The parallax of a star is the angle at the star subtended by the radius of the earth's orbit, 92,900,000 miles, or the astronomical unit. It is, in other words, the angular distance between the earth and sun as viewed from the star. The larger the parallax the nearer the star. The largest known stellar parallax is that of Alpha Centauri and its value is 0".75.
The light-year is the distance that light travels in one year. It is equal to about 63,000 astronomical units or nearly six trillion (6,000,000,000,000) miles. The velocity of light is 186,000 miles per second.
The parsec is equal to 3.26 light-years. It is the distance of a star that has a parallax of one second of arc.
The apparent magnitude of a star is its apparent brightness estimated on a scale in which a difference of one magnitude corresponds to a difference in brightness of 2.51, or the fifth root of one hundred. A difference of five magnitudes corresponds to a difference one one hundredfold in brightness, of ten magnitudes to ten thousandfold in brightness. In exact measurements on this scale magnitudes are estimated to tenths.
Stars that are one magnitude brighter than stars of the standard first magnitude are of the zero magnitude and stars still brighter are of negative magnitudes.
Sirius is a star of the -1.6 magnitude. Jupiter at opposition is of -2.0 magnitude and Venus at greatest brilliancy of -4.0 magnitude. The sun on this scale of comparative brightness is of the -26.7 apparent magnitude. The faintest stars visible in the most powerful telescope in the world—the 101-inch Mt. Wilson Hooker telescope—are of the twentieth magnitude.
The absolute magnitude of a star is its apparent magnitude at the standard distance of ten parsecs or 32.6 light years. The absolute magnitude of the sun is five. That is, the sun would be a fifth-magnitude star at the standard distance of 32.6 light-years. The absolute magnitudes of stars indicate how bright they would be relatively if they were all at the same standard distance. Apparent magnitudes indicate how bright the stars appear to be at their true distances.
The mean distance of the moon from the earth is approximately 240,000 miles or sixty times the earth's radius.
The sun is four hundred times farther away than the moon and its diameter is about four hundred times greater than the moon's diameter.