Autumnal Equinox: All about this natural phenomenon
Every six months, once in March and once in September, an equinox happens. On that day, we get equal halves of light and darkness, i.e.12 hours of daylight and 12 of night. We had the vernal or spring equinox this year on March 21 in the Northern Hemisphere. And on September 23, we will have the autumnal equinox. The dates are opposite in the Southern Hemisphere, following their seasonal cycle. Also, some countries will experience equinox on the 22nd of September, and not 23rd, following local calendars.
How does an equinox happen?
Our planet orbits the sun on an axis tilted at 23.5 degrees. Twice in the year, Earth’s orbit and its axial tilt combine to put the sun just above Earth’s Equator. This casts the dividing line between the light and dark parts of the planet, called the terminator or twilight zone, right through the North and South Poles. The terminator doesn’t divide the planet into perfect dark and light halves though. Earth’s atmosphere bends sunlight by 60 kilometres, which equals half a degree. So even on equinox, one half of our planet stays brighter a little longer than the other.
Types of equinoxes
There are two kinds of equinoxes, the vernal (spring) equinox and the autumnal (autumn or fall) equinox.
Vernal equinox: The word ‘vernal’ means anything that is vibrant and fresh like spring. During March, the equinox that occurs in the northern hemisphere is called the vernal equinox. In September, when it’s spring in the southern hemisphere, then the vernal equinox happens there. This marks the onset of spring in both the hemispheres but at very different times. The sun both rises earlier and sets later than usual, with its rays directly falling on the equator because of earth’s slanting.
Autumnal equinox: Like the vernal equinox, the autumnal equinox also happens twice a year and follows the same process. In the northern hemisphere, this phenomenon occurs on 22nd or 23rd September, while in the southern hemisphere it occurs on 20th or 21st March. This marks the start of autumn or fall in both the hemispheres respectively.
Unique facts about the equinoxes
- During an equinox, people living close to the equator experiences the sun directly above their heads.
- While the north pole welcomes six months of continuous daylight, the south pole has to spend six continuous months in darkness.
- During equinoxes, a divider or the twilight zone, as its name suggests separates the line between the illuminated and darkened zones, through the north and south poles directly.
- In the northern hemisphere, when the autumnal equinox happens, a clear visual of the northern lights, also known as aurora borealis, can be seen.
Does equinox happen only on Earth?
No, every planet in our solar system has them. In 2009, the Cassini space shuttle captured an equinox happening on Saturn. As on Earth, equinoxes happen every half-year on Saturn. But 1 Saturn year is equal to 15 Earth years, so Cassini's photo session was really lucky.
Equinox and human civilization
Ancient cultures were aware of the equinoxes and tracked them. Pyramids, sun dials, stone engravings that acted as calendars and clock-style sun architecture in churches marked equinoxes. This was very important as the spring equinox marks the beginning of spring, just like the one in September will usher in autumn. Apart from planning seasonal farming and festival activities, some old cultures celebrated equinoxes. The Lakota tribe of USA smoke sacred tobacco to mark seasonal change. At Stonehenge's equinox celebrations in England, people still gather to mark how druids (tribal doctors) used to come there to mark the passage of seasons.
(With inputs from Pallavi Kanungo)