New Delhi: The fall season begins on September 1, according to meteorologists. However, it’s the Autumn Equinox that marks the beginning of the astronomical fall season in the Northern Hemisphere, and the astronomical spring season in the Southern Hemisphere. The Autumn Equinox can occur any time between September 21 and September 24. This year, it will occur at 12:51 am, September 23, according to NASA.
What Is Autumn Equinox?
Equinox is an astronomical event caused by Earth's tilt on its axis as it revolves around the Sun. On this day, Earth's axis is tilted 23.5 degrees from the plane of its orbit.
In the Autumn Equinox, there is fall or autumn north of the equator.
On the first day of the fall in the Northern Hemisphere, the Earth is perfectly aligned sideways to the Sun, or in other words, the Sun will be exactly above Earth's equator, moving from north to south. As a result, day and night will be nearly equal in length, throughout the world.
According to NASA solar scientist Mitzi Adams, a kind of twilight will be prevalent at the North Pole from now until sometime in October because over the next few days after the Autumn Equinox, the Sun will go below the horizon.
While it is Autumn Equinox in the Northern Hemisphere, there will be Spring Equinox in the Southern Hemisphere. The Spring or Vernal Equinox occurs in the Northern Hemisphere on March 20 or 21.
Equinoxes happen only twice a year. Because of the Earth's axis being tilted with respect to the Sun-Earth plane, the Sun shines unevenly over the Northern and Southern Hemispheres and it is the tilt which results in seasons. However, on the spring and autumn equinoxes, the Sun shines almost equally on the Northern and Southern Hemispheres.
Interesting Facts About Autumn Equinox
1. Equinox is an instantaneous phenomenon
Equinox is usually thought of as a phenomenon lasting an entire day. However, it is a single moment in time, when the Sun crosses the celestial equator, which is an imaginary line in the sky above Earth's equator. Equinox is a moment when Earth's axis neither tilts away nor towards the Sun. After Autumn Equinox, there will be earlier sunsets and later sunrises in the Northern Hemisphere.
2. Day and Night are not precisely 12 hours each
Aequus, meaning 'equal', and nox, meaning 'night', are the Latin words from which the word equinox has been derived. However, day and night are not precisely 12 hours each on the day of fall or spring equinox. In some places in the Northern Hemisphere, the Sun will appear to be above the horizon when it is actually below it, because of the refraction of sunlight, according to National Weather Service.
People living far from the Equator will experience longer days.
Though the centre of the Sun sets 12 hours after sunrise, the day begins a little before the centre of the Sun rises, because the upper edge of the Sun rises above the horizon before the centre. Similar is the case with sunset, because it doesn't occur until the entire Sun completely sinks below the horizon. This is why the days are still a bit longer when the Autumn Equinox begins, and a few days after that, equal days and nights start occuring.
3. Equinox does not occur on a fixed day
The Autumn Equinox usually occurs on September 22 or 23, but occasionally, it can even occur on September 21 or 24. This happens because the Earth takes 365.25 days or 365 days and 6 hours to orbit the Sun, which means the Equinox occurs 6 hours later than the time at which it happened the previous year. The Gregorian calendar does not consider those six hours. Instead, these six hours account for the additional day in a leap year. Because of leap years, the day of the equinox is reset to September 22 or 23. 1931 was the last time Autumn Equinox happened on September 24, and it will happen next in 2303.
4. Equinoxes signal the start of Northern Lights
Aurora Borealis refers to a spectacular view of colourful lights in the night sky. Autumn Equinox is the prime time for viewing Aurora Borealis or Northern Lights. This is because geomagnetic storms happen twice as frequently than the annual average. During the equinox, solar winds or the particles of plasma are better able to reach Earth's atmosphere, due to the Earth's axial tilt, through our geomagnetic field, and this interaction results in the strongest geomagnetic storms. The particles of plasma interact with atoms of oxygen, nitrogen and other elements in the atmosphere, and release photons of different wavelengths, causing beautiful colours of aurora.
5. Harvest Moon in the Autumnal Equinox
The first full moon occuring after the Autumn Equinox is termed as the Harvest Moon. This is the time when moonrise occurs earlier in the evening, as a result of which farmed are able to work longer. When the harvest moon occurs in October, it is known as the full corn moon because thay is the time of the corn harvest.
In Britain, and in churches, Harvest Festival is celebrated on the Sunday closest to the Harvest Moon— people donate non-perishable food items to the underprivileged.
6. Autumn Equinox once marked the start of the new year
According to the French Republican Calender, the Autumn Equinox marked the official start of every new year, between 1793 and 1805. This is because the French monarchy was eradicated one day before the equinox in 1792. From 1793 to 1805, the first day of each year began when the autumnal equinox fell at the Paris Observatory.
7. Sun rises due east and sets due west
During the Autumn and Spring Equinoxes, the Sun rises due east and sets due west, which means that the cardinal compass point is at 90 degrees, or straight towards the east and west, respectively.
8. Defining colours of autumn leaves
Chemicals such as flavonoids, carotenoids and anthocyanins become more prominent in leaves after the Autumn Equinox, while Chlorophyll decreases, and this results in the vibrant ambers, reds and yellows of autumn.
9. Autumn Equinox in Greek mythology
When Persephone, a Greek Goddess and the daughter of Zeus and Demeter, was abducted by Hades, the God of the Underworld, to make her his Queen, autumn began, according to Greek mythology. This is because Demeter, the Goddess of Harvest, destroyed all crops on Earth until her daughter was allowed to return. When Hades returned Persephone to Demeter, the goddess of nature allowed crops to grow again, marking the beginning of Spring.
10. Origin of the word autumn
The word autumn is derived from the French word Autumne. Also, fall, which is used to describe the Autumn season, is a shortening of the phrase "fall of the leaf".
11. Snake of Sunlight in Mexico
On the day of the Autumnal Equinox and the Spring Equinox, afternoon sunlight bathes the western side of the main stairway of Mayan Pyramid of El Castillo, in Mexico. This creates a shadow which imitates the body of a 120 foot long rattlesnake, creeping downwards.
12. Apple Magic practised during Autumn Equinox
Pagans celebrate Autumn Equinox with a harvest festival known as Mabon, which is a time for practising Apple Magic. In Apple Magic, a person peels an apple, keeping it in one long piece, and drops it on the floor when it comes off. The letter formed is the first initial of the person's true lover's name. The apple is also associated with immortality by the Pagans.