The Astronomy Unit

Astronomers use many of the same units of measurement as other scientists — meters for length, kilograms for mass, and seconds for time. But because the scale of the universe is so large, astronomers have had to invent some new units of measure. One of these is the astronomical unit, or au. An astronomical unit is the average distance of Earth from the Sun. It is equal to 149,597,870,700 m exactly, or about 150 million kilometers (93 million miles).

The astronomical unit has a specific significance for astronomers because it is used in the calculation of orbits and trajectories of planets and satellites within our solar system. The value of the astronomical unit has been determined by radar-ranging studies of the Moon and the planets. These measurements are made by measuring the time it takes for a radio wave to reach a planet, and then to return. The distance between the planet and Earth can then be calculated from the planetary coordinates in the spacecraft’s “ephemerides.”

Other units of measure used by astronomers include the light year and the parsec. A light year is the distance that light travels in a vacuum in one Julian year (or, on the alternative definition, 3.26 light years). A light-year is generally the unit of length used to express distances to nearby stars in non-specialist and popular science publications. Astronomers also use kilo-parsecs and megaparsecs, which are multiples of the light-year, to express larger distances in the solar system and galaxies.

Another astronomical unit is the Jupiter mass, a standard way of expressing the mass of gas giants such as Jupiter and Saturn. The mass of Jupiter is equal to 1.898×1024 kg. Astronomers sometimes refer to masses of brown dwarfs and Neptune-mass planets using the Jupiter mass, as well.

The dimensions of the solar system and the value of the astronomical unit were first established in the late 19th century by a combination of measurements: transits of Venus across the face of the Sun; laser ranging on the surface of the Moon, based on the reflection of sunlight off the surface by lunar reflectors placed there by Apollo astronauts; and the measurement of the tiny shift in position that nearby stars exhibit as they move relative to each other (a phenomenon known as parallax).

The astronomical unit is still the main unit of distance for describing the size and motion of objects within our solar system. However, astronomers often describe distances to other stars in terms of light years and parsecs, and the distances of distant galaxies in terms of their redshift. All of these units of measurement are defined differently, but all are useful to astronomers.