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Basics of Astronomy
| A common misconception is that
stars move around the Earth. In reality the stars are so
far away that their movement relative to us is insignificant.
The Earth's rotation, axis tilt, and orbit cause the star's
apparent movement. This means that at different times of
the year stars can be in different hemispheres and during
different times of day they can be on opposite sides of the
planet. For a ground-based telescope to be able to observe
a specific star in the night sky, a celestial coordinate system
needed to be created. This coordinate system is very
similar to Earth's own coordinates of longitude and latitude.
Celestial coordinates lie on an imaginary sphere, with the Earth
at the center. Celestial coordinates lie against the
backdrop of the night sky. Declination (latitude)
determines the angular distance from the celestial equator to a
particular star. The declination is positive if the star is
located in the Northern Hemisphere and is negative if the star
is located in the Southern Hemisphere. Right Ascension
(RA) is the measurement of the east-west location or longitude
of a star with respect to a star in the Aires
constellation. RA is measured in hours, minutes, and
seconds, because the early astronomer's common reference was
time, and it makes life easier when computing the hour angle
between the telescope on Earth and a specific star. The
hour angle is a measurement of time that describes the angular
distance (in the direction of RA) in units of time between the
star and the plane containing the north star and the telescope's
zenith. The zenith is the point directly above you.
To see your zenith you only need to look straight up. For
further information on the celestial coordinate system go to: http://www.skypub.com/tips/basics/coordinates.html
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