Light-Years: Units of Linear Distance
This is a re-post for unlocking purposes.
I run across this more often than I should when reading fanfiction.
A light-year is a measure of distance, not of time. It is the distance that light will travel during one earth year (365.25 days, if you want to be picky). One of Earth's nearest neighbors, Alpha Centauri, is 4.3 light-years away. Light being emitted by that star right now will take 4.3 years to arrive here. In contrast, light from our own sun takes about 8 minutes to arrive on earth.
So. Let's be scientific about this, and use the international notation. The speed of light is a constant at 3.00x10^8 m/s in a vacuum. A light-year is then:
3.00x10^8 meters/second x 365.25 days/year x 24 hours/day x 60 min/hour x 60 seconds/minute
=9.47x10^12 kilometers/year. That's 9,470,000,000,000 kilometers or about 5,880,000,000,000 miles.
Notice with all the cross-cancelling of units how this has now become a unit of linear measurement. Of DISTANCE. We'll skip all the tedious stuff about defining a meter and calculating to X number of significant figures and all the other crap that sciencey types get all bent about. I'm a sciencey type and only get bent when I really have to.
Alpha Centauri is therefore 4.3 x 9.47x10^12 = 4.07x10^13 (40,700,000,000,000) km away. (That's about 25,300,000,000,000 miles).
Bottom line. Light-years are measurements of extreme interstellar and intergalactic distance, not of time. Try substituting another distance unit in the sentence (say, miles) and see if it works. Research is your friend. And there are really quite a few of us fannish sciencey-types out here. I can't speak for other sciencey-types, but while I won't do your research for you (you can do your own grunt-work, as I have enough of my own, thank you) if you catch me right and ask nice, I generally like to talk about science.
You can also toss your questions over to the lovely and under-used science_beta community.
I run across this more often than I should when reading fanfiction.
A light-year is a measure of distance, not of time. It is the distance that light will travel during one earth year (365.25 days, if you want to be picky). One of Earth's nearest neighbors, Alpha Centauri, is 4.3 light-years away. Light being emitted by that star right now will take 4.3 years to arrive here. In contrast, light from our own sun takes about 8 minutes to arrive on earth.
So. Let's be scientific about this, and use the international notation. The speed of light is a constant at 3.00x10^8 m/s in a vacuum. A light-year is then:
3.00x10^8 meters/second x 365.25 days/year x 24 hours/day x 60 min/hour x 60 seconds/minute
=9.47x10^12 kilometers/year. That's 9,470,000,000,000 kilometers or about 5,880,000,000,000 miles.
Notice with all the cross-cancelling of units how this has now become a unit of linear measurement. Of DISTANCE. We'll skip all the tedious stuff about defining a meter and calculating to X number of significant figures and all the other crap that sciencey types get all bent about. I'm a sciencey type and only get bent when I really have to.
Alpha Centauri is therefore 4.3 x 9.47x10^12 = 4.07x10^13 (40,700,000,000,000) km away. (That's about 25,300,000,000,000 miles).
Bottom line. Light-years are measurements of extreme interstellar and intergalactic distance, not of time. Try substituting another distance unit in the sentence (say, miles) and see if it works. Research is your friend. And there are really quite a few of us fannish sciencey-types out here. I can't speak for other sciencey-types, but while I won't do your research for you (you can do your own grunt-work, as I have enough of my own, thank you) if you catch me right and ask nice, I generally like to talk about science.
You can also toss your questions over to the lovely and under-used science_beta community.