2.Understanding Celestial Motion and Coordinates
Understanding where to locate celestial objects and how these objects move through the sky is fundamental to fully appreciating astronomy as a hobby. Most amateur astronomers use the same visual path (or
Celestial North Pole (close to the North Star (Polaris))
Earth's rotation
Right ascension
Celestial South Pole
Figure 2: Celestial Sphere
Understanding how astronomic objects move
Given the earth's rotation, celestial bodies (stars) appear to move from east to west along a curved trajectory across the sky. The trajectory that they follow is known as the right ascension line (R.A.). The angle of the trajectory that they follow is known as the declination line (Dec.). The right ascension and the declination form a system that is similar to
the terrestrial system of latitude and longitude.
In the system of R.A. and Dec. coordinates, stars are projected onto the "celestial sphere", i.e. onto the imaginary sphere where all of the stars appear to be located.
Understanding celestial coordinates
In the system of celestial coordinates, the poles are defined as being the two points where the earth's rotation axis, when prolonged infinitely to the north and to the south, intersects with the celestial sphere. Consequently, the celestial North Pole is the point in the sky where the prolongation of the earth's axis passing through the North Pole intersects the celestial sphere. In fact this point in the sky is located close to the North Star or pole star (Polaris).
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The celestial equivalent to terrestrial latitude is called "Declination" or simply "Dec", expressed in degrees, minutes, and seconds north (“+”) or south
celestial equator.
Consequently any point located along the celestial equator (e.g.
passing through the constellations of Orion, Virgo and Aquarius) is specified by its declination of 0º0’0”. The declination of the North Star or pole star located very close to the North Celestial Pole is +89.2º.
The celestial equivalent to terrestrial longitude is called "Right Ascension" or "R.A.", expressed in hours, minutes and seconds from a "zero" R.A. line defined arbitrarily and which passes through the
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