Illumination of the Earth by the Sun
on the day of an equinox (ignoring twilight).
The Earth in its orbit around the Sun
causes the Sun to appear on the celestial sphere moving over the
ecliptic (red), which is tilted on the Equator (white).
Day arc at 0° latitude (Equator)
Day arc at 20° latitude
Day arc at 50° latitude
Day arc at 70° latitude
Day arc at 90° latitude (Pole)
occurs twice a year, when the tilt of
's axis is inclined neither away from
nor towards the Sun
, the Sun being vertically
above a point on the Equator
. The term
can also be used in a broader sense, meaning the
date when such a passage happens.The name "equinox" is derived from
the Latin aequus
(equal) and nox
around the equinox, the night and day are approximately equally
long. It may be better understood to mean that latitudes
south of the equator experience nights of equal length.
The word is also used for the same event happening on other planets
and in setting up a celestial coordinate system
At an equinox, the Sun is at one of two opposite points on the
where the celestial equator
(i.e. declination 0) and
intersect. These points of
intersection are called equinoctial points:
and the autumnal
. By extension, the term equinox
an equinoctial point.
An equinox happens each year at two specific moments in time
(rather than two whole days), when there is a location on the
the centre of the Sun
can be observed to be
vertically overhead, occurring around March 20/21 and September
22/23 each year.
- Vernal equinox and autumnal
equinox: these classical names are direct derivatives of
Latin (ver = spring and autumnus =
- March equinox and September
equinox: a usage becoming the preferred standard by
technical writers choosing to avoid Northern Hemisphere bias
(implied by assuming that March is in the springtime and September
is autumnal—true for those in the Northern Hemisphere but exactly
opposite in the Southern Hemisphere).
- Northward equinox and southward
equinox: names referring to the apparent motion of the Sun
at the times of the equinox.
- Vernal point and autumnal
point are the points on the celestial sphere where the Sun
is located on the vernal equinox and autumnal
equinox respectively (again, the seasonal attribution is that
of the Northern Hemisphere).
- First point (or cusp)
of Aries and
first point of Libra are archaic names used by
navigators and astrologers. Navigational
ephemeris tables record the geographic position of the First Point
of Aries as the reference for position of navigational stars. Due
to the precession of the
equinoxes, the astrological signs where
these equinoxes are located no longer correspond with the actual
constellations once ascribed to
Length of equinoctial day and night
On a day of the equinox, the centre of the Sun spends a roughly
equal amount of time above and below the horizon at every location
on the Earth, night and day being of roughly the same length. The
derives from the Latin words aequus
(equal) and nox
(night); in reality, the day is longer
than the night at an equinox. Commonly, the day
is defined as the period when sunlight reaches the ground in the
absence of local obstacles. From the Earth, the Sun appears as a
disc rather than a single point of light, so when the centre of the
Sun is below the horizon, its upper edge is visible. Furthermore,
the atmosphere refracts light, so even when the upper limb of the
Sun is below the horizon, its rays reach over the horizon to the
ground. In sunrise
tables, the assumed semidiameter (apparent
) of the Sun is 16 minutes of arc
and the atmospheric refraction
is assumed to
be 34 minutes of arc. Their combination means that when the upper
limb of Sun is on the visible horizon, its centre is 50 minutes of
arc below the geometric horizon, which is the intersection with the
celestial sphere of a horizontal plane through the eye of the
observer. These cumulative effects make the day about 14 minutes
longer than the night at the Equator and longer still towards the
Poles. The real equality of day and night only happens in places
far enough from the equator to have a seasonal difference in day
length of at least 7 minutes, actually occurring a few days towards
the winter side of each equinox.
The date at which the time between sunset and sunrise crosses 12
hours , is known as the equilux
. Because sunset and
sunrise times vary with an observer's geographic location
(longitude and latitude), the equilux likewise depends on location
and does not exist for locations sufficiently close to the equator.
The equinox, however, is a precise moment in time which is common
to all observers on Earth.
Heliocentric view of the seasons
The Earth's seasons
are caused by the
rotation axis of the Earth not being perpendicular to its orbital
plane. The Earth's axis is tilted at an angle of approximately
23.44° from the orbital plane; this tilt is called the axial tilt
. As a consequence, for half of the
year (i.e. from around March 20 to around September 22), the
northern hemisphere tips toward the Sun, with the maximum around
June 21, while for the other half of the year, the southern
hemisphere has this honor, with the maximum around December 21. The
two instants when the Sun is directly overhead at the Equator are
the equinoxes. Also at that moment, both the North and South Poles
of the Earth are just on the terminator
and day and night are divided
equally between the hemispheres.
The table above
gives the dates and times of
equinoxes and solstices
over several years.
A few remarks can be made about the equinoxes:
- Because the Sun is a spherical (rather than a single-point)
source of light, the actual crossing of the Sun over the Equator
takes approximately 33 hours.
- At the equinoxes, the rate of change for the length of daylight
and night-time is the greatest. At the Poles, the equinox marks the
start of the transition from 24 hours of nighttime to 24 hours of
daylight. High in the Arctic Circle, Longyearbyen, Svalbard, Norway has
an additional 15 minutes more daylight every day around the time of
the Spring equinox, whereas in Singapore (which is virtually on the Equator), the
amount of daylight each day varies by just seconds.
- It is 94 days from the June solstice to the September equinox,
but only 89 days from the December Solstice to the March equinox.
The seasons are not of equal length, because of the variable speed
of the Earth in its orbit around the Sun.
- The instances of the equinoxes are not fixed, but fall about
six hours later every year, amounting to one full day in four
years. They are reset by the occurrence of a leap year. The
Gregorian calendar is designed to follow the seasons as accurately
as is practical, which is good, but not absolutely perfect.
Also see: Gregorian calendar
- Smaller irregularities in the times are caused by perturbations
of the Moon and the other planets.
- Currently, the most common equinox and solstice dates are March
20, June 21, September 22 and December 21; the four-year average
will slowly shift to earlier times in coming years. This shift is a
full day in about 70 years (compensated mainly by the century "leap
year" rules of the Gregorian calendar). This also means that in
many years of the twentieth century, the dates of March 21, June
22, September 23 and December 22 were much more common, so older
books teach (and older people may still remember) these dates.
that the times are given in UTC (roughly speaking, the time
at Greenwich, ignoring British Summer Time). People living farther
to the east (Asia and Australia), whose local times are in advance,
will see the seasons apparently start later; for example, in
Tonga (UTC+13), an equinox occurred on September 24,
1999, a date which will not crop up again until 2103. On the
other hand, people living far to the west (America) whose clocks
run behind UTC may experience an equinox as early as March 19.
Geocentric view of the seasons
In the half year centred on the June solstice, the Sun rises and
sets towards the north, which means longer days with shorter nights
for the Northern Hemisphere and shorter days with longer nights for
the Southern Hemisphere. In the half year centred on the December
solstice, the Sun rises and sets towards the south and the
durations of day and night are reversed.
Also on the day of an equinox, the Sun rises everywhere on Earth
(except the Poles) at 06:00 in the morning and sets at 18:00 in the
evening (local time). These times are not exact for several
reasons, one being that the Sun is much larger in diameter than the
Earth that more than half of the Earth could be in sunlight at any
one time (due to unparallel rays creating tangent points beyond an
equal-day-night line); other reasons are as follows:
- Most places on Earth use a time zone
which is unequal to the local time, differing by up to an hour or
even two hours, if daylight saving
time (summer time) is included. In that case, the Sun could
rise at 08:00 and set at 20:00, but there would still be 12 hours
- Even those people fortunate enough to have their time zone
equal to the local time will not see sunrise and sunset at 06:00
and 18:00 respectively. This is due to the variable speed of the
Earth in its orbit, and is described as the equation of time. It has different values
for the March and September equinoxes (+8 and −8 minutes
- Sunrise and sunset are commonly defined for the upper limb of
the solar disk, rather than its centre. The upper limb is already
up for at least one minute before the centre appears, and likewise,
the upper limb sets one minute later than the centre of the solar
disk. Due to atmospheric
refraction, the Sun, when near the horizon, appears a little
more than its own diameter above the position than where it is in
reality. This makes sunrise more than another two minutes earlier
and sunset the equal amount later. These two effects add up to
almost seven minutes, making the equinox day 12 h 7 min
long and the night only 11 h 53 min. In addition to that,
the night includes twilight. When dawn and dusk are added to the
daytime instead, the day would be almost 13 hours.
- The above numbers are only true for the tropics. For moderate
latitudes, this discrepancy increases (for example, 12 minutes
in London) and closer to the Poles it gets very large. Up to about
100 km from either Pole, the Sun is up for a full 24 hours on
an equinox day.
- Height of the horizon on both the sunrise and sunset sides
changes the day's length. Going up into the mountains will lengthen
the day, while standing in a valley with hilltops on the east and
the west can shorten the day significantly. This is why settlements
in east-west running valleys are more favourable (daylight-wise)
than north-south running valleys.
Day arcs of the Sun
Some of the above statements can be made clearer when picturing the
day arc (i.e. the path the Sun tracks along the celestial dome in
movement). The pictures
show this for every hour on equinox day. In addition, some 'ghost'
suns are also indicated below the horizon, up to 18° down. The Sun
in this area still causes twilight
pictures can be used for both Northern and Southern hemispheres.
The observer is supposed to sit near the tree on the island in the
middle of the ocean; the green arrows give cardinal directions.
- On the northern hemisphere, north is to the left, the Sun rises
in the east (far arrow), culminates in
the south (right arrow) while moving to the right and setting in
the west (near arrow).
- On the southern hemisphere, south is to the left, the Sun rises
in the east (near arrow), culminates in the north (right arrow)
while moving to the left and setting in the west (far arrow).
The following special cases are depicted:
- The day arc on the Equator, passing through the zenith, has almost no shadows at high noon.
- The day arc on 20° latitude: the Sun culminates at 70° altitude
and also its daily path at sunrise and sunset occurs at a steep 70°
angle to the horizon. Twilight is still about one hour.
- The day arc on 50° latitude: twilight is almost two hours
- The day arc on 70° latitude: the Sun culminates at no more than
20° altitude and its daily path at sunrise and sunset is at a
shallow 20° angle to the horizon. Twilight is more than four hours;
in fact, there is barely any night.
- The day arc at the Pole: if it were not for atmospheric
refraction, the Sun would be on the horizon all the time.
Celestial co-ordinate systems
The vernal point (vernal equinox) — the one the Sun passes in March
on its way from south to north — is used as the origin of some
Because of the precession of the
, the position of the vernal point changes over
time and as a consequence, both the Equatorial and the ecliptic
co-ordinate systems change over time. Therefore, when specifying
celestial co-ordinates for an object, one has to specify at what
time the vernal point (and also the celestial equatorial) are
taken. That reference time is also called equinox
The autumnal equinox is at ecliptic longitude 180° and at right
The upper culmination
of the vernal
point is considered the start of the sidereal day
for the observer. The hour angle
of the vernal point is, by definition,
the observer's sidereal time
For western tropical astrology
the same thing holds true; the vernal equinox is the first point
(i.e. the start) of the sign of Aries
. In this system, it is of no
significance that the fixed stars and equinox shift compared to
each other due to the precession
of the equinoxes
Cultural aspects of the equinox
- The traditional East Asian calendars divide a year into 24
solar terms (節氣, literally "climatic
segments"), and the vernal equinox (Chūnfēn,
) and the autumnal equinox (Qiūfēn, ) mark
the middle of the spring and autumn seasons, respectively. In this context, the Chinese character 分 means "(equal)
division" (within a season).
- In Japan, (March) Vernal Equinox Day (春分の日 Shunbun no
hi) is an official national
holiday, and is spent visiting family graves and holding family
reunions. Similarly, in September, there is an Autumnal Equinox Day
(秋分の日 Shūbun no hi).
- Wiccans and many other Neopagans hold religious celebrations of
"Ostara" on the spring equinox, and
"Mabon" on the autumnal equinox.
Vernal equinox commemorations
Chichen Itza pyramid during the spring
equinox - Kukulkan, the famous descent of the snake
- The March equinox marks the first day of various calendars
including the Iranian calendar and
the Bahá'í calendar.
Persian (Iranian) new year's
festival of Nowruz is celebrated then.
According to the ancient Persian mythology Jamshid, the
mythological king of Persia, ascended to the throne on this day and
each year this is commemorated with festivities for two weeks.
These festivities recall the story of creation and the ancient
cosmology of Iranian and Persian people. It is also a holiday
for Azerbaijan, Afghanistan, India, Turkey, Zanzibar, Albania, and various
countries of Central Asia, as well as
among the Kurds. As well as
being a Zoroastrian holiday, it is
also a holy day for adherents of the Bahá'í Faith and the Nizari Ismaili Muslims.
- Sham El Nessim was an ancient Egyptian holiday which can be traced
back as far as 2700 B.C. It is still one of the public holidays in Egypt. Sometime
during Egypt's Christian period (c. 200-639) the date moved to
Easter Monday, but before then it
coincided with the vernal equinox.
- The Jewish Passover usually falls on the first full moon after the Northern Hemisphere vernal
equinox, although occasionally (7 times every 19 years) it will
occur on the second full moon.
- The Christian churches calculate Easter as the first
Sunday after the first full moon on or after the March equinox. The
official church definition for the equinox is March 21; however, as
the Eastern Orthodox
Churches use the older Julian
calendar, while the Western Churches use the Gregorian calendar, both of which
designate March 21 as the equinox, the actual date of Easter differs. The earliest possible
Easter date in any year is therefore March 22 on each calendar. The
latest possible Easter date in any year is April 25.
- Tamil and Bengali New Years follow
the Hindu zodiac and are celebrated according
to the sidereal vernal equinox (April 14). The former is
celebrated in the South Indian state of Tamil Nadu, and the latter in Bangladesh and the East Indian state of
- Andhra Pradesh, Karnataka and Maharastra people celebrate new year ugadi set by Satavahana on
the first morning after first new moon from March equinox.
Also the calculations of the great Indian Mathematician Bhaskaracharya proclaim the Ugadi day as the
beginning of the New Year, New month and New day.
- In many Arab countries, Mother's Day is celebrated on the March
- World Storytelling Day is
a global celebration of the art of oral storytelling, celebrated
every year on the spring equinox in the northern hemisphere, the
first day of autumn equinox in the southern.
- World Citizen Day occurs on the
Maryland, USA, boatyard
employees and sailboat owners celebrate the spring equinox with the
Burning Of The Socks festival.
Traditionally, the boating community wears socks only during the
winter. These are burned at the approach of warmer weather, which
brings more customers and work to the area. Officially, nobody then
wears socks until the next equinox.
- Kerala, a state of
India celebrates the celestial vernal equinox as their
New year around April 14. It is known as 'Vishu' meaning equal in Sanskrit.
- Earth Day was initially celebrated on
March 21, 1970, the equinox day. It is currently celebrated in
various countries on April 22.
Autumnal equinox commemorations
- The September equinox marks the first day of Mehr or Libra in
the Persian calendar. It is one of
the Iranian festivals called
Jashne Mihragan, or the festival of sharing
or love in Zoroastrianism.
- In Korea, Chuseok is a major harvest
festival and a three-day holiday celebrated around the Autumn
- The Mid-Autumn Festival is
celebrated on the 15th day of the 8th lunar
month, oftentimes near the autumnal equinox day, and is an
official holiday in China and in many countries with a significant
Chinese minority. As the lunar calendar is not synchronous with the
Gregorian calendar, this date could be anywhere from mid-September
to early October.
- The traditional harvest
festival in the United Kingdom was celebrated on the Sunday of
the full moon closest to the September equinox.
- The September equinox was "New Year's
Day" in the French
Republican Calendar, which was in use from 1793 to 1805. The
French First Republic was
proclaimed and the French
monarchy was abolished on September 21, 1792, making the
following day (the equinox day that year) the first day of the
"Republican Era" in France. The start of every year was to be
determined by astronomical calculation, (that is: following the
real Sun and not the mean Sun as all other calendars).
Myths, fables and facts
- One effect of equinoctial periods is the temporary disruption
satellites. For all geostationary satellites, there are a
few days near the equinox when the sun goes directly behind the
satellite relative to Earth (i.e. within the beamwidth of the
groundstation antenna) for a short period each day. The Sun's
immense power and broad radiation spectrum overload the Earth
station's reception circuits with noise and, depending on antenna
size and other factors, temporarily disrupt or degrade the circuit.
The duration of those effects varies but can range from a few
minutes to an hour. (For a given frequency band, a larger antenna
has a narrower beamwidth, hence experiences shorter duration
"Sun outage" windows).
- A modern urban legend claims that
on the March equinox day (some may also include the September
equinox day rather than leaving it out), one can balance an egg on
its point. However, one can balance an egg on its point any day of
the year...if one has enough patience.
- Although the word equinox is often understood to mean
"equal [day and] night," as is noted elsewhere, this is not
strictly true. For most locations on earth, there are two distinct
identifiable days per year when the length of day and night are
closest to being equal; those days are commonly referred to as the
"equiluxes" to distinguish them from the equinoxes. Equinoxes are
points in time, but equiluxes are days. By convention, equiluxes
are the days where sunrise and sunset are closest to being exactly
12 hours apart. This way, you can refer to a single date as being
the equilux, when in reality, it spans from sunset on one day to
sunset the next, sunrise on one to sunrise the next or lunchtime on
one day to lunchtime on the next.
- What is true about the equinoxes is that two observers at the
same distance north and south of the equator will experience nights
of equal length.
- The equilux counts times when some direct sunlight could be
visible, rather than all hours of usable daylight (which is any
time when there is enough natural light to do outdoor activities
without needing artificial light). This is due to twilight; a
particular type of twilight which is officially defined as civil twilight. This amount of twilight can
result in more than 12 hours of usable daylight up to a few weeks
before the spring equinox, and up to a few weeks after the fall
- In a contrary vein, the daylight which is useful for
illuminating houses and buildings during the daytime and is needed
to produce the full psychological benefit of daylight, is
shorter than the nominal time between sunrise and sunset.
So in that sense, "useful" daylight is present for 12 hours
only after the vernal equinox and before the
autumnal equinox, because the intensity of light near sunrise and
sunset, even with the sun slightly above the horizon, is
considerably less than when the sun is high in the sky.
- It is perhaps valuable for people in the Americas and Asia to
know that the equinoxes listed as occurring on March 21, which
occurred frequently in the 20th century and which will occur
occasionally in the 21st century, are presented as such using
UTC, which is at least
four hours in advance of any clock in the Americas and as much as
twelve hours behind Asian clocks. Thus, there will be no spring
equinox later than March 20 in the Americas in the coming
- Baha'i calendar
- Keith's Moon Facts
- "the utmost global citizen". Global Culture
- Annapolis Welcomes Spring by Burning Socks
- Infernal Egguinox Standing an egg on end on the spring
- Equinox Means Balanced Light, Not Balanced
- De-Fact-o article on the egg equinox myth