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On Earth\,
there are two equinoxes every year: one around March 21 and another around
September 22.
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Sometimes\, the equinoxes are nick
named the “
vernal equinox” (
spring equinox) and the “
autum
nal equinox” (fall equinox)\, although these have different dates in t
he Northern and Southern Hemispheres. The March equinox is the vernal equ
inox in the Northern Hemisphere\, and the autumnal equinox in the Southern
. The September equinox is the autumnal equinox in the Northern Hemisphere
and the vernal in the Southern.
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The Scie
nce of the Equinoxes
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During the equinoxe
s\, solar declination is 0°.
Solar declination describes the
la
titude of the Earth where the sun is directly overhead at noon. (The E
quator\, of course\, is 0° latitude.)
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So\, equino
xes are the only times of the year when the subsolar point is directly on
the Equator. The subsolar point is an area where the sun’s rays shine
perpendicular to the Earth’s surf
ace—a
right angle. Only during
an equinox is the Earth’s 23.5°
axis <
em>not tilting toward or away from the sun: the perceived center of t
he Sun’s disk is in the same
plane as
the Equator.
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Before and after the equinox\, the
subsolar point migrates north or south. After the March equinox\, the subs
olar point migrates north as the Northern Hemisphere tilts toward the sun.
Around June 21\, the subsolar point hits the
Tropic of Cancer\, (23.5°N). This is the June
solstice\, after which the subsolar point
begins to migrate south. After the September equinox\, the subsolar point
continues to move south as the Southern Hemisphere tilts toward the sun. A
round December 21\, the subsolar point hits the
Tropic of Capricorn (23.5°S). This is the Decembe
r solstice.
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As its name suggests\, an equinox ind
icates equally
illuminated
hemispheres\, with the solar terminat
or equally dividing the Earth from north to south. (The
solar terminator is the shadowed line indica
ting daylight and sunlight on a globe.)
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A true eq
uinox would indicate 12 hours of both day and night. Although the equinoxe
s are as close to this
phenomenon as happens on Earth\, even during the equinoxes day and night aren’t exa
ctly equal. This is largely due to atmospheric
refraction.
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Atmospheric refraction
describes the way light seems to bend or
deviate from a straight line as it passes through Earth’s
atmosphere. Atmospheric refraction is a r
esult of increasing air
density\, w
hich decreases the
velocity of lig
ht through the air. Due to atmospheric refraction\, we are able to see the
sun minutes before it actually rises and sets.
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<
em>Equatorial Regions
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The Equator\, at 0° latitude\, rece
ives a maximum intensity of the sun’s rays all year.
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div>As a result\, areas near Earth’s Equator experience relatively constan
t sunlight and little
equinoctial
a> variation. Equinoxes and celestial seasons generally have less impact t
han climate-driven patterns such as
precipitation (
rainy seasons and
dry seasons).
Seasonal variation increases with latitude. Atmospheric r
efraction also increases the
disparit
y in the “equinox” length of day and night. At about 30° latitude\, da
y is about eight minutes longer than night.
“Midnight sun” describes the phenomenon in which the sun never dip
s below the
horizon\, keeping the r
egion bathed in sunlight 24 hours a day. “Polar night” describes the oppos
ite phenomenon\, a time in which the sun never rises\, keeping the region
dark for 24-hour periods.
During an equinox\, the sun is
aligned directly behind satellites in
geostationary orbit at the
Equator. Situated directly above the subsolar point\, the satellites are
flooded with direct
solar radia
tion. This solar radiation can interfere with and even stop satellites
from
transmitting signals.
Saturn’s spectacular
ring system orbits in the same
plane as the planet’s equator. Although the rings extend thousands of kil
ometers into space\, they are actually very thin\, only about a kilometer
wide. During Saturn’s equinoxes\, the rings (and Saturn’s equator) line up
perfectly with the sun. Photos taken from the solar perspective reveal th
e rings as a razor-thin line.
Like the solstices\,
equinoxes are historic markers of seasonal change.