Sky & Telescope - March 2025 - 74

BEGINNER'S SPACE by Gary Seronik
What is
Twilight?
Civil twilight
FOR NIGHT-SKY ENTHUSIASTS, twilight
is a time of anticipation - a seemingly
interminable interval between
sunset and when it's dark enough to
start productively using a telescope. But
there's so much more to it than that.
I'll bet even some experienced observers
don't know that there are three types
of twilight or fully appreciate how it
unfolds in different parts of the world.
Let's delve into the topic a little deeper.
As with so much of astronomy,
specific definitions apply to the condition
we call twilight. The initial phase
is known as civil twilight and is defined
as the time between sunrise or sunset
and when the center of the solar disk is
as much as 6° below the horizon. Typically,
you'll only see the Moon or Venus
(assuming either is above the horizon)
during this period. When the Sun is
between 6° and 12° below the horizon,
we experience nautical twilight. This
is what most non-astronomers would
simply refer to as nighttime. Towards
the end of nautical twilight it's dark
enough that you need a flashlight to see
what you're doing, and you can trace
out most of the brighter constellations.
Lastly, there's astronomical twilight,
which occurs when the Sun is 12° to
18° below the horizon. This is when the
impatient among us uncover our scopes
and start observing - it's not as dark as
it can get, but it's still dark enough that
planets and bright deep-sky objects look
pretty good in the eyepiece. And when
the Sun is more than 18° below the
horizon, we enjoy astronomical night.
How long each phase of twilight lasts
depends on your latitude. As a Canadian,
I'm accustomed to long, lingering
twilights. Indeed, from my home in
74 MARCH 2025 * SKY & TELESCOPE
6°
FADING LIGHT The
transition between day
and night includes three
different phases of
twilight. Sunset occurs
when the top of the Sun
is below a 0° horizon.
Night begins when the
center of the Sun is 18° or
more below the horizon.
Morning twilight follows
the sequence shown here,
but in reverse. Daytime
begins when the top of the
Sun first appears above
the horizon.
southern British Columbia, astronomical
twilight doesn't end at all for a few
weeks around the summer solstice -
the Sun is never 18° or more below the
horizon. As a result, evening twilight
merges seamlessly into morning twilight,
skipping true night entirely. Yes, I
can still use my telescope productively,
but the observing window is very narrow.
Indeed, it's not much of an accomplishment
to pull an all-nighter during
summer in southernmost Canada.
Of course, things could be worse too
- the greater your latitude, the nearer
you are to the Land of the Midnight
Sun. Locations north of the Arctic
Circle (or south of the Antarctic Circle)
don't experience any twilight around
the summer solstice - the solar disk
never slips below the horizon.
Indeed, high-latitude spots are
among the toughest places on the globe
to be an amateur astronomer. For a
good portion of the year it doesn't get
dark, and for the rest of the time, the
weather is often terrible! Conversely,
for those who enjoy the night sky from
locations near the equator, the three
phases of twilight pass in rapid succession.
For example, if you travel to
Costa Rica to drink in the night sky
around the summer solstice, you'll find
that astronomical night arrives just 70
minutes after sunset. That same day-tonight
interval takes a full 150 minutes
from mid-northern (45° latitude) locations.
Of course, this situation plays out
south of the equator, too.
These differences are governed by the
angle of the ecliptic (S&T: Sept. 2022,
p. 74) in relation to the horizon. Far
from the equator, that angle is very
shallow. As a result, from my home in
Canada, it takes much longer for the
Sun to reach 18° below the horizon so
that night can begin. However, from
the tropics, the Sun seems to sink like a
stone, following a path that's only a few
degrees from being perfectly perpendicular
to the horizon. ¢
Sunset
Horizon (0°)
BEATRIZ INGLESSIS / S&T
Nautical twilight
18°
Astronomical twilight
Astronomical night
12°
Sky & Telescope - March 2025

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