Earth Science

Measuring Distances in Space
Astronomical Unit (AU) and Light-Year (LY)

	Key Points
	A Light-Year (LY) is a unit of distance. It is the distance light travels in one Earth year.
	An Astronomical Unit (AU) is the distance between the center of the Earth and the center of the Sun when they are at their greatest distance from each other.


	Teacher Blogs
	I try to reward students do silly things like The Happy Teacher Dance...or when I get a good answer or comment or reflection I’ll rush over and shake that student’s hand. I’m always telling kids not just that an answer was good but also why it was good. I have gone around the room during an entire class block and at different times I have told each and every kid in that class, “You are my favorite child.” Yes, every kid hears me tell every other kid – be very very VERY sure that you tell every single kid all in one class period. Mrs. Thomas 6^th Grade


	Tips From Other Parents
	When my kid comes home I make a snack and say, “Tell me about your homework.” I find a lot of times we can talk out the solutions to assignments before she even starts her homework. Ginny’s Mom


	Tutorial Video



	A light-year is a unit of distance. It is the distance that light can travel in one year. Light moves at a velocity (speed) of about 300,000 kilometers (km) each second. So in one year, it can travel about 10 trillion kilometers. More precisely, one light-year is equal to 9,500,000,000,000 kilometers.
	Why would you want such a big unit of distance? Well, on Earth, a mile or a kilometer may be just fine. It is a few hundred kilometers from New York City to Washington DC; It is a few thousand miles from Los Angeles to London. In the Universe, the kilometer is just too small to be useful. For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km. That's 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance.
	In our solar system, we describe distances in terms of the Astronomical Unit (AU). The AU is defined as the average distance between the Earth and the Sun. It is aproximately 150 million km (93 million miles). Mercury can be said to be about 1/3 of an AU from the Sun and Pluto averages about 40 AU from the Sun. The AU, however, is not big enough of a unit when we start talking about distances to objects outside of our solar system.