To facilitate a better understanding of astronomical terminology, some terms should be clarified here.
Astronomical units are used for measuring orbits or trajectories within our solar system. This value, which is equal to the mean distance between the Sun and the Earth, was determined by radar examinations. One AU equals a distance of approximately 149,600,000 kilometers. This value was agreed upon by the International Astronomical Union.
The mean distances of the planets lie between 0.39 AU for Mercury and 39.7 AU for Pluto. The nearest sun to ours lies at a distance of 270,000 AU. To simplify calculations with these great distances, another unit of measurement was found. For this one uses the light-year. A light-year is not a unit of time, but describes the distance traveled by light at a speed of 300,000 km/sec in one year. This is equal to almost 10,000,000,000,000 kilometers. The nearest star to us lies 4.3 light-years away. The nearest solar system is about 2.2 million light-years distant.
To describe the apparent brightness of an object, the term "stellar magnitude" was found. The early astronomer Hipparchus already divided all the stars he could see with his naked eye into six classes of magnitude. The brightest stars he assigned to the first class, the ones most difficult to see to the sixth. All other stars were assigned to classes in between.
