Astronomical Unit

space · measurement · planets · distance

Gigantic measurement. An astronomical unit (or AU) is about 150 million kilometers long. That's the distance light takes roughly 8 minutes to travel from the Sun to Earth.

Earth to Sun distance. Scientists use the AU to measure how far planets orbit from the Sun. Mercury is about 0.4 AU away, while Neptune is about 30 AU away.

Official definition set. In 2012, scientists officially decided that 1 AU equals exactly 149,597,870,700 meters. Before that, they measured it as the average distance from Earth to the Sun.

Comparing space distances. Instead of using huge numbers like millions of kilometers, astronomers use AU to make comparisons easier. It's like using your hand width instead of centimeters to compare sizes.

Solar system ruler. The AU helps us understand our entire solar system on a simple scale. It shows why planets take much longer to orbit the farther they are from the Sun.