Last reviewed: June The point in the sky directly above an observer. If the observer is at a pole, then the observer's zenith is a celestial pole; but for observers at midlatitudes the zenith is a point in the sky that corresponds to a changing right ascension but a constant declination as the sky rotates overhead.
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The top of that dome, the point directly above your head, is called the zenith , and where the dome meets Earth is called the horizon. 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. Of course, during some of those hours, the Sun is too bright for us to see them.
What would an observer in the latitudes of the United States or Europe see? For those in the continental United States and Europe, the north celestial pole is neither overhead nor on the horizon, but in between. As Earth turns, the whole sky seems to pivot about the north celestial pole.
They are always above the horizon, day and night. This part of the sky is called the north circumpolar zone. For observers in the continental United States, the Big Dipper, Little Dipper, and Cassiopeia are examples of star groups in the north circumpolar zone. That part of the sky is the south circumpolar zone. To most U.
It is called Polaris , the pole star, and has the distinction of being the star that moves the least amount as the northern sky turns each day. Astronomers measure how far apart objects appear in the sky by using angles. This will alert our moderators to take action. Nifty 18, Zomato Ltd. Market Watch. ET NOW. Brand Solutions.
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ET Markets Conclave — Cryptocurrency. Reshape Tomorrow Tomorrow is different. Let's reshape it today. Corning Gorilla Glass TougherTogether. ET India Inc. ET Engage. ET Secure IT. Suggest a new Definition Proposed definitions will be considered for inclusion in the Economictimes. Wavelength Definition: Wavelength can be defined as the distance between two successive crests or troughs of a wave. It is measured in the direction of the wave. Description: Wavelength is the distance from one crest to another, or from one trough to another, of a wave which may be an electromagnetic wave, a sound wave, or any other wave.
Crest is the highest point of the wave whereas the trough is the lowest. The following equation is known as the wavelength formula or the wavelength equation:? Red colour has the longest wavelength while violet has the least.
The wavelength of UV radiation is shorter than violet light. Similarly, the wavelength of infra red radiation is longer than the wavelength of red light. Wavelength is inversely proportional to frequency. This means the longer the wavelength, lower the frequency.
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