Remember:
distance = speed x time |
Light travels very fast
How long does it take light to cross a lecture hall measuring 50 feet from front to back? | speed = 1 foot/billionth of a second
distance = 50 feet and so time = distance/speed = 50/1 = 50 billionths of a second |
How long does it take light to get to us from the Sun? (The Sun is 93 million miles away) | distance = 93,000,000 miles
speed = 186,000 miles/second and so time = distance/speed = 93,000,000/186,000 = 500 seconds (equal to 8 minutes 20 seconds) |
How far is a light year? (A light year is defined as the distance light travels in one year. It is a convenient unit for distance in astronomy where interstellar and intergalactic distances are too large to be conveniently expressed in miles.) | speed = 186,000 miles/second
time = 1 year = 365 x 24 x 60 x 60 = 31,500,000 seconds so distance = 186,000 x 31,500,000 = 5,859,000,000,000 miles (a little less than 6 trillion miles) |
How far is it to the nearest star? | The nearest star is about 4 light years away, so taking the result
of the previous calculation
distance = 4 x 5,859,000,000,000 miles = 23,436,000,000,000 miles (a long way!) |
How long would it take you to get to the Andromeda Galaxy (2.7 million light years away) if you had a spaceship capable of traveling 100,000 mph? | This is a harder problem. We have to first find out how far it is and
then calculate the time.
From the definition of a light year, the distance to the Andromeda Galaxy is 2,700,000 light years = 2,700,000 x 5,859,000,000,000 miles = 1.58 x 10^{19} miles (that is 158 followed by 17 zeroes!) distance = 1.58 x 10^{19} miles speed = 100,000 miles/hour time = distance/speed = 1.58 x 10^{19}/100000 = 1.58 x 10^{14} hours (158 trillion hours, or about 18 billion years) |