stars rise and set with clockwork regularity. Today, we know that these celestial objects are not really on a dome, but at greatly varying distances from us in space. Nevertheless, it is sometimes still convenient to talk about the celestial dome or sphere to help us keep track of objects in the sky.
There is even a special theater, called a planetarium , in which we project a simulation of the stars and planets onto a white dome. As the celestial sphere rotates, the objects on it maintain their positions with respect to one another.
A grouping of stars such as the Big Dipper has the same shape during the course of the night, although it turns with the sky. During a single night, even objects we know to have significant motions of their own, such as the nearby planets, seem fixed relative to the stars. This is because they are not stars at all.
We can use the fact that the entire celestial sphere seems to turn together to help us set up systems for keeping track of what things are visible in the sky and where they happen to be at a given time. Figure 3: Circling the South Celestial Pole. This long-exposure photo shows trails left by stars as a result of the apparent rotation of the celestial sphere around the south celestial pole.
In reality, it is Earth that rotates. Imagine a line going through Earth, connecting the North and South Poles. If we extend this imaginary line outward from Earth, the points where this line intersects the celestial sphere are called the north celestial pole and the south celestial pole.
As Earth rotates about its axis, the sky appears to turn in the opposite direction around those celestial poles Figure 3.
The apparent motion of the celestial sphere depends on your latitude position north or south of the equator. If you stood at the North Pole of Earth, for example, you would see the north celestial pole overhead, at your zenith.
As you watched the stars during the course of the night, they would all circle around the celestial pole, with none rising or setting.
Only that half of the sky north of the celestial equator is ever visible to an observer at the North Pole. Similarly, an observer at the South Pole would see only the southern half of the sky. As the sky turns, all stars rise and set; they move straight up from the east side of the horizon and set straight down on the west side. During a hour period, all stars are above the horizon exactly half the time. You can estimate smaller angles with your hand as well. To measure angular separations more accurately, we will use a device called a cross-staff , which is basically a stick, The length of the stick was deliberately chosen; if the ruler is If you know trigonometry, note that To use a cross-staff close one eye and place the end of the stick without the ruler just under the other eye.
Sight along the stick towards the two stars you want to measure and adjust the markers on the ruler to line up with these stars. Finally, read off the positions of the markers on the ruler; the difference between them is the angular separation between the two stars. It's often fairly easy to see a constellation when it's pointed out in the sky, but harder to remember it so you can find it yourself. The best way to really learn the constellations is to draw them; when you do this, your eyes pick out geometrical patterns which will help you remember.
Our drawings will be made to scale; this gives you a feeling for the sizes of constellations. Many people confuse the Pleiades with the Little Dipper; one is about ten times the size of the other! Here's how to make an accurate drawing:. Because different constellations are visible at different times, we will return to the study of constellations throughout the semester.
These constellations are easy to see; you may already know some of them. We call groups of stars which form a picture in our imagination, a constellation. Ancient cultures all over the world created their own constellations and told stories about them. You might have seen parts of constellations in the sky at night.
Maybe you've spotted the 3 bright stars which make a "belt" in the constellation, Orion. Or you've seen the part of the Great Bear which contains the "big dipper".
Did you know we have a name for these smaller parts of constellations? We call them asterisms. They don't divide it just like a puzzle, th fifteen grouping of stars are galexys between th galaxies the is nothing so their not dividing the sky it's self they're grouping the different places also known as galaxies. Stare at the sky.
I've never heard anybody use the phrase "on the sky" in any context. Divide the Blackened Sky was created on Hundreds of different constellations have been proposed, by groups of people from every walk of life.
Currently there are 88 officially recognized constellations that divide up our night sky, which aid astronomers in locating objects and events.
A Telescope. Many people consider the first astronomers to be the first humans that ever looked up at the night sky and wondered what was really up there. So cavemen were probably the first astronomers. Catalogue of stars or star catalog. The sky is divided into 88 constellations. The answer is totally dependent on what their specialty is. Log in. Study now. See Answer. Best Answer. Study guides. Astronomy 20 cards. How long does it take for the solar system to make one orbit around the Milky Way galactic center.
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