[轉貼]Introduction to Spherical Astronomy

[yfsum]

http://www.physics.csbsju.edu/astro/CS/CS.01.html

Terms: constellation, stick figure, magnitude
The below drawing shows a piece of the celestial sphere (dome of stars) that you might see if you faced North at CSB/SJU.

In this figure, stars that are brighter in the sky are shown as bigger white circles. (In fact, as seen from Earth all stars except the Sun just look like points of light, some of the points looks brighter than the rest because the star is unusually close or unusually bright.) Hipparchos (in the second century B.C.) called the brightest stars in the sky first magnitude stars, fairly bright stars (like those in the Big Dipper) second magnitude stars, etc. He called the dimmest stars he could see sixth magnitude stars That is, the brighter the star the smaller its magnitude. With telescopes we can now “see” stars much dimmer than sixth magnitude; very dim stars with magnitudes near 25 can now be measured.

Since the stars in the sky are fairly randomly arrayed, it seems to be a rather difficult task to learn where certain stars are. The star Vega, for example, will be in the northwest on December nights, nearly overhead on August nights, and in the northeast on April nights. The process of learning the stars is made possible by the discovery that, while stars move around in the sky, they keep the same relationship to each other year after year. Thus the star Vega can be identified because it is the bright star that has a dim parallelogram of stars within 10° of it. The parallelogram of stars follows Vega around where ever it goes. We can then invent patterns of neighboring stars by imagining certain (bright) stars to be “joined”. The joined figures (stick figures of constellations) can then be learned separately from how one figure is positioned relative to another.

Here we have (from top to bottom), the stick figures for the Big Dipper, Draco, the Little Dipper, Cepheus, and Cassiopeia. Can you find the Big Dipper in the first picture? Did you notice it before you saw the stick figure?

Since different people see different stick figures in the sky, astronomers have developed a standard set of sky regions: the 88 official constellations cover the entire celestial sphere. The stick figures typically have the same sort of relationship to the official constellation as a highway map has with a country’s border. Just as a small town, while certainly part of some country, may not be on any highway map, so a dim star may not be a vertex of any stick figure, but it is certainly part of some constellation.

[yfsum]

http://www.physics.csbsju.edu/astro/CS/CS.02.html

Term: celestial pole
The stars keep the same position relative to each other. For example, the stars forming the lip of the Big Dipper always point toward the tail star of the Little Dipper (which is the star Polaris). Thus we can consider the stars as attached to the dome of stars (celestial sphere). The fact that a star is not always in the same position in the sky tells us that the celestial sphere is not fixed overhead. It is moving. The motion of the celestial sphere in the sky is particularly simple. The fact that the motion is as simple as possible, was an attractive fact to the ancient Greek philosophers: it demonstrated design deserving an explanation.
Lets see if you can discover how the stars seem to move. Below find four pictures of the sky facing North, taken at 9 p.m., 12 midnight, 3 a.m., and 6 a.m. How are they related?

[yfsum]

http://www.physics.csbsju.edu/astro/CS/CS.03.html

Term: celestial pole, zenith, meridian
Did you notice that the tail of the Little Dipper (the star Polaris) remained fixed while the other stars just rotated around it? If not, go back and check.
What fraction of a rotation did the sky undergo in the 9 hours between 9 p.m. and 6 a.m.? go back and check!
The points of rotation are called celestial poles. There is one at the north end of the celestial sphere (near the star Polaris) and one at the south end (not near anything in particular).

The below picture shows where the north celestial pole is located in our sky. Here at CSB/SJU the north celestial pole sits about 45° directly above the north point on the horizon. The line that starts at the north point, goes through the north celestial pole through the point directly overhead and back to the south point on the horizon is called the meridian. We’ll also need a name for “the point directly overhead”; it’s called the zenith.

[yfsum]

http://www.physics.csbsju.edu/astro/CS/CS.04.html

Term: altitude, vertical circle
Here at CSB/SJU the north celestial pole sits about 45° directly above the north point on the horizon. Thus the altitude of the north celestial pole is about 45°. Altitude describes the angle between the nearest part of the horizon and a star. Zenith has an altitude of 90° above every part of the horizon. The “line” (really part of a great circle) that goes straight down from the star to the horizon, and straight up through zenith and back down to the other horizon, is called a vertical circle. Vertical circles rise perpendicular to the horizon. Technically altitude is the angle along the vertical circle between the horizon and the star.

[Emma]

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