Glossary of Astronomical Terms/astronomical unit
From Wikibooks, the open-content textbooks collection
The astronomical unit (AU) is the unit of distance where the Gaussian Gravitational Constant takes on the value k = 0.01720209895 AU3/2S-1/2D-1, where S is the mass of the Sun and D is a mean solar day (86,400 seconds). It is roughly 149,597,870.691 km (92,955,807.267 mi). The AU used to be defined as the average distance of the Earth from the sun - under the new definition, the average distance of the Earth from the Sun is very slightly more than 1AU. For estimating purposes, 93 million miles is often the figure used. These distances have been recently affirmed by radar ranging techniques.
The AU is most often used for expressing distances within the solar system. There are approximately 63,240 AUs in one light year, thus for larger distances, the light year is more generally useful.
[edit] Constructing a Scale Model
The most distant planet, Pluto, is approximately 39 AUs from the sun. Thus, if we construct a scale model of the solar system in which the earth is placed ten feet from the sun, Pluto would be placed 380 feet beyond earth, more than the length of a football field. Comets often swing as far as 50,000 AUs from the sun. In our model, this would mean that at their farthest point they would be placed 94.7 miles away.
- Question for students: How far away in our model would we need to place the nearest star (Proxima Centauri, one of three stars in the Alpha Centauri system) which is approximately 4.3 light years away? (Hint: all of the data you need is contained within this article.)
