The Celestial Sphere - Right Ascension and Declination
- Introduction to Astronomy
- The Celestial Sphere - Right Ascension and Declination
- What is Angular Size?
- What is the Milky Way?
- The Magnitude Scale
- Sidereal Time, Civil Time and Solar Time
- Parallax, Distance and Parsecs
- Apparent Magnitude, Absolute Magnitude and Distance
- Variable Stars
- Spectroscopy and Spectrometry
- Redshift and Blueshift
- Spectral Classification of Stars
- Hertzsprung-Russell Diagram
- Kepler's Laws of Planetary Motion
- The Lagrange Points
- What is an Exoplanet?
- Glossary of Astronomy & Photographic Terms
The celestial sphere is a projected sphere centred on the Earth that we can imagine all the stars are painted on. As the Earth rotates on its axis, this gives the impression of the sphere continually rotating overhead and creates the effect of stars rising in the East and setting in the West.
In order to identify the position of an object on the celestial sphere we need a coordinate system that can cope with an objects position on the sphere at a given time from a given location. Imagine that you are stood on the North Pole and when you look directly up you will see the Moon. Now imagine another observer stood on the Equator, where would they see the Moon? It certainly won't be overhead; instead it will be on the horizon. In the same manner an objects position will appear to change if one observer was in Europe and another in America, however over time the object will move into the position observed as the Earth rotates.
For these reasons the celestial coordinate system is based on the latitude and longitude system used on the surface of Earth. The coordinate system consists of two figures, Right Ascension (RA) and Declination (Dec). Right Ascension is a measure of an objects position from the First Point of Aries and can be thought of as the celestial sphere equivalent of longitude, while Declination is a measure of the position relative to the celestial equator and is similar to latitude. The celestial equator is a projection of the Earth's equator onto the celestial sphere.
In the diagram above, declination is marked on the blue lines and is analogous to latitude as it measures the angular distance in degrees from the equator, from 0° at the equator to +90° at the North Pole and -90° at the South Pole.
First Point of Aries and Right Ascension
Right Ascension is analogous longitude in that it measures angular distance around the equator, however this is where the comparison ends. The First Point of Aries acts as the zero point for the Right Ascension scale instead of starting from the Greenwich meridian like longitude does.
As the Earth orbits the Sun, we see that the Suns position changes with respect to the background stars over the period of a year. If you were able to the position of the Sun compared with fixed background stars, the Sun would appear to move in a large ellipse on the surface of the celestial sphere. This ellipse is imaginatively called the elliptic. As the Sun moves along the elliptic, it crosses the celestial equator twice per year - in March we know it as the Vernal Equinox and in September the Autumnal Equinox. The First Point of Aries is the point in the sky where the Celestial Meridian, the Celestial Equator and the Ecliptic all meet. It is presently in the southwest of Pisces, moving slowly towards Aquarius.
When this point was first conceived the point was located within the constellation Aries, hence its name, however over time this position has moved westwards due to the precession of the equinoxes.
Right Ascension is measured in hours, minutes and seconds from the local sidereal time.