Email: nexusmail at this Web site address
If you've done a lot of astronomical observing or calculations, you're familiar with the equatorial coordinate system of right ascension and declination (if not, go to the Introduction to Astronomical Terms page for an explanation). Though this is probably the most widely used coordinate system in astronomy, it has its limitations. Since it's closely related to Earth-based latitude and longitude, it works best from the Earth. 3D star mappers and astrometry buffs often work from vantage points many light years away, so equatorial coordinates don't make much sense as a general system. For example, suppose you want to calculate the positions of a bunch of stars as seen from a very distant location (say, the Pleiades). There is no simple way to do this using only equatorial coordinates. Some other coordinate system is needed.
For our purposes, the most flexible coordinate system is Cartesian coordinates, which are simply the components of the star's position along three perpendicular axes (x,y, and z). Cartesian coordinates have two big advantages over most other systems:
With this in mind, let's define the three axes needed for our coordinate system:
The origin -- the zero point -- of this coordinate system is at the Sun.
Though the locations of these axes are defined from certain values of and , they point to specific, fixed regions in space that are independent of the Earth. With this in mind, we don't need to worry about the Earth anymore, and can locate stars in this coordinate system anywhere in space.
If you know the equatorial coordinates (, , d) of a star, you can get its Cartesian coordinates (x,y,z) by three simple formulas.
Remember, since right ascension is normally given in hours, you must multiply it by 15 to get degrees before using these formulas.
The x,y,z coordinates are distances, with respect to the origin (i.e., Earth) and have the same units as the original distance, d.
The closest star to the Sun is Proxima Centauri, a faint red star only about 0.01% as bright as the Sun. Its coordinates are:
First, convert to degrees: 14.4966 * 15 = 217.45 degrees. Then:
which results in: