1 00:00:01,920 --> 00:00:06,480 Speaker 1: Welcome to brain Stuff, the production of I Heart Radio. Hey, 2 00:00:06,519 --> 00:00:10,280 Speaker 1: brain Stuff, Lauren fog Obam. Here. As you watch a 3 00:00:10,320 --> 00:00:14,200 Speaker 1: setting sun, you see the sky go darker. Some people 4 00:00:14,240 --> 00:00:16,920 Speaker 1: call that period dusk, while others may call it twilight. 5 00:00:17,560 --> 00:00:21,480 Speaker 1: But these words aren't arbitrary. In the astronomical world, they 6 00:00:21,520 --> 00:00:26,360 Speaker 1: have specific meanings. Astronomers have classified three types or phases 7 00:00:26,400 --> 00:00:30,680 Speaker 1: of twilight, dusk, and dawn, so today let's talk about 8 00:00:30,720 --> 00:00:32,840 Speaker 1: what they are and how you can tell them apart. 9 00:00:34,520 --> 00:00:38,720 Speaker 1: The three phases of twilight are civil, nautical, and astronomical twilight. 10 00:00:39,479 --> 00:00:42,239 Speaker 1: Civil twilight occurs when the sun is between zero and 11 00:00:42,400 --> 00:00:46,199 Speaker 1: six degrees below the horizon. It's the brightest phase of twilight. 12 00:00:47,159 --> 00:00:49,920 Speaker 1: Nautical twilight occurs when the sun is between six and 13 00:00:50,040 --> 00:00:54,720 Speaker 1: twelve degrees below the horizon, and astronomical twilight occurs when 14 00:00:54,720 --> 00:00:58,360 Speaker 1: the sun is between twelve and eighteen degrees below the horizon. 15 00:00:58,800 --> 00:01:02,480 Speaker 1: It's the darkest phase and occurs just before or after 16 00:01:02,800 --> 00:01:06,199 Speaker 1: night night, being classified as when the sun is eighteen 17 00:01:06,240 --> 00:01:11,040 Speaker 1: degrees or farther below the horizon. These phases of twilight 18 00:01:11,080 --> 00:01:14,040 Speaker 1: occur in the same order anywhere on Earth, but how 19 00:01:14,080 --> 00:01:16,720 Speaker 1: long they last is based on your location on Earth. 20 00:01:17,480 --> 00:01:20,360 Speaker 1: That's because how long each phase lasts depends on the 21 00:01:20,400 --> 00:01:23,640 Speaker 1: season and latitude that you're viewing the sun from. Each 22 00:01:23,680 --> 00:01:25,759 Speaker 1: phase of twilight can be as short as twenty four 23 00:01:25,800 --> 00:01:28,720 Speaker 1: minutes if you're near the equator, or last several weeks 24 00:01:28,760 --> 00:01:30,800 Speaker 1: if you're at or near one of the poles. During 25 00:01:30,800 --> 00:01:35,800 Speaker 1: the summer or winter solstice. When most people say it's twilight, 26 00:01:36,000 --> 00:01:39,640 Speaker 1: they're referring to civil twilight, that first phase of twilight 27 00:01:39,680 --> 00:01:43,800 Speaker 1: that occurs just before sunrise or just after sunset. At 28 00:01:43,800 --> 00:01:46,560 Speaker 1: this point, the sun is barely below the horizon and 29 00:01:46,560 --> 00:01:48,920 Speaker 1: there's still a good amount of light in the sky. 30 00:01:49,560 --> 00:01:52,360 Speaker 1: Many countries and cities that laws based on this stage 31 00:01:52,360 --> 00:01:55,240 Speaker 1: of twilight, such as requiring street lights and headlights on 32 00:01:55,320 --> 00:01:59,880 Speaker 1: cars to turn on. Unfortunately, there's no hard and faster 33 00:02:00,080 --> 00:02:02,680 Speaker 1: rule for telling the difference between each type of twilight 34 00:02:02,760 --> 00:02:06,280 Speaker 1: by eye, because after all, you can't see how far 35 00:02:06,400 --> 00:02:09,120 Speaker 1: below the horizon the sun is from your point of view. 36 00:02:10,560 --> 00:02:13,320 Speaker 1: But one technique is to determine how well you can 37 00:02:13,360 --> 00:02:17,000 Speaker 1: distinguish shapes in the dark. Obviously, we know that when 38 00:02:17,000 --> 00:02:19,680 Speaker 1: the sun breaks above or dips below the horizon, the 39 00:02:19,720 --> 00:02:23,720 Speaker 1: time just before or after that is civil twilight. When 40 00:02:23,760 --> 00:02:27,960 Speaker 1: the horizon itself becomes difficult to distinguish. That indicates that 41 00:02:28,000 --> 00:02:31,079 Speaker 1: you're in a nautical twilight. The term dates back to 42 00:02:31,280 --> 00:02:34,600 Speaker 1: mariners navigating the world by ship. After the sun dips 43 00:02:34,639 --> 00:02:38,240 Speaker 1: below twelve degrees, a sailor usually can't distinguish between the 44 00:02:38,280 --> 00:02:42,560 Speaker 1: sea and the sky. Meanwhile, when you're in astronomical twilight, 45 00:02:42,800 --> 00:02:46,480 Speaker 1: the darkness is almost complete, The sky is an inky black, 46 00:02:46,760 --> 00:02:49,000 Speaker 1: and it's hard to distinguish any shapes at all unless 47 00:02:49,040 --> 00:02:53,000 Speaker 1: the moon is lighting up the night. Another technique is 48 00:02:53,040 --> 00:02:56,079 Speaker 1: to note the stars you can see. In civil twilight, 49 00:02:56,200 --> 00:02:58,920 Speaker 1: only the brightest stars and visible planets can be seen. 50 00:02:59,600 --> 00:03:02,840 Speaker 1: It became ms increasingly easy to spot stars throughout nautical 51 00:03:02,880 --> 00:03:06,880 Speaker 1: twilight until astronomical twilight, when even the faintest stars and 52 00:03:06,960 --> 00:03:11,960 Speaker 1: sometimes even the milky Way can be seen. While we 53 00:03:12,040 --> 00:03:15,720 Speaker 1: often use the terms dawn, dusk, and twilight interchangeably depending 54 00:03:15,760 --> 00:03:18,400 Speaker 1: on the time of day, both dawn and dusk have 55 00:03:18,520 --> 00:03:23,000 Speaker 1: specific astronomical meanings too. A civil dusk occurs when the 56 00:03:23,040 --> 00:03:26,520 Speaker 1: sun is exactly six degrees below the horizon in the evening. 57 00:03:27,120 --> 00:03:29,440 Speaker 1: Nautical dusk occurs when the sun has twelve degrees below 58 00:03:29,480 --> 00:03:33,280 Speaker 1: the horizon and astronomical dusk occurs when it's eighteen degrees 59 00:03:33,400 --> 00:03:39,040 Speaker 1: below the horizon. Conversely, civil, nautical and astronomical dawn occur 60 00:03:39,160 --> 00:03:41,800 Speaker 1: when the sun is at those respective degrees below the 61 00:03:41,840 --> 00:03:45,920 Speaker 1: horizon before sunrise. In other words, the three types of 62 00:03:46,000 --> 00:03:49,360 Speaker 1: dawn and dusk are specific points. The three types of 63 00:03:49,400 --> 00:03:53,800 Speaker 1: twilight are the phases between those points. To get a 64 00:03:53,840 --> 00:03:56,600 Speaker 1: sense of how far six degree increments are in the sky, 65 00:03:56,960 --> 00:03:59,960 Speaker 1: you can use your hand by holding your arms straight 66 00:04:00,040 --> 00:04:03,200 Speaker 1: out and putting three fingers up from your closed fist. 67 00:04:03,600 --> 00:04:05,840 Speaker 1: The distance from one side of your fingers to the 68 00:04:05,840 --> 00:04:09,880 Speaker 1: other is roughly five degrees. This may help you understand 69 00:04:09,920 --> 00:04:12,280 Speaker 1: how far the sun might be below the horizon at 70 00:04:12,320 --> 00:04:20,320 Speaker 1: each phase of twilight. Today's episode is based on the 71 00:04:20,400 --> 00:04:23,320 Speaker 1: article what's the difference between Twilight and dust? On how 72 00:04:23,360 --> 00:04:26,600 Speaker 1: stuff works dot com written by Valerie Steama. Brain Stuff 73 00:04:26,640 --> 00:04:28,480 Speaker 1: is production of by Heart Radio and partnership with how 74 00:04:28,480 --> 00:04:30,680 Speaker 1: stuff works dot Com, and it's produced by Tyler Klang. 75 00:04:31,120 --> 00:04:33,360 Speaker 1: Four more podcasts from my heart Radio, visit the i 76 00:04:33,440 --> 00:04:36,000 Speaker 1: heart Radio app, Apple Podcasts, or wherever you listen to 77 00:04:36,040 --> 00:04:36,880 Speaker 1: your favorite shows.