26
M
Mo
ot
ti
io
on
no
of
ft
th
he
eS
St
ta
ar
rs
s
Thedaily motion of the Sun across the sky is familiar to even the most casual observer. Thisdail y trekis not the Sun
movingas early astronomers thought, but the result of the Earth's rotation. TheEarth's rotation also causes the stars to
do thesame, scribing out a large circle as the Earth completes one rotation. Thesize of the circular path a star follows
dependson where it is in the sky. Starsnear the celestial equator form the largest circles rising in the east and setting in
thewest. Movingtoward the north celestial pole, the point around which the stars in the northernh emisphereappear to
rotate, these circlesbecome smaller. Stars in the mid-celestial latitudes rise in the northeast and set in the northwest.
Starsat h igh celestial latitudes are always above the horizon, and are said to be circu mpolar because they never rise and
nevers et. Youwill never see the stars complete one circl e because the sunlight during the day washes out the starlight.
However, part of this circularmotion of stars in this region of the sky can be seen by setting up a camera on a tripod
and opening the shutterfor a couple hours. The processed film will reveal semicircles that revolve around the pole.
(Thisdescription of stellar motions also applies to the southern hemisphereexcept all stars south of the celestial equator
movearound the south celestial pole.)
Figure6-2
Allstars appear to rota te around the celestial poles. However,the appearance of this motion
variesdepending on where you arelooking in the sky. Nearthe northcelestial pole the stars
scribeout recognizable circlescentered on the pole (1). Starsnearthe celestial equator also
followcircular pa ths around the pole. But,the complete path is interrupted by the horizon.
Theseappear to rise in the east and set in the west (2). Looking toward the opposite pole, stars
curveor arc in the opposite direction scribinga circlearoundthe opposite pole (3).