Sky & Telescope - June 2021 - 49

DAY TO NIG H T DIAG R A M: G REGG DINDER M A N / S&T;
TR A FFIC AT T WILIG H T: BOB K ING

sunset position. The shadow appears
as a purple-gray band about 5° wide,
fringed by a diffuse, pink glow. Known
as the Belt of Venus, it's caused by reddened sunlight scattering off dust high
in the atmosphere.
Nautical twilight covers the interval
when the center of the Sun is between
6° and 12° below the horizon. During
this time, sailors can still discern the
horizon and make reliable readings of
star positions at sea with a sextant. On
terra ﬁrma, the brighter stars and constellations are visible, and most outdoor
activities require artiﬁcial lighting.
The general public would consider the
end of nautical twilight " night " - but
not astronomers. For them, true night
only arrives at the end of astronomical
twilight, when the center of the solar
disk has dipped 18° below the horizon.
With all trace of twilight gone, observers far from city lights can seek the
faintest deep-sky objects and comets. Of
course, the night sky is never truly dark.
Starlight, airglow, aurorae, and zodiacal
light continue to provide feeble illumination, making it possible to see your
hand silhouetted against the sky even
from the darkest locations on Earth.
Nights around the solstice offer only
a brief voyage into darkness before our
craft arrives at dawn's shore, where each
stage of twilight plays out in reverse
before the Sun peeps over the horizon.
How long twilight lasts depends on
how quickly the Sun drops below the

Day
Horizon

Sunset

Civil twilight

autical

Ast

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rono

Nig

6°

t
twiligh
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12 °

iligh

18°

S DAY TO NIGHT Twilight has three phases,
each defined by the Sun's position relative to
the horizon. The sky is only fully dark when the
Sun's center lies 18° below the horizon at the
end of astronomical twilight (evening) or before
the start of astronomical twilight (morning).

horizon, which varies according to the
season and your latitude. Dusk and
dawn zip by in about 70 minutes at the
equator, where the solar disk quickly
drops to -18°. Northern visitors to the
tropics are often shocked at how quickly
darkness falls.
Not surprisingly, twilight lingers
longest at the poles, where the Sun's
path is nearly parallel to the horizon
around the equinoxes. All of us have
heard of the Land of the Midnight Sun,
where daylight lingers continuously
during the summer solstice for locations north of the Arctic Circle. However, you might be surprised to learn
that if you live north of 48.5° the Sun's
center never dips to -18° on the June
solstice. As most Canadian stargazers are keenly aware, at least a trace of
twilight lingers in the northern sky all
night long.

S FADING LIGHT The blue-gray band of Earth's shadow rises in the eastern sky beneath the
rosy-hued Belt of Venus. Both features lie opposite the setting Sun and remain visible for 20 to 25
minutes after sunset.

The Sun's declination varies during
the year from 23.5° north of the celestial equator on the summer solstice, to
23.5° south on the ﬁrst day of winter.
Near the equator, the Sun's path is
almost perpendicular to the eastern
and western horizons all year long.
However, at mid-Northern latitudes in
summer, the Sun intersects the horizon
at a much shallower angle. That means
the Earth has to spin longer for the Sun
to sink to the requisite -18°, increasing
twilight's duration by an hour or more.
June is, of course, the month of the
summer solstice and brings a bounty
of eventide. Use the time to relish the
progression of colors and emerging stars
as our planet languorously unfurls its
shadow across the sky.

A Nunki
Cover-up
The Moon eclipses 2.1-magnitude Sigma (σ) Sagittarii (better
known as Nunki) 11 times in
2021. Nunki is the most northern
star in the Handle of the Sagittarius Teapot and the brightest
star the Moon will occult this
year. (The star is shown on our
Northern Hemisphere Map, near
the " Facing SE " label.)
On the morning of June 25th,
if you live in the western half of
the U.S. or in Mexico, at last it
will be your turn to see an occultation. From most of the western
half of the U.S. and Hawai'i, the
15.5-day-old Moon will cover the
star at its bright limb in a dark
sky, while for locations farther
east the event occurs in morning
twilight or around sunrise.
A narrow slice of the far
southern U.S., Central America,
and northern South America
will see the star disappear again
behind the 11-day-old Moon
on August 18-19. Additional
information is available at
https://is.gd/2021occultations.

sk yandtelescope.org * JUNE 2 021

https://www.is.gd/2021occultations http://www.skyandtelescope.org

Sky & Telescope - June 2021

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