The Mathematics of the Longitude - Department of Mathematics

(RA) and is measured in hours, minutes, and seconds rather than in the
more familiar degrees, minutes and seconds. (There are 360 degrees or 24
hours in a full circle.) The right ascension is always measured eastward
from the vernal equinox.
Next measure along the body's hour circle and the angle between the
celestial equator and the position of the body. This angle is called the
declination (Dec) of the body and is measured in degrees, minutes and
seconds north or south of the celestial equator, analogous to latitude on the
earth (Figure 2.7).
Right ascension and declination together determine the location of a body
on the celestial sphere.
Figure 2.7 shows the Dec and RA of a celestial body.
2.5 Geographical Position of a Celestial Body
Now consider a line connecting the center of a celestial body and the center
of the earth. The point where this line crosses the surface of the earth is
called the geographical position (GP) of the body (Figure 2.8). An