Expert: Jayendra Upadhye - 10/23/2004

Question
I need to know if I did this right.   I need to know the number of miles in light years using 1.86X10 to the 5th mi/s as the speed of light  IS the answer 3.15x10 to the 7th s/y?

Answer
Hi Mary!
Tell you what! (if you are in a hurry, go directly to end of the answer else take time and browse thru the whole of it)
When you need such details fast, there is another source at hand which is infallible, accurate and precise.
just type "define: light year" [without the quotes] in google.
You will be delighted to see the rersults!
This way you can get any definitions immediately, even build a small data base in xl on your pc for future quick looks! [do it when you have spare time, it will also add to your knowledge of building useful data tables using spread-sheets].
Coming to your question:-
1 - speed of light is 300,000 km / sec or 1,86,000 miles /sec.
2 - in a year ie: 24 hrs/day * 3600 seconds/hr * 365.25 days /year = 31557600 seconds/year as other quantities cancel out as under:
[(hr/day) * (sec/hr) * (day/year) = (sec/year) units]
thus 1,86,000 miles /sec * 31557600 sec/year = 5869713600000 miles per year.
checking in google as i told you earlier: we get
"The distance light travels in a year, at the rate of 300,000 kilometers per second (671 million miles per hour); 1 light-year is equivalent to 9.46053e12 km, 5,880,000,000,000 miles or 63,240 AU. " [quotes mine]
Our answer is only
[(5880000000000 - 5869713600000)/5880000000000]*100 = 0.175
percent wrong! (as based on their answer which is taken as base!)
YOUR 3.15*10E7 IS RIGHTLY, THE NUMBER OF SECONDS PER YEAR
(31557600 sec/year APPROX = 3.156 * 10E7 WITH VERY LOW ERROR if you consider 365.25 as tru number of days per year. Aftearall that ".25" gives us a leap year every 4 years does it not?)
You mention it as s/y right?
Jayen