To explain the celestial system, lets first suppose that the earth is a glass sphere, with meridians and parallels traced in black and a light placed at the center. Suppose, too, that this sphere is placed at the center of another infinitely larger sphere, as shown in figure 15-3. This larger sphere is the imaginary celestial sphere on which all the heavenly bodies are presumed to be located.

The celestial sphere is a mathematical concept of a sphere of infinite radius whose center is at the center of the earth The points at which the earths prolonged axis of rotation pierces the celestial sphere are known as the celestial poles. The plane of the earths equator, extended to the celestial sphere, coincides with the celestial equator. Great circles through the celestial poles, comparable to the earths meridians, are called hour circles. The angle between hour circles is the hour angle. Even though the earth rotates and the stars appear stationary among themselves, it is easier to think of the earth as being stationary, while the celestial sphere, with the celestial bodies attached, rotates from east to west, This is actually its apparent motion. When reference is made to a stars path or motion, it is this apparent motion that is referred to.

DECLINATION. Similar to latitude, the declination of a celestial body (star, sun, or planet) is its angular distance north or south of the celestial equator. As with latitude, declination is expressed in degrees and is measured horn 0 to 90 north or south from the celestial equator. North and south declination values are given plus and minus signs, respectively. The conventional symbol for declination is the Greek letter d (delta).

RIGHT ASCENSION. The vernal equinox, also known as the first point of Aries, is an imaginary point on the celestial sphere where the ecliptic (or apparent path of the sun) crosses the equator from south to north on or about 21 March of each year. The vernal equinox moves westward along the equator about 50 seconds of arc per year. The right ascension of the sun or any star is the angular distance measured eastward along the celestial equator between the vernal equinox and the hour circle passing through the celestial body. Right ascension is normally expressed in units of time from 0 to 24 hours, although it can be expressed in degrees with 1 hour of time corresponding to 15. The conventional symbol for right ascension is the Greek letter a (alpha), or it can be abbreviated RA.

HOUR ANGLE. Right ascension and declination are independent coordinates of the celestial system, whereas the hour angle is a dependent coordinate. Hour angle is the angle between celestial meridians, or hour circles; but its origin is the meridian that passes through the observers zenith (or point on the celestial sphere directly above the observer). The hour angle of a star is defined as the angular distance, measured westward along the celestial equator, between the observers meridian and the hour circle or meridian of the star. This angle is often called the local hour angle (LHA), which will be discussed later.

GREENWICH HOUR ANGLE. The coordi-nate for a heavenly body that corresponds to longitude is called the Greenwich hour angle (GHA). The Greenwich hour angle is the angular distance from the Greenwich meridian to the meridian of the heavenly body. It is always measured westward from the Greenwich meridian and is expressed in degrees from 0 to 360. Another point to remember is that, while the longitude of a point on the earth always remains the same, the GHA of the celestial object is constantly increasing as the body moves westward on the celestial sphere.

Horizon System of Coordinates

To connect the celestial and terrestrial coordinates, you must have a third system, descriptive of the observers position. The fundamental reference of this

Figure 15-3.Terrestrial and celestial coordinate system.

Figure 15-4.-Horizon system of coordinates.

system is the observers horizon. Figure 15-4 illustrates the horizon system. In this figure, O represents both the earth and the location of the observer.

The horizon is a plane through the observers position that is perpendicular to the direction of gravity at that point and that intercepts the celestial sphere in a great circle. The direction of gravity, commonly called the direction of the plumb line, does not necessarily pass through the earths center. The horizon plane is considered tangent to the surface of the earth at the observers position For most star observations, the distance from this plane to the center of the earth is too small to affect the computations. However, observations of the sun, planets, and some of the nearer stars, when used in the more precise computations, must account for the displacement of the horizon plane. This is called the correction for parallax.

The point where the plumb line, extended overhead, pierces the celestial sphere is known as the zenith. The point opposite this and underneath is the nadir. Great circles drawn through the zenith and nadir (with their planes perpendicular to that of the horizon) are called vertical circles. The angular distance of a celestial body measured along a vertical circle from the horizon is the altitude (h) of the body. The complement of the altitude is the coaltitude, or zenith distance, and is measured along the vertical circle from the zenith to the body.

The vertical circle through the poles, which also passes through the zenith, is called the observers meridian. The azimuth of an object is the angle measured clockwise in the plane of the horizon from the observers meridian to the vertical circle passing through the object. The northern intersection of the meridian with the horizon is used as the zero azimuth point. Azimuth is measured in degrees from 0 to 360. The conventional symbol for azimuth is the letter A or Z