1 00:00:00,560 --> 00:00:03,600 Speaker 1: Welcome to brain Stuff from how Stuff Works dot com 2 00:00:03,600 --> 00:00:07,200 Speaker 1: where smart Happens. This podcast is brought to you by 3 00:00:07,240 --> 00:00:10,360 Speaker 1: Go to Meeting, the best way to hold meetings over 4 00:00:10,400 --> 00:00:14,640 Speaker 1: the Internet. Readuce travel expenses, save time. Just hold an 5 00:00:14,640 --> 00:00:17,720 Speaker 1: online meeting with go to Meeting. Try it free, is 6 00:00:17,760 --> 00:00:24,320 Speaker 1: it go to meeting dot com slash brain Stuff. Hi. 7 00:00:24,400 --> 00:00:28,560 Speaker 1: I'm Marshall brain with today's question, what is a light year? 8 00:00:29,240 --> 00:00:32,280 Speaker 1: A light year is a way of measuring distance that 9 00:00:32,320 --> 00:00:36,080 Speaker 1: doesn't make much sense because light year contains the word year, 10 00:00:36,240 --> 00:00:39,680 Speaker 1: which is normally a unit of time. Even so, light 11 00:00:39,760 --> 00:00:43,920 Speaker 1: years measure distance. When astronomers use their telescopes to look 12 00:00:43,920 --> 00:00:48,440 Speaker 1: at stars, the distances are gigantic. For example, the closest 13 00:00:48,479 --> 00:00:51,879 Speaker 1: star to Earth besides our Sun is something like twenty 14 00:00:51,920 --> 00:00:57,080 Speaker 1: four trillion miles or thirty eight trillion kilometers away. That's 15 00:00:57,160 --> 00:01:00,440 Speaker 1: the closest star. There are stars that are millions of 16 00:01:00,520 --> 00:01:03,800 Speaker 1: times further away than that. When you start talking about 17 00:01:03,800 --> 00:01:07,319 Speaker 1: those kind of distances, a mile or kilometer just isn't 18 00:01:07,319 --> 00:01:10,840 Speaker 1: a practical unit to use because the numbers get too big. 19 00:01:11,240 --> 00:01:13,880 Speaker 1: No one wants to write or talk about numbers that 20 00:01:13,959 --> 00:01:18,000 Speaker 1: have twenty digits in them. So to measure really long distances. 21 00:01:18,040 --> 00:01:21,360 Speaker 1: People use a unit called a light year. Light travels 22 00:01:21,400 --> 00:01:24,880 Speaker 1: at a hundred eighty six thousand miles per second or 23 00:01:24,959 --> 00:01:29,520 Speaker 1: three hundred thousand kilometers per second. Therefore, a light second 24 00:01:29,959 --> 00:01:32,720 Speaker 1: is a hundred eighty six thousand miles or three hundred 25 00:01:32,720 --> 00:01:36,800 Speaker 1: thousand kilometers. A light year is the distance that light 26 00:01:36,880 --> 00:01:41,160 Speaker 1: can travel in a year, or roughly six trillion miles 27 00:01:41,360 --> 00:01:45,000 Speaker 1: or ten trillion kilometers. That's a long way. So the 28 00:01:45,080 --> 00:01:49,360 Speaker 1: closest star is about four light years away. Using a 29 00:01:49,520 --> 00:01:53,480 Speaker 1: light year as a distance measurement has another advantage. It 30 00:01:53,520 --> 00:01:56,840 Speaker 1: helps you determine age. Let's say that a star is 31 00:01:56,880 --> 00:01:59,680 Speaker 1: a million light years away. The light from that star 32 00:02:00,080 --> 00:02:03,160 Speaker 1: has traveled at the speed of light to reach us. Therefore, 33 00:02:03,520 --> 00:02:06,600 Speaker 1: it's taken the star's light one million years to get here, 34 00:02:06,720 --> 00:02:10,240 Speaker 1: and the light we're seeing was created one million years ago. 35 00:02:10,840 --> 00:02:13,519 Speaker 1: So the star we're seeing is really how the star 36 00:02:13,680 --> 00:02:16,440 Speaker 1: looked a million years ago, not how it looks right now. 37 00:02:16,919 --> 00:02:19,600 Speaker 1: In the same way, our sun is eight or so 38 00:02:19,840 --> 00:02:23,799 Speaker 1: light minutes away. If the sun were to suddenly explode 39 00:02:23,880 --> 00:02:26,960 Speaker 1: right now, we wouldn't know about it for eight minutes, 40 00:02:27,000 --> 00:02:29,079 Speaker 1: because that's how long it would take for the light 41 00:02:29,120 --> 00:02:32,679 Speaker 1: of the explosion to get here. Do you have any 42 00:02:32,760 --> 00:02:36,399 Speaker 1: ideas or suggestions for this podcast. If so, please send 43 00:02:36,400 --> 00:02:39,720 Speaker 1: me an email at podcast at how stuff works dot com. 44 00:02:39,840 --> 00:02:42,120 Speaker 1: For more on this and thousands of other topics, go 45 00:02:42,200 --> 00:02:45,960 Speaker 1: to how stuff works dot com.