Equinox

An equinox is an eve nt in which a planet’s subsolar point passes through its Equator.

GRADES

5 – 12\, Higher Ed

SUBJEC TS

Earth Scienc e\, Astronomy\, Meteorology\, Geography\, Human Geography\, Physics

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An e quinox is an event in which a planet’s subsolar point passes through its E quator. The equinoxes are the only time when both the Northern and Souther n Hemisphere experience roughly equal amounts of daytime and nighttime.

ILLUSTRATION BY PRZEMYSLAW\, COU RTESY WIKIMEDIA. CC-BY-SA-2.0

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An equinox is an event in which a planet’s subsolar point passes through its Equator. The equinoxes are the only time when both the Northern and Southern Hemispheres experience roughly eq ual amounts of daytime and nighttime.

On Earth\, there are two equinoxes every year: one around March 21 and another around September 22.

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.

The Scie nce of the Equinoxes

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.)

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.

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.

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.)

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.

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.

< em>Equatorial Regions

The Equator\, at 0° latitude\, rece ives a maximum intensity of the sun’s rays all year.

\n< div>As a result\, areas near Earth’s Equator experience relatively constan t sunlight and little equinoctial variation. Equinoxes and celestial seasons generally have less impact t han climate-driven patterns such as precipitation (rainy seasons and dry seasons).

Mid-latitudes< /em>

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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.
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P olar Regions
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Atmospheric refraction is most dramatic in t he Arctic and Antarctic\, with daylight extending about 12 hours and 16 minutes. In this way\, the equinoxes in polar regions signal the slow c hange from “midnight sun” to “ polar night.”
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“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.
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Equinoctial Disrupt ions
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Satellites are vulnerable to disruptions in the days before and after an equin ox. “Sun outages” describe this disruption.
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In the Northern Hemisphere\, sun outages occur in the days b efore the March equinox and after the September equinox. In the Southern H emisphere\, sun outages occur after the March equinox and before the Septe mber equinox.
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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.
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Many communications satellites orbit around the Equator\, and consumers may experience slow Internet connections\, radio stat ic\, or frozen television screens during equinoctial sun outages.
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Extraterrestrial Equinoxes
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Most planets experience equinoxes. The timing of equinoxes is determined by the planet’s axial tilt and orbital characteristics.
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On the gas giant Saturn\, for example\, equinoxes are particular ly dramatic. About 15 years separate the equinoxes on Saturn\, and the equ inoxes last about four days.
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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.
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The Culture of the Equinoxes
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Like the solstices\, equinoxes are historic markers of seasonal change.
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\nMarch Equinox

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The March equinox\, unofficially mark ing the spring season\, is traditionally observed as a time of rebirth and renewal. For this reason\, many cultures have celebrated the March equino x as the first day of the new year. The ancient Babylonian calendar began on the first full moon after the March equinox\, and today\, many cultural and religious calendars continue to celebrate new year in the spring.
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Perhaps the most widespread and well-known equinoctial new year is Nowruz\, the first day of the year in the Persian calendar. For more than 3\,000 years\, N owruz has been a religious holiday in Zoroastrianism\, but today it is marked by secular celebrations throughout Eastern Europe and Centr al Asia. The Nowruz holiday includes m ythical figures (such as Amu Nowruz\, sometimes nicknamed the “Iranian Santa Claus”)\, traditional family gatherings\, and the items of “haft-si n.”
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Haft-sin< /a> are seven symbolic foods associated with Nowruz\, all beginning with t he Arabic or Persian letter sin: sabze (sprouts\, symbol izing rebirth)\; samanu (sweet pudding\, symbolizing wealth)\; senjed (dried\, date-like fruits\, symbolizing love)\; seer (garlic \, symbolizing health)\; seeb (apples\, symbolizing beauty)\; somac (red sumac fruit\, symbolizing the color of sunrise)\; and serkeh (vinegar\, symbolizing age and wisdom).

\n< div>Outside new year celebrations\, the March equinox is celebrated in hol idays all over the world. Vernal Equinox Day is a national holiday in Japa n. The Jewish festival of Passover begins the night of a full moon after the March equinox. The date of Easter\, one of the most important holid ays in the Christian calendar\, i s calculated using the March equinox.

Septemb er Equinox

Fewer events mark the September equinox. Perha ps the most familiar of these are Rosh Hashanah and Yom Kippur\, Judaism’s “High Holy Days.”

Rosh Hashanah is the Jewish new year\, marked around the September equinox. Rosh Has hanah is calculated as 163 days after the first day of Passover (which its elf is calculated by the March equinox.) The sounding of the shofar\, a ram’s horn used as a trumpet for thous ands of years of Jewish ritual\, wel comes Rosh Hashanah.

Yom Kippur\, the holiest day in the Jewish calendar\, falls about 10 days after Rosh Hashanah. Yom Kippur is a solemn holiday known as the “Day of Atonement.” It is traditionally observed with a daylong fast and prayers for forgiveness.

Most cultural e vents associated with the September equinox are tied to autumnal harvest festivals. Chuseok\, celebrated over a three-day period in the Koreas\, is one of the most familiar of these folk festivals. Sometimes nicknamed “Korean Thanksgiving\,” Chuseok is a celebration of family and Korea’s ric h agricultural heritage.

The solar terminator appears vert ical during a March equinox.

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Shadowy Snake
\nAncient Mayan architects may have honor ed the equinoxes in the design the enormous stone pyramid nicknamed “El Ca stillo” at Chichen-Itza\, Mexico. Every equinox\, the light of the sun cas ts shadows down the steps of the pyramid\, making it look like a slitherin g serpent. “El Castillo” is a temple honoring the serpent god Kukulcan.

Archaeologists and anthropologists have not determined if this pheno menon was intentional or a happy accident.

Burning of the Socks
\nNot all festivals surrounding the equinox are an cient. Annapolis\, Maryland\, celebrates The Burning of the Socks every Ma rch equinox. Chesapeake Bay boaters traditionally only wear socks between winter and spring. The Burning of the Socks signifies the warmer\, sock-le ss spring season.

Chuseok Spam

Chuseok\, the huge Korean holiday around the September equinox\, is a time for family gatherings and gift-giving. In addition to traditional autumn foods such as apples and nuts\, one of the most popular Chuseok gifts are boxed sets of Spam.

Songpyeon
\nSongpyeon are an iconic food associated with Chuseok\, the Kor ean holiday around the September equinox. Songpyeon are rice cake s made with freshly harvested rice and filled with sesame seeds\, chestnut s\, or red bean paste. The treats are steamed and traditionally served on layers of pine needles. Songpyeon symbolize important elements of Chuseok. The bite-sized food encourages sharing\, and the rice and fillin gs are all harvested in late summer and autumn. Finally\, the scent of pin e\, a sturdy evergreen\, symbolizes autumn and the slow onset of the cold season on the Korean peninsula.

Source: https://www.nation algeographic.org/encyclopedia/equinox/

As Earth revolves around the Su n\, there are two moments each year when the Sun is exactly above the equator. These moments — called equinoxes — occur around March 20 or 21 and September 22 or 23. Equinoxliterally means “equal night\,” since the leng th of day and night is nearly equal in all parts of the world during the equinoxes.

The March equinox marks when the Northern He misphere starts to tilttoward the sun\, which means longer\, sunnier days. In the Northern Hem isphere\, the March equinox is called the vernal equinox\, because it sign als the beginning of spring (vernal means fresh or new like the spring). The September equinox is called the a utumnal equinox\, because it marks the first day of fall (autumn).

When the Northern Hemisphere starts to tilt towar d the sun in spring\, the Southern Hemisphere star ts to tilt away from the sun\, signaling the start o f fall. Thus\, in the Southern Hemisphere\, the March equino x is called the autumnal equinox\, and the September equinox is called the vernal equinox.

People have celebrated the vernal equinox for centuries. For ancient cultures\, the vernal equinox signaled that their food supplies would soon return. Early Egyptian s even built the Great Sphinx so that it points directly toward the ri sing Sun on the day of the vernal eq uinox. In Christianity\, the vernal equinox is significant\, because Easter always falls on the first Sunday after the first full moon after the vern al equinox.

If you keep your e yes and ears open around the time of the vernal equinox\, you’re likely to hear or see people talki ng about a magical phenomenonthat only occur s on that day. According to legend\, the special astr onomicalproperties of the vernal equinox make it possible to < /a>balance eggs on end.

So is there any truth to this popular legend? Nope! It’s actually possible to ~~balance~~ eggs on end on any day of the year. It just takes a lot of patienceand det ermination. There’s nothing magical about the ve rnal equinox that makes it any easie r to balance an egg on end.

You might b e wondering how such an interesting and widespread legend got started. No one knows for sure\, but some believe the Ch inese may have started the practice of balancing eggs on end during the vernal equinox. Given t hat day and night are balanced at the time of the vernal equinox\, it’s possible that the Chine se chose a balanced egg as a symbolic representation of this astro nomical phenomenon.

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Try It Out

Spring forward to find a few friends you can always fall back on! Ask them to help you expl ore one or more of the following fun activities:

If you’re more fascinated by the thought of balancing eggs on end\, give it a try. M ake sure you get your parents’ permission before giving egg-balancing a wh irl on your kitchen counters. It’s not as easy as it looks. Take it slow a nd you’ll do fine! How many eggs can you balance? If you’re feeling eggs-t ra inspired\, try one of these egg-ceptional craft ideas:

Up for a challenge? Let’s say you just hit the lottery. You have a gazillion dol lars in your bank account and you can do what you want\, when you want\, w here you want. You want to never see Old Man Winter again. You don’t mind spring and you love summer. Your goal is to always be living\, visiting or staying in a place where it’s spring or summer. Find a map of the world a nd plot out the places you’d go and when you’d go there\, keeping in mind the dates of the equinoxes and what seasons these dates signal in differen t hemispheres. Have fun dreaming of the never-ending summer!

Does it feel like spring in your part of the world right now? Ev en if Old Man Winter is still hanging around\, you can still get excited a bout the warmer weather to come with these crafts inspired by spring flowe rs:\n

Tissue Paper Flowers\n
Flower Cards

Equinox

Science: Earth and Space

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