1. Objectives: Learn what units scientists measure distances in space. Define and use an astronomical unit to measure distances in space. Define and use a light year to measure distances in space.

2. For example:  Earth is 93,000,000 miles from the sun.  Earth is 46,000,000 miles from Mars.  Pluto is 3,720,000,000 miles from the sun.  Our sun is 25,800,000,000,000 miles from the nearest star, Proxima Centauri.  Our galaxy, The Milky Way, is 600,000,000,000,000,000 miles in diameter (across).

3. Scientists do not want to work with large numbers, so they make up new units to measure distances in space with. The two units used in astronomy are: Astronomical Units (A.U.s) Light Years

4. Used to measure distances within the solar system. 1 A.U. = the average distance between the Sun and Earth. 1 A.U. = 93 million miles / 149.6 million kilometers

5. PlanetDistance from Sun in kms Distance in A.U.s Mercury57,900,0000.387 Venus108,200,0000.723 Earth149,600,,0001.000 Mars227,900,0001.524 Jupiter778,400,0005.203 Saturn1,427,000,0009.539 Uranus2,896,600,00019.182 Neptune4,520,000,00030.214 Pluto5,899,900,00039.439 Distances Within the Solar System

6. A light year is the distance light can travel in one year. Light travels at the speed of 300, 000 km/s (186,000 mi/s) 1 LY = 6 trillion miles (9.5 trillion kilometers) 6 x 10 12 mi (9.5 x 10 12 km) Used to measure distances outside our solar system Example: Distance between the sun and another star or galaxy

7. Proxima Centauri is the closest star to our sun. Proxima Centauri is 4.6 light years away. How many miles away is Proxima Centauri? 4.6 light years x 6 trillion mi= 27.6 trillion miles away (27,600,000,000,000 miles or 2.76 x 10 13 ) Since Proxima Centauri is 4.6 light years away, it takes 4.6 years for the light from Proxima Centauri to reach us.

8. How can we tell how far away a star is? PARALLAX = APPARENT CHANGE IN POSITION OF AN OBJECT WITH RESPECT TO A MORE DISTANT BACKGROUND. As Earth moves in its orbit, stars are viewed from different angles and appear to move against the background of more distant stars. The degree of “apparent” motion can be converted into LYs. ASTRONOMERS MEASURE THE PARALLAX OF NEARBY STARS TO DETERMINE THEIR DISTANCE. (Used for stars w/in a few hundred LY) Degrees parallax can be converted to light years. http://instruct1.cit.cornell.edu/courses/astro101/java/parallax/parallax.html

10. M13 is about 145 light-years in diameter, and it is composed of several hundred thousand stars.diameterstars M13 is 25,100 light-years away from Earth.Earth

12. Use parallax to determine which star is closer to Earth? FebruaryAugust