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### Light Years

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Date: 11/19/97 at 21:26:57
From: Vehbi Tasar
Subject: light years

How many miles is a light year just for the purpose of planning?

Thank you,
Vehbi
```

```
Date: 11/25/97 at 12:18:16
From: Doctor Mark
Subject: Re: light years

Hi Vehbi,

A more interesting question is:  What are you planning?  Can I come?

More seriously, a light year is the distance that light travels in one
year. Since distance is equal to speed times time, you can find how
many miles there are in a light year by multiplying the speed of light
by the time of one year.

In doing this, you have to make sure that the units work out
correctly, so if you give the speed of light in miles per second, you
have to find the length of a year in seconds. The way this works out
is:

speed of light = 186,000 mi/sec (approximately)

number of seconds in one year =
(number of days in one year)(number of hours in one day)
(number of minutes in one hour)(number of seconds in one minute)

= (365)(24)(60)(60) = 31,536,000

So to find a light year in miles, you multiply 186,000 mi/sec by
31,536,000 sec to find:

one light year  = about 6 trillion miles.

That's a whole boatload of miles. To get some sense of how big it is,
if you were to fill a football stadium with sand to a depth of about
3 feet (one meter), that would be about 6 trillion grains of sand.
Take all that sand and drop one grain of sand every mile, and you
would have to travel a distance of one light year to get rid of all
the sand.

Another way of thinking about a light year is that if you were to make
a model of the universe (you'd better have lot of free time for that!)
on a scale so that the earth (8000 miles in diameter) were the size of
a pea (about 1/4" in diameter), a light year would be about the
distance across the United States. On that same scale, light would
move about as fast as a baby crawls (and about the time it got to
Denver, it would start walking).

-Doctor Mark,  The Math Forum
Check out our web site!  http://mathforum.org/dr.math/
```
Associated Topics:
Middle School Calendars/Dates/Time
Middle School Definitions
Middle School Terms/Units of Measurement

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