DK Eyewitness - Astronomy

Where is the pole star? Pole Star
To find a town on Earth, a map
is used. To find a star in the night
sky, astronomers need to use the
celestial coordinates. The Pole Star
is one useful marker in the northern
hemisphere because it indicates the
northern celestial pole. Since the
north–south axes of both Earth and
the sky run at right angles to the
terrestrial and celestial equators,
which are measured as 0°, the Pole
Star is measured as 90° North. An
observer looking at the Pole Star Arctic circle
near the Arctic Circle sees it very
high in the sky; near the equator,
the Pole Star barely rises above Pole Star
the horizon. In the South Pacific,
it is never seen at all.
Great Bear
Pole Star
80° Latitude (Greenland) 30° Latitude (Egypt) 0° Latitude (at the equator)
Celestial sphere
Pole
Star
Tropic of Cancer
North/South axis
North Pole
A static Earth,
surrounded by the
crystalline sphere
of the fixed stars
Sun
Saturn
Tropic of
Cancer
Terrestrial
equator
Celestial
equator
Ecliptic
Tropic of
Capricorn
South Pole
To a distant star
Sun
Noon
on first
day
Measuring time
With solar time, one day equals the time it takes Earth to make one
full rotation on its axis, from noon to noon. But because Earth is also
orbiting the Sun as well as spinning, the solar day is
Second noon
for sidereal time
Second noon for
solar time
not accurate in relation to distant stars, and it is the
stars that concern astronomers. They measure time
in relation to a distant star. This “day” is the time that
passes between two successive “noons” of a star, noon
being the moment when that star passes directly over
the local meridian (p.27). This is called a sidereal day.
Antarctic
circle
The celestial sphere
This model of the celestial sphere
records how the ancients viewed the
universe. All the planets seemed to
travel along the same band as the
Sun. Since eclipses happened along
this path, it was called the ecliptic.
The ecliptic seemed to run at an angle
of 23½° from the plane of Earth’s
equator. When the Sun passed along
the ecliptic, it turned back as it passed
through the signs of Cancer in the
north and Capricorn in the south.
These points where the Sun turned in
its path were called tropics.
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