Chapter 2 Celestial Motions

Fig. 2.1

The Celestial Sphere To understand the idea of the celestial sphere first think of the earth in space. The stars all around us look as if they could be on the inside of a huge distant sphere. Even though the stars are really at different distances this idea of the celestial sphere is very useful for getting positions on the sky. Now imagine you are close to the north pole. Here it is like being on a rowing boat, because the earth is spinning on its axis once a day. Every day, each star circles all around you and around the point overhead, which is the celestial north pole. There is of course a celestial south pole as well and the celestial equator circles the sky half way between the poles.north polecelestial south pole Positions on the celestial sphere are specified by two coordinates. Declination, like latitude on earth is measured in degrees north or south of the celestial equator. Positive declinations are north of the celestial equator, negative declinations are south of the equator. Declination Right ascension, the equivalent of longitude on earth, is measured hours from the zero line and each hour corresponds to 15 degrees.Right ascension Because you are on the earth's surface, half of the sky is not visible. The earth gets into your way and creates your horizon. In effect your personal sky is like a hemispherical dome.hemispherical dome Imagine now you move to the earth's equator. The celestial equator runs from east to west through the zenith, the overhead point. The celestial poles are your north and south on the horizon. The north-south line through the zenith is called the meridian.earth's equator It is sometimes useful to know some star's altitude its height above the horizon and its azimuth, its compass direction. Azimuth is measured across the horizon from north. As the stars move their altitude and azimuth constantly change.constantly change At an intermediate latitude, the altitude of the celestial pole on the sky is equal to your latitude on earth.equa Some stars are circumpolar and never set.

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Fig. 2.8 Star trails at the earth’s equator

The Ecliptic and the Zodiac Constellations

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Solar day and sidereal day

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Phases of the moon

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The motion of the moon

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Retrograde motion of planets

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Fig. 2.28