The first day of spring. The Vernal Equinox: Everything you need to know
A guide to what the vernal equinox means and how you can make the most of it
When does spring start in 2016?
The astronomical spring in the Northern Hemisphere begins today, Sunday, March 20.
The Spring (vernal) equinox in the Northern Hemisphere is also known as the March equinox. It’s called the “autumnal (fall) equinox” in the Southern Hemisphere.
Spring begins in
The March equinox marks the moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from south to north.
This happens on March 19, 20 or 21 every year.
Photo: Keith Tsuji/Getty Images
Why is it Called “Equinox”?
Since night and day are nearly exactly the same length – 12 hours – all over the world the event is called the equinox, which in Latin, literally means ‘equal night’ (equi – equal and nox – night).
In reality though, equinoxes do not have exactly 12 hours of daylight.
Solstices and equinoxes mark key stages in the astronomical cycle of the earth. In a year there are two equinoxes (spring and autumn) and two solstices (summer and winter).
The dates of the equinoxes and solstices aren’t fixed due to the Earth’s elliptical orbit of the sun. The Earth’s orbit around the sun means that in early January, the sun is closest (known as perihelion) and in early July it is most distant (aphelion).
What happens on an equinox?
The Earth’s axis always tilts at an angle of about 23.5° in relation to the ecliptic, i.e the imaginary plane created by the Earth’s orbit around the Sun.
On any other day of the year, either the Northern Hemisphere or the Southern Hemisphere tilts a litte towards the Sun but on the two equinoxes, the tilt of the Earth’s axis is perpendicular to the Sun’s rays.
The equinox happens at exactly the same time around the world.
The equinox occurs at the exact moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s Equator – from south to north. At this moment, the Earth’s axis is neither tilted away from nor towards the Sun.
In 2016, this happens at 4:30 am UTC (GMT).
The first day of spring 2016 is celebrated by Google with a Doodle.
Last year’s Doodle marked the day with flowers. Because what better way is there to mark spring?
Celebrating new Beginnings
Photo: Jo Whitworth
The March equinox has long been celebrated as a time of rebirth in the Northern Hemisphere. Many cultures celebrate spring festivals and holidays around the March equinox, like Easter and Passover.
Japan’s cherry blossom
In Japan, as spring approaches, the arrival of cherry blossom (Sakura) is hotly anticipated as the nation turning a shade of pink with the arrival. Months before they arrive, retailers switch into sakura mode, Danielle Demetriou writes. The countdown excitement is heightened further by the televised Cherry Blossom Forecast which offers a petal-by-petal analysis of the advance of the blooms – known as the cherry blossom front – as they sweep from the south to the north of the archipelago.
The colourful Hindu festival of Holi is celebrated as the vernal equinox approaches. The ancient religious festival has become popular across the world, including celebrations across the UK, though it is mainly observed in India and Nepal. The celebrations begin with a bonfire the previous evening where revellers sing and dance. The following day involves participants throwing colour over each other, either with dry powder and water or coloured water-filled balloons. The festival signifies the victory of good over evil with the onset of spring and the end of winter.
Easter also has associations with the Spring break enjoyed by college students in the US, and has connotations of beach holidays and raucous parties. The 2012 film Spring Breakers starring Selena Gomez, Vanessa Hudgens and James Franco takes this to an extreme as it follows James Franco’s gangster rapper as he takes four bikini-clad young women on a tour of Florida.
Easter Spring Lamb recipes
- Jose Pizarro’s take on Spring Lamb involves a roast rack of lamb with braised peas and lemon-thyme salsa.
Plump lamb chops cured in garlic, ginger and chilli are smoked over green tea and wood chips before grilling. Delicious with a spicy Korean miso.
A simple and quick dish of pan-fried lamb teamed with warm broad beans and fresh mint.
A rich and vibrant stew of chunky lamb with red peppers and coriander.
A beautiful stuffed roast saddle, served with a saffron jus.
The Easter Bunny
Rabbits and hares have been associated with spring since ancient times. It is thought that the Ango-Saxon Goddess of Spring, Eostre, had a hare as her companion, which symbolised fertility and rebirth.
It’s hardly surprising that rabbits and hares have become associated with fertility as they are both prolific breeders and give birth to large litters in early spring.
The legend of the Easter Bunny is thought to have originated among German Lutherans, where the ‘Easter Hare’ judged whether children had been good or bad in the run-up to Easter.
Over time it has become incorporated into Christian celebrations and became popular in Britain during the 19th century.
Many children believe that the Easter Bunny lays and hides baskets of coloured eggs, sweets and sometimes toys in their homes or around the garden the night before Easter Sunday – much like Father Christmas delivering gifts on Christmas Eve.
This has given rise to the tradition of the Easter egg hunt which is still popular among children today.
Vernal Equinox Date
Vernal Equinox Local Time
Sunset = 18:32:26
Day Duration = 12 Hours 07 Mins 37 Secs
Previous Day Duration = 12 Hours 05 Mins 53 Secs
Next Day Duration = 12 Hours 09 Mins 20 Secs
An Equinox is an astronomical event that happens twice, once in spring and once in autumn, each year when the tilt of the Earth’s axis is inclined neither away from nor towards the Sun. During Equinoxes the tilt of the Earth (with respect to the Sun) is 0° and because of it duration of the day and the night are almost equal on Equinox day i.e. 12 hours.
Equinoxes occur on 20th or 21st March and 22nd or 23rd September each year and both days has equal length of the day and the night.
During March it is spring time in the Northern hemisphere and March Equinox is called Vernal Equinox and Spring Equinox while it is autumn time in the Southern hemisphere and March Equinox is called Autumnal Equinox and Fall Equinox. In other words on March Equinox it is spring time in the UK, the USA, Canada, Russia, India and China while it is autumn time in Australia, Argentina, Chile, New Zealand and South Africa.
Similarly, during September it is autumn time in the Northern hemisphere and September Equinoxis called Autumnal Equinox and Fall Equinox while it is spring time in the Southern hemisphere and September Equinox is called Vernal Equinox and Spring Equinox. In other words on September Equinox it is autumn time in the UK, the USA, Canada, Russia, India and China while it is spring time in Australia, Argentina, Chile, New Zealand and South Africa.
To avoid any confusion Equinoxes are preferably referred as March Equinox (Northern Equinox) and September Equinox (Southern Equinox).
In Hindu astrology Vernal Equinox is known as Vasant Vishuva or Vasant Sampat. Due to precession Hindu equivalent of Vernal Equinox has drifted apart and celebrated on Mesha Sankranti.
|UT date and time of
equinoxes and solstices on Earth
On an equinox, day and night are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction. To avoid this ambiguity, the word equilux is sometimes used to mean a day in which the durations of light and darkness are equal.[note 1] SeeLength of equinoctial day and night for further discussion.
The oldest meaning of the word “equinox” refers to a day when daytime and nighttime are of approximately equal duration.
The word “equinox” comes from this definition and is derived from the Latin aequinoctium, aequus (equal) and nox (genitivenoctis) (night).
Equinoxes on Earth
The equinoxes are the only times when the solar terminator (the “edge” between night and day) is perpendicular to the equator. As a result, the northern and southern hemispheres are equally illuminated.
In other words, the equinoxes are the only times when the subsolar point is on the equator, meaning that the Sun is exactly overhead at a point on the equatorial line. The subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox.
The equinoxes, along with solstices, are directly related to the seasons of the year. In the northern hemisphere, the vernal equinox (March) conventionally marks the beginning of spring in most cultures and is considered the New Year in the Persian calendar or Iranian calendars as Nouroz (means new day), while the autumnal equinox (September) marks the beginning of autumn. In the southern hemisphere, the vernal equinox occurs in September and the autumnal equinox in March.
Diagram of the Earth’sseasons as seen from the north. Far right: December solstice.
When Julius Caesar established the Julian calendar in 45 BC, he set 25 March as the date of the spring equinox. Because the Julian year (365.25 days) is slightly longer than the tropical year, the calendar “drifted” with respect to the two equinoxes — such that in 300 AD the spring equinox occurred about 21 March. By 1500 AD, it had drifted backwards to 11 March.
This drift induced Pope Gregory XIII to create a modern Gregorian calendar. The Pope wanted to continue to conform with the edicts concerning the date of Easter of the Council of Nicaea of AD 325, which means he wanted to move the vernal equinox to 21 March, which is the day allocated to it in the Easter table of the Julian calendar. However, the leap year intervals in his calendar were not smooth (400 is not an exact multiple of 97). This causes the equinox to oscillate by about 53 hours around its mean position. This in turn raised the possibility that it could fall on 22 March, and thus Easter Day might theoretically commence before the equinox. The astronomers chose the appropriate number of days to omit so that the equinox would swing from 19 to 21 March but never fall on the 22nd (although it can in a handful of years fall early in the morning of that day in the Far East).
- Spring equinox and fall (or autumn) equinox: colloquial names based on the seasons. However, these can be ambiguous since the northern hemisphere‘s spring is the southern hemisphere‘s autumn, and vice versa. The Latinatenames vernal equinox (spring) and autumnal equinox (fall) are often used to the same effect.
- March equinox and September equinox: names referring to the months of the year they occur, with no ambiguity as to which hemisphere is the context. They are still not universal, however, as not all cultures use a solar-based calendar where the equinoxes occur every year in the same month (as they do not in the Islamic calendar and Hebrew calendar, for example).
- Northward equinox and southward equinox: names referring to the apparent direction of motion of the Sun. The northward equinox occurs in March when the sun crosses the equator from south to north, and the southward equinox occurs in September when the sun crosses the equator from north to south. These terms can be used unambiguously for other planets.
- First Point of Aries and first point of Libra: names referring to the astrological signs the sun is entering. Due to theprecession of the equinoxes, however, the constellations where the equinoxes are currently located are Pisces andVirgo, respectively.
Length of equinoctial day and night
Day is usually defined as the period when sunlight reaches the ground in the absence of local obstacles. On the day of the equinox, the center of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, so night and day are about the same length. In reality, the day is longer than the night at an equinox. There are two reasons for this:
First, from the Earth, the Sun appears as a disc rather than a point of light, so when the centre of the Sun is below the horizon, its upper edge is visible. Sunrise, which begins daytime, occurs when the top of the Sun’s disk rises above the eastern horizon. At that instant, the disk’s center is still below the horizon.
Second, Earth’s atmosphere refracts sunlight. As a result, an observer sees daylight before the top of the Sun’s disk rises above the horizon. Even when the upper limb of the Sun is 0.4 degrees below the horizon, its rays curve over the horizon to the ground.
In sunrise/sunset tables, the assumed semidiameter (apparent radius) 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 the 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 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.
Because the Sun is an extended (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 (or vice versa). Far north of the Arctic Circle, at Longyearbyen, Svalbard, Norway, there is an additional 15 minutes more daylight every day about the time of the spring equinox, whereas in Singapore (which is just one degree of latitude north of the equator), the amount of daylight in each daytime varies by just a few seconds.
Geocentric view of the astronomical seasons
|This section does not cite any sources. (December 2011)|
In the half-year centered on the June solstice, the Sun rises north of east and sets north of west, 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 centered on the December solstice, the Sun rises south of east and sets south of west and the durations of day and night are reversed.
Also on the day of an equinox, the Sun rises everywhere on Earth (except at the poles) at about 06:00 and sets at about 18:00 (local time). These times are not exact for several reasons:
- The Sun is much larger in diameter than the Earth, so 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).
- Most places on Earth use a time zone which differs from the local solar time by minutes or even hours. For example, if the Sun rises at 07:00 on the equinox, it will set 12 hours later at 19:00.
- Even people whose time zone is equal to local solar time will not see sunrise and sunset at 06:00 and 18:00. This is due to the variable orbital speed of the Earth and the inclination of its orbit, and is described as the equation of time. It has different values for the March and September equinoxes (+8 and −8 minutes respectively).
- Sunrise and sunset are commonly defined for the upper limb of the solar disk, rather than its center. The upper limb is already up for at least a minute before the center appears, and the upper limb likewise sets later than the center of the solar disk. Also, when the Sun is near the horizon, atmospheric refraction shifts its apparent position above its true position by a little more than its own diameter. This makes sunrise more than two minutes earlier and sunset an equal amount later. These two effects combine to make the equinox day 12 h 7 min long and the night only 11 h 53 min. Note, however, that these numbers are only true for the tropics. For moderate latitudes, the discrepancy increases (e.g., 12 minutes in London); and closer to the poles it becomes very much larger (in terms of time). Up to about 100 km from either pole, the Sun is up for a full 24 hours on an equinox day.
- Night includes twilight. If dawn and dusk are instead considered daytime, the day would be almost 13 hours near the equator, and longer at higher latitudes.
- Height of the horizon changes the day’s length. For an observer atop a mountain the day is longer, while standing in a valley will shorten the day.
Day arcs of the Sun
Some of the statements above can be made clearer by picturing the day arc (i.e., the path the Sun tracks along the celestial dome in its diurnal 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° below it; the Sun in such areas still causes twilight. The depictions presented below can be used for both the northern hemisphere and the southern hemisphere. The observer is understood to be sitting near the tree on the island depicted in the middle of the ocean; the green arrows give cardinal directions.
- In 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).
- In 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:
Day arc at 0° latitude (equator)
The arc passes through the zenith, resulting in almost no shadows at high noon.
Celestial coordinate systems
The vernal equinox occurs in March, about when the Sun crosses the celestial equator south to north. The term “vernal point” is used for the time of this occurrence and for the direction in space where the Sun is seen at that time, which is the origin of some celestial coordinate systems:
- in the ecliptic coordinate system, the vernal point is the origin of the ecliptic longitude;
- in the equatorial coordinate system, the vernal point is the origin of the right ascension.
Strictly speaking, at the equinox the Sun’s ecliptic longitude is zero. Its latitude will not be exactly zero since the Earth is not exactly in the plane of the ecliptic. Its declination will not be exactly zero either. (The ecliptic is defined by the center of mass of the Earth and Moon combined.) The modern definition of equinox is the instants when the Sun’s apparent geocentric longitude is 0° (northward equinox) or 180° (southward equinox). See the diagram to the right.
Because of the precession of the Earth’s axis, the position of the vernal point on the celestial sphere changes over time, and the equatorial and the ecliptic coordinate systems change accordingly. Thus when specifying celestial coordinates for an object, one has to specify at what time the vernal point and the celestial equator are taken. That reference time is called the equinox of date.
The autumnal equinox is at ecliptic longitude 180° and at right ascension 12h.
The same is true in western tropical astrology: 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 equinoxes shift over time with respect to the fixed stars.
Using the current official IAU constellation boundaries – and taking into account the variable precession speed and the rotation of the celestial equator – the equinoxes shift through the constellations as follows (expressed in astronomical year numbering in which the year 0 = 1 BC, −1 = 2 BC, etc.):
- The March equinox passed from Taurus into Aries in year −1865, passed into Pisces in year −67, will pass into Aquariusin year 2597, will pass into Capricornus in year 4312. It passed along (but not into) a ‘corner’ of Cetus on 0°10′ distance in year 1489.
- The September equinox passed from Libra into Virgo in year −729, will pass into Leo in year 2439.
A number of traditional spring and autumn (harvest) festivals are celebrated on the date of the equinoxes.
Equinoxes on other planets
|This section does not cite any sources. (September 2012)|
Equinox is a phenomenon that can occur on any planet with a significant tilt to its rotational axis. Most dramatic of these is Saturn, where the equinox places its ring system edge-on facing the Sun. As a result, they are visible only as a thin line when seen from Earth. When seen from above – a view seen by humans during an equinox for the first time from the Cassini space probe in 2009 – they receive very little sunshine, indeed more planetshine than light from the Sun.
This lack of sunshine occurs once every 14.7 years. It can last a few weeks before and after the exact equinox. The most recent exact equinox for Saturn was on 11 August 2009. Its next equinox will take place on 30 April 2024.
One effect of equinoctial periods is the temporary disruption of communications satellites. For all geostationary satellites, there are a few days around the equinox when the sun goes directly behind the satellite relative to Earth (i.e. within the beam-width of the ground-station 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 beam-width and hence experiences shorter duration “Sun outage” windows.)
- Aphelion – occurs around 5 July (see formula)
- Lady Day
- Songkran (disambiguation)
- Sun outage – a phenomenon that occurs around the time of an equinox
- This meaning of “equilux” is rather modern (c. 2006) and unusual. Technical references since the beginning of the 20th century (c. 1910) use the terms “equilux” and “isophot” to mean “of equal illumination” in the context of curves showing how intensely lighting equipment will illuminate a surface. See for instance John William Tudor Walsh, Textbook of Illuminating Engineering (Intermediate Grade), I. Pitman, 1947.
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