below the equator. Latitudes are expressed in degreesand are measured from 0° to 90° north or south. Theconventional symbol for latitude used in computation isthe Greek letter @.As shown also in figure 15-2, the longitude of apoint is the angular distance measured along the equatorbetween the meridian passing through a point and areference meridian. The chosen reference meridian isthe Greenwich meridian that passes through Greenwich,England. That meridian is known as the primary orprime meridian. Longitude is also expressed in degreesbut is measured from 0° to 180° west or east from theprime meridian. The conventional symbol for longitudeis the Greek letter k (lambda).Celestial system ofCoordinatesTo explain the celestial system, let’s first supposethat the earth is a glass sphere, with meridians andparallels traced in black and a light placed at the center.Suppose, too, that this sphere is placed at the center ofanother infinitely larger sphere, as shown in figure 15-3.This larger sphere is the imaginary celestial sphere onwhich all the heavenly bodies are presumed to belocated.The celestial sphere is a mathematical concept of asphere of infinite radius whose center is at the center ofthe earth The points at which the earth’s prolonged axisof rotation pierces the celestial sphere are known as thecelestial poles. The plane of the earth’s equator,extended to the celestial sphere, coincides with thecelestial equator. Great circles through the celestialpoles, comparable to the earth’s meridians, are calledhour circles. The angle between hour circles is the hourangle. Even though the earth rotates and the stars appearstationary among themselves, it is easier to think of theearth as being stationary, while the celestial sphere, withthe celestial bodies attached, rotates from east to west,This is actually its apparent motion. When reference ismade to a star’s path or motion, it is this apparent motionthat is referred to.DECLINATION.— Similar to latitude, thedeclination of a celestial body (star, sun, or planet) isits angular distance north or south of the celestialequator. As with latitude, declination is expressed indegrees and is measured horn 0° to 90° north or southfrom the celestial equator. North and south declinationvalues are given plus and minus signs, respectively. Theconventional symbol for declination is the Greekletter 6 (delta).RIGHT ASCENSION.— The vernal equinox,also known as the first point of Aries, is an imaginarypoint on the celestial sphere where the ecliptic (orapparent path of the sun) crosses the equator fromsouth to north on or about 21 March of each year. Thevernal equinox moves westward along the equatorabout 50 seconds of arc per year. The right ascensionof the sun or any star is the angular distance measuredeastward along the celestial equator between thevernal equinox and the hour circle passing through thecelestial body. Right ascension is normally expressedin units of time from 0 to 24 hours, although it can beexpressed in degrees with 1 hour of timecorresponding to 15°. The conventional symbol forright ascension is the Greek letter a (alpha), or it canbe abbreviated RA.HOUR ANGLE.— Right ascension and declinationare independent coordinates of the celestial system,whereas the hour angle is a dependent coordinate. Hourangle is the angle between celestial meridians, or hourcircles; but its origin is the meridian that passes throughthe observer’s zenith (or point on the celestial spheredirectly above the observer). The hour angle of a star isdefined as the angular distance, measured westwardalong the celestial equator, between the observer’smeridian and the hour circle or meridian of the star. Thisangle is often called the local hour angle(LHA), whichwill be discussed later.GREENWICH HOUR ANGLE.— The coordi-nate for a heavenly body that corresponds to longitudeis called the Greenwich hour angle (GHA). TheGreenwich hour angle is the angular distance from theGreenwich meridian to the meridian of the heavenlybody. It is always measured westward from theGreenwich meridian and is expressed in degrees from0° to 360°. Another point to remember is that, while thelongitude of a point on the earth always remains thesame, the GHA of the celestial object is constantlyincreasing as the body moves westward on the celestialsphere.Horizon System ofCoordinatesTo connect the celestial and terrestrial coordinates,you must have a third system, descriptive of theobserver’s position. The fundamental reference of this15-4