F N I TO I IN … TY N O Y E B D N A ! D

F N I TO I IN TY N O Y E B D N A ! D

H G LI A ! S R A E Y T light-year is a unit of distance. It is the distance that light can travel Light moves at a velocity of about 300,000 kilometers (km) each second. in one year SO in ONE YEAR, it can travel about 10 trillion km!! More precisely, one light-year is equal to 9,500,000,000,000 kilometers!! With materials from http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html

WHY WOULD YOU WANT SUCH A BIG UNIT OF DISTANCE? Well, on Earth, a kilometer may be just fine It is a few hundred kilometers from New York City to Washington, DC It is a few thousand kilometers from California to Maine. With materials from http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html

For example, the distance from our galaxy (the Milky Way) to the next nearest big galaxy, the Andromeda Galaxy, Is 21 Quintillion km. That's 21,000,000,000,000,000,000 km!!! With materials from http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html

This is a number that is so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance. With materials from http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html

Astronomical Units In our solar system, we tend to 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 approximately 150 million km (93 million miles) With materials from http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html

Astronomical Units Mercury can be said to be about 1/3 of an AU from the Sun Pluto averages about 40 AU from the Sun. With materials from http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html

Astronomical Units AU The , however, is not big enough of a unit when we start talking about distances to objects outside our solar system. With materials from http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html

For distances to other parts of the Milky Way Galaxy (or even further), astronomers use units of the light-year or the parsec. The light-year we have already defined. The parsec is equal to 3.3 light-years. light-year, Using the we can say that: The Crab supernova remnant is about 4,000 light-years away. The Milky Way Galaxy is about 150,000 light-years across. The Andromeda Galaxy is 2.3 million light-years away With materials from http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html

Pop Quiz!! 1. A light year is a unit of time or distance? 2. A light year is equal to about a. 10 million kilometers b. 10 billion kilometers c. 10 trillion kilometers 3. In our solar system, we tend to describe distances in terms of a. light years b. Astronomical Units c. parsecs 4. True or False? Light years must be used for measuring distances outside of our solar system because the AU is not big enough