Page 3, Introduction: [Previous Page] [Next Page] [Class Home Page]

Units of Measurement used in Astronomy

Units of Length (Distance) in Astronomy
The basic unit of length is the meter (metric system), equal to one ten-millionth the distance from the Earth's North Pole to the Equator, or about 39 1/3 inches.

The average Earth-Sun Distance: 1.5 x 10¹¹ meters = 150 million km = 93 million miles. This distance is defined as the astronomical unit (AU), and is used for measuring distances between planets.

For interstellar distances, we use the light year, the distance traveled by light during the course of a year. One light year (ly) = 9.5 x 10¹⁵ m, or 63,000 AU. Proxima Centauri, the nearest star to the Sun, is 4.2 ly = 260,000 AU. Distances between stars are much greater than the size of the Solar System.

---

Units of TIME used in Astronomy
Our method of measuring time (60 seconds per minute, 60 minutes per hour, 24 hours per day) goes back to the ancient Babylonians, who were fond of the numbers 6, 12, and 60.

The other frequently used unit of time is the year, the time it takes the Earth to revolve once around the Sun. A year is approximately 365.25 days (hence the need for leap years), or 31.6 million (3.16 x 10⁷) seconds.

---

Units of MASS used in Astronomy
The basic unit of mass in the metric system is the kilogram, equal to the mass of water contained within a cube 0.1 meters on a side. PLEASE NOTE: mass and weight are not the same thing.

Mass: amount of matter present
Weight: force with which gravity tugs on matter.

A one-liter bottle of water has a mass of 1 kg, ALWAYS. At the surface of the Earth, gravity will create a force of 2.2 lbs. On the Moon, however, where gravity is smaller, a 1-kg mass has a weight of 0.37 lbs. The same 1-kg mass has a weight of 0.8 lbs on Mars and 5.6 lbs in the cloud layer of Jupiter.