1. Minds On
Read Mysteries Beyond the Planets (Pg. 353) and summarize the article in one paragraph.

2. To the Stars …

3. Learning Goals
I will be able to explain what a light year is and why we use it.
I will be able to calculate distances in both km and light years.

4. The Light Year
We used the Astronomical Unit (AU) to measure distances within our solar system. Recall:
1 AU = km
The nearest star to Earth is Proxima Centauri which is 40 trillion (4.01 x 1013) km or AU away.
Most stars are more than 100 trillion (1 x 1014) km away.

5. The Light Year
The AU is much too small to measure these distances so Astronomers use a larger unit.
In space, light travels at a constant speed of km/s.
Light Year: The distance light travels in a vacuum in a year ( days).
1 ly = km
9.46 x 1012 km
Therefore, 1 light year ≈ 10 trillion km!

6. Example 1 Polaris is 400 ly from Earth. How far away is it in km?
= (400 ly) X (9.46 x1012 km/ly)
= x 1015 km
Polaris is x 1015 km away from our planet!!!!!

7. Example 2
2. If the Andromeda Galaxy is approximately 2.46 x 1019 km from Earth, how many light years away is it? = (2.46 x 1019 km ) ÷ (9.46 x1012 km/ly) = ly

8. Learning Goals
I will be able to explain what a light year is and why we use it.
I will be able to calculate distances in both km and light years.

9. Your Turn Please complete light year worksheet!
Study for your Quest on Monday!
Research for your Unit Task!