chapter 2 celestial motions. fig. 2.1 the celestial sphere to understand the idea of the celestial...

Chapter 2

Celestial Motions

Fig. 2.1

              The Celestial SphereThe 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.

• 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.

• Right ascension, the equivalent of longitude on earth, is measured hours  from the zero line and each hour corresponds to 15 degrees.

• 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.

• 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.

• 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.

• At an intermediate latitude, the altitude of the celestial pole on the sky is equal to your latitude on earth.

• Some stars are circumpolar and  never set.

Fig. 2.2

Fig. 2.3

Fig. 2.4

Fig. 2.5

Fig. 2.6

Fig. 2.7

Fig. 2.8

Star trails at the earth’s equator

The Ecliptic and the Zodiac Constellations

Fig. 2.9

Fig. 2.10

Fig. 2.11

Fig. 2.12

Fig. 2.13

Fig. 2.14

Fig. 2.15

Fig. 2.16

Solar day and sidereal day

Fig. 2.17

Phases of the moon

Fig. 2.19

Fig. 2.21

Fig. 2.22

The motion of the moon

Fig. 2.23

Fig. 2.25

Fig. 2.26

Retrograde motion of planets

Fig. 2.27

Fig. 2.28