Light-years and astronomical units

Space is too big for kilometres. Inside the solar system astronomers use the astronomical unit, the Earth's average distance from the Sun, about 150 million km. For the stars they use the light-year: the distance light, at about 300,000 km a second, travels in a year, about 9.5 million million km. A light-year measures distance, not time. Because light takes time to arrive, we see things as they were: the Sun about 8 minutes ago, the nearest other star, Proxima Centauri, about 4.2 years ago. Our galaxy, the Milky Way, is about 100,000 light-years across, and the universe holds billions of galaxies.

Depth 1 · Foundation

You know what an astronomical unit and a light-year are, that a light-year is a distance and not a time, and that because light takes time to travel we see the Sun and stars as they were in the past; and you can give the rough size of the Milky Way.

Depths of understanding

1
Foundation
You know what an astronomical unit and a light-year are, that a light-year is a distance and not a time, and that because light takes time to travel we see the Sun and stars as they were in the past; and you can give the rough size of the Milky Way.
Opens after
GalaxiesLight-years are the ruler for the distances between the stars of a galaxy.The size of the solar systemThe astronomical unit and the light-year go on from the scale of the solar system.

Your path here

What's still between you and Light-years and astronomical units, depth 1, in the order you can learn it.

Where it fits

Builds on

Galaxies The size of the solar systemThe Solar System

This concept

Light-years and astronomical unitsStars

Builds onThis conceptGalaxies The size of the solar systemThe Solar System Light-years and astronomical unitsStars