What's the Difference Between Dusk and Twilight?

We often use the terms 'dusk' and 'twilight' interchangeably, but they have distinct astronomical meanings. Learn the real definitions of twilight, dusk, and dawn in this episode of BrainStuff, based on this article: https://science.howstuffworks.com/nature/climate-weather/atmospheric/twilight-dusk.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff, the production of I Heart Radio. Hey,
brain Stuff, Lauren fog Obam. Here. As you watch a
setting sun, you see the sky go darker. Some people
call that period dusk, while others may call it twilight.
But these words aren't arbitrary. In the astronomical world, they
have specific meanings. Astronomers have classified three types or phases

(00:26):
of twilight, dusk, and dawn, so today let's talk about
what they are and how you can tell them apart.
The three phases of twilight are civil, nautical, and astronomical twilight.
Civil twilight occurs when the sun is between zero and
six degrees below the horizon. It's the brightest phase of twilight.

(00:47):
Nautical twilight occurs when the sun is between six and
twelve degrees below the horizon, and astronomical twilight occurs when
the sun is between twelve and eighteen degrees below the horizon.
It's the darkest phase and occurs just before or after
night night, being classified as when the sun is eighteen
degrees or farther below the horizon. These phases of twilight

(01:11):
occur in the same order anywhere on Earth, but how
long they last is based on your location on Earth.
That's because how long each phase lasts depends on the
season and latitude that you're viewing the sun from. Each
phase of twilight can be as short as twenty four
minutes if you're near the equator, or last several weeks
if you're at or near one of the poles. During
the summer or winter solstice. When most people say it's twilight,

(01:36):
they're referring to civil twilight, that first phase of twilight
that occurs just before sunrise or just after sunset. At
this point, the sun is barely below the horizon and
there's still a good amount of light in the sky.
Many countries and cities that laws based on this stage
of twilight, such as requiring street lights and headlights on
cars to turn on. Unfortunately, there's no hard and faster

(02:00):
rule for telling the difference between each type of twilight
by eye, because after all, you can't see how far
below the horizon the sun is from your point of view.
But one technique is to determine how well you can
distinguish shapes in the dark. Obviously, we know that when
the sun breaks above or dips below the horizon, the
time just before or after that is civil twilight. When

(02:23):
the horizon itself becomes difficult to distinguish. That indicates that
you're in a nautical twilight. The term dates back to
mariners navigating the world by ship. After the sun dips
below twelve degrees, a sailor usually can't distinguish between the
sea and the sky. Meanwhile, when you're in astronomical twilight,
the darkness is almost complete, The sky is an inky black,

(02:46):
and it's hard to distinguish any shapes at all unless
the moon is lighting up the night. Another technique is
to note the stars you can see. In civil twilight,
only the brightest stars and visible planets can be seen.
It became ms increasingly easy to spot stars throughout nautical
twilight until astronomical twilight, when even the faintest stars and

(03:06):
sometimes even the milky Way can be seen. While we
often use the terms dawn, dusk, and twilight interchangeably depending
on the time of day, both dawn and dusk have
specific astronomical meanings too. A civil dusk occurs when the
sun is exactly six degrees below the horizon in the evening.

(03:27):
Nautical dusk occurs when the sun has twelve degrees below
the horizon and astronomical dusk occurs when it's eighteen degrees
below the horizon. Conversely, civil, nautical and astronomical dawn occur
when the sun is at those respective degrees below the
horizon before sunrise. In other words, the three types of
dawn and dusk are specific points. The three types of

(03:49):
twilight are the phases between those points. To get a
sense of how far six degree increments are in the sky,
you can use your hand by holding your arms straight
out and putting three fingers up from your closed fist.
The distance from one side of your fingers to the
other is roughly five degrees. This may help you understand

(04:09):
how far the sun might be below the horizon at
each phase of twilight. Today's episode is based on the
article what's the difference between Twilight and dust? On how
stuff works dot com written by Valerie Steama. Brain Stuff
is production of by Heart Radio and partnership with how
stuff works dot Com, and it's produced by Tyler Klang.

(04:31):
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