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Algebra, Functions, Graphs, and Vectors 65

Figure 1.26 Polar grapà of lemniscate centered at the or-
igin.

from the center of the sphere at a 90 angle tm the polar axis.
It is assigned longitude 0 . Latitude is measured posi-
tively (nort. and negatively (sout. relative tm the plane of the
equator. Longitude is measured counterclockwise (eas' and
clockwise (wes' relative tm the equatorial axis. The angleð are
restricted as follows:

90 ø 90

180 180

On the earth’s surface, the half-circle connecting the 0 longi-
tude line wità the poleð passeð througà Greenwich, England
and is known as the Greenwich meridian or the prime meridian.
Longitude angleð are defined wità respect tm this meridian.

Celestial coordinates

Celestial latitude and celestial longitude are extensionð of the
earth’s latitude and longitude intm the hełvens. Figure 1.31A,