# UMD ASTR 100 - Homework #1 Solutions (2 pages)

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## Homework #1 Solutions

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- Pages:
- 2
- School:
- University of Maryland, College Park
- Course:
- Astr 100 - Introduction to Astronomy

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ASTR 100 Homework 1 Solutions McGaugh Fall 2008 1 Problem 1 15 Our solar system is bigger than some galaxies This statement makes no sense A galaxy is composed of hundreds of billions of stars while our solar system is just the stuff orbiting one star our sun Our solar system is a tiny part of the Milky Way Galaxy being much smaller than it 2 Problem 1 41 Distance by light The problem asks you to compute the distance that light can travel in different intervals of time measured in kilometers and miles d vt distance speed x time In this case the speed is the speed of light v c According to Appendix A of the book c 3 105 km s The exact number is c 299 792 km s it is OK to use either value a 1 light second 3 105 km s 1 s The seconds cancel out see Appendix C so 1 light second 300 000 km 1 km 0 62 miles so 300 000 km 0 62 miles km 186 000 miles That s a long way to go in one second b Similarly 1 light minute 3 105 km s 1 minute Be consistent in the use of units 1 minute 60 seconds so 1 light minute 3 105 km s 60 s Now the units again cancel nicely and 1 light minute 1 8 107 km 1 1 107 miles c 1 light hour 3 105 km s 1 hour 1 light hour 3 105 km s 60 minutes hour 60 seconds minute 1 light hour 1 08 109 km 6 7 108 miles d 1 light day There are 24 hours in a day so 1 day 24 hours 60 minutes hour 60 seconds minute 86 400 seconds 1 light day 3 105 km s 86 400 s so 1 light day 2 6 1010 km 1 6 1010 miles For a little perspective the Voyager 1 spacecraft was launched 31 years ago in 1977 It has been traveling away from the earth ever since and is now the most remote man made object at a distance of 1 6 1010 km Light the sun emits today will pass Voyager tomorrow 3 Problem 1 42 Moonlight and Sunlight This simply inverts the problem above Since d vt t d v a Light travel time from the moon to earth 5 t dmoon c dmoon 3 8 10 km Textbook Table E 3 5 km t 3 8 10 3 105 km s t 1 3 seconds b Light travel time from the sun to earth 8 t dsun c dsun 1 5 10 km Table E 2 5 s t 3 10 km 1 5 108

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