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North celestial pole

Earth

Celestial
equator

South celestial pole

FIGURE 14.3 The celestial sphere with the celestial equator
A
directly above Earth’s equator and the celestial poles directly above
Earth’s poles.

Midnight
75° 90°
60°
45° Celestial meridian
30°
6:00 A.M. 6:00 P.M.
North 15° Horizon
Star
Observer
180° 0° north
south
B
45°E
azimuth 30° altitude
FIGURE 14.1 Ancient civilizations used celestial cycles of from due south
motion as clocks and calendars. (A) This photograph shows the path
of stars around the North Star. (B) A “snapshot” of the position of the FIGURE 14.4 Once you have established the celestial equator,
Big Dipper over a period of 24 hours as it turns around the North Star the celestial poles, and the celestial meridian, you can use a
one night. This shows how the Big Dipper can be used to help you two-coordinate horizon system to locate positions in the sky. One
keep track of time. popular method of using this system identifies the altitude angle
(in degrees) from the horizon up to an object on the celestial
sphere and the azimuth angle (again in degrees) of the object on
the celestial sphere is east or west of due south, where the celestial
meridian meets the horizon. The illustration shows an altitude of
30° and an azimuth of 45° east of due south.

You can only see one-half of the overall celestial sphere
from any one place on the surface of Earth. Imagine a point
on the celestial sphere directly above where you are located.
An imaginary line that passes through this point, then passes
north through the north celestial pole, continuing all the way
around through the south celestial pole and back to the point
directly above you makes a big circle called the celestial merid-
ian (Figure 14.4). Note that the celestial meridian location is
determined by where you are on Earth. The celestial equator
FIGURE 14.2 The stone pillars of Stonehenge were positioned
so they could be used to follow the movement of the Sun and Moon and the celestial poles, on the other hand, are always in the same
with the seasons of the year. place no matter where you are.

14-3 CHAPTER 14 The Universe 353