4.c Distances between astronomical objects are enormous. Measurement units such as centimeters, meters, and kilometers used in the laboratory or on field trips are not useful for expressing those distances. Consequently, astronomers use other units to describe large distances. The astronomical unit (AU) is defined to be equal to the average distance from Earth to the Sun: 1 AU = 1.496 × 1011 meters. Distances between planets of the solar system are usually expressed in AU. For distances between stars and galaxies, even that large unit of length is not sufficient. Interstellar and intergalactic distances are expressed in terms of how far light travels in one year, the light year (ly): 1 ly = 9.462 × 1015 meters, or approximately 6 trillion miles. The most distant objects observed in the universe are estimated to be 10 to 15 billion light years from the solar system. Teachers need to help students become familiar with AUs by expressing the distance from the Sun to the planets in AUs instead of meters or miles. A good way to become familiar with the relative distances of the planets from the Sun is to lay out the solar system to scale on a length of cash register tape.

### Distances Within the Solar System

The most common unit of measurement for distances within the solar system is the astronomical unit (AU). The AU is based on the mean distance from the sun to Earth, roughly 150,000,000 km. JPL's Deep Space Network refined the precise value of the AU in the 1960s by obtaining radar echoes from Venus. This measurement was important since spacecraft navigation depends on accurate knowledge of the AU. Another way to indicate distances within the solar system is terms of light time, which is the distance light travels in a unit of time. Distances within the solar system, while vast compared to our travels on Earth's surface, are comparatively small-scale in astronomical terms. For reference, Proxima Centauri, the nearest star at about 4 light years away, is over 265,000 AU from the sun.

Light Time Approximate Distance Example
3 seconds 900,000 km ~Earth-Moon Round Trip
3 minutes 54,000,000 km ~Sun to Mercury
8.3 minutes 149,600,000 km Sun to Earth (1 AU)
1 hour 1,000,000,000 km ~1.5 x Sun-Jupiter Distance
12.5 hours 90 AU Voyager-1 (January, 2004)
1 year 63,000 AU Light Year
4 years 252,000 AU ~Next closest star

Temperatures Within the Solar System

Select the above link for the Solar System Temperature Reference showing examples and comparing temperatures of objects and conditions from absolute zero through planet temperatures, to those of stars. The reference also includes temperature conversion factors and links to a conversion engine.