Speaker 1: Welcome to brain Stuff, the production of I Heart Radio. Hey,
Speaker 1: brain Stuff, Lauren fog Obam. Here. As you watch a
Speaker 1: setting sun, you see the sky go darker. Some people
Speaker 1: call that period dusk, while others may call it twilight.
Speaker 1: But these words aren't arbitrary. In the astronomical world, they
Speaker 1: have specific meanings. Astronomers have classified three types or phases
Speaker 1: of twilight, dusk, and dawn, so today let's talk about
Speaker 1: what they are and how you can tell them apart.
Speaker 1: The three phases of twilight are civil, nautical, and astronomical twilight.
Speaker 1: Civil twilight occurs when the sun is between zero and
Speaker 1: six degrees below the horizon. It's the brightest phase of twilight.
Speaker 1: Nautical twilight occurs when the sun is between six and
Speaker 1: twelve degrees below the horizon, and astronomical twilight occurs when
Speaker 1: the sun is between twelve and eighteen degrees below the horizon.
Speaker 1: It's the darkest phase and occurs just before or after
Speaker 1: night night, being classified as when the sun is eighteen
Speaker 1: degrees or farther below the horizon. These phases of twilight
Speaker 1: occur in the same order anywhere on Earth, but how
Speaker 1: long they last is based on your location on Earth.
Speaker 1: That's because how long each phase lasts depends on the
Speaker 1: season and latitude that you're viewing the sun from. Each
Speaker 1: phase of twilight can be as short as twenty four
Speaker 1: minutes if you're near the equator, or last several weeks
Speaker 1: if you're at or near one of the poles. During
Speaker 1: the summer or winter solstice. When most people say it's twilight,
Speaker 1: they're referring to civil twilight, that first phase of twilight
Speaker 1: that occurs just before sunrise or just after sunset. At
Speaker 1: this point, the sun is barely below the horizon and
Speaker 1: there's still a good amount of light in the sky.
Speaker 1: Many countries and cities that laws based on this stage
Speaker 1: of twilight, such as requiring street lights and headlights on
Speaker 1: cars to turn on. Unfortunately, there's no hard and faster
Speaker 1: rule for telling the difference between each type of twilight
Speaker 1: by eye, because after all, you can't see how far
Speaker 1: below the horizon the sun is from your point of view.
Speaker 1: But one technique is to determine how well you can
Speaker 1: distinguish shapes in the dark. Obviously, we know that when
Speaker 1: the sun breaks above or dips below the horizon, the
Speaker 1: time just before or after that is civil twilight. When
Speaker 1: the horizon itself becomes difficult to distinguish. That indicates that
Speaker 1: you're in a nautical twilight. The term dates back to
Speaker 1: mariners navigating the world by ship. After the sun dips
Speaker 1: below twelve degrees, a sailor usually can't distinguish between the
Speaker 1: sea and the sky. Meanwhile, when you're in astronomical twilight,
Speaker 1: the darkness is almost complete, The sky is an inky black,
Speaker 1: and it's hard to distinguish any shapes at all unless
Speaker 1: the moon is lighting up the night. Another technique is
Speaker 1: to note the stars you can see. In civil twilight,
Speaker 1: only the brightest stars and visible planets can be seen.
Speaker 1: It became ms increasingly easy to spot stars throughout nautical
Speaker 1: twilight until astronomical twilight, when even the faintest stars and
Speaker 1: sometimes even the milky Way can be seen. While we
Speaker 1: often use the terms dawn, dusk, and twilight interchangeably depending
Speaker 1: on the time of day, both dawn and dusk have
Speaker 1: specific astronomical meanings too. A civil dusk occurs when the
Speaker 1: sun is exactly six degrees below the horizon in the evening.
Speaker 1: Nautical dusk occurs when the sun has twelve degrees below
Speaker 1: the horizon and astronomical dusk occurs when it's eighteen degrees
Speaker 1: below the horizon. Conversely, civil, nautical and astronomical dawn occur
Speaker 1: when the sun is at those respective degrees below the
Speaker 1: horizon before sunrise. In other words, the three types of
Speaker 1: dawn and dusk are specific points. The three types of
Speaker 1: twilight are the phases between those points. To get a
Speaker 1: sense of how far six degree increments are in the sky,
Speaker 1: you can use your hand by holding your arms straight
Speaker 1: out and putting three fingers up from your closed fist.
Speaker 1: The distance from one side of your fingers to the
Speaker 1: other is roughly five degrees. This may help you understand
Speaker 1: how far the sun might be below the horizon at
Speaker 1: each phase of twilight. Today's episode is based on the
Speaker 1: article what's the difference between Twilight and dust? On how
Speaker 1: stuff works dot com written by Valerie Steama. Brain Stuff
Speaker 1: is production of by Heart Radio and partnership with how
Speaker 1: stuff works dot Com, and it's produced by Tyler Klang.
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