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Autumnal Equinox 2021 Is Here!

September 21, 2021    7 min read

The next season is upon us! The September (or autumnal) equinox is the beginning of the astronomical autumn season in the Northern Hemisphere and astronomical spring season in the Southern Hemisphere. This happened on September 23, 2021 in India.

An equinox is the point in time when the plane of Earth's equator passes through the geometric center of the Sun's disk. During an equinox, both hemispheres of the Earth experience the same amount of sunlight and darkness – 12 hours each. This occurs twice each year, around 20 March (Spring equinox) and 23 September.

In the Northern hemisphere, the September equinox marks the start of a period of later sunrises and earlier sunsets. We will also feel days getting colder and trees shedding leaves. This will be true for 90% of the world’s population as the Northern hemisphere houses 90% of the world!

Why do equinoxes happen?

Equinox only happens twice in the Earth’s year-long trip around the Sun. The rest of the year, the Sun shines unevenly over the Northern and Southern Hemispheres. That’s because the Earth’s axis is tilted with respect to the Sun-Earth plane.

The axis around which the Earth spins isn't straight vertical line - it's about 23.4 degrees tilted. Because of that, different parts of the Earth get more or less sunlight as the planet rotates around the sun. That's why we have seasons too and why the two hemispheres experience opposite seasons!

Source: NASA

Difference between Solstice & Equinox

We have covered solstices in the past in the Practically blog, so you may know that the solstices and equinoxes signal the changing of the seasons on Earth, but do you remember if they are just different names for the same thing? Actually, a solstice and an equinox are opposites.

There are two solstices which happen in June (20 or 21) and December (21 or 22). These are the days when the Sun’s path in the sky is the farthest north or south from the Equator respectively. A hemisphere’s winter solstice is the shortest day of the year and its summer solstice the longest. In the Northern Hemisphere, the June or summer solstice marks the start of summer. This is when the North Pole is closest to the Sun, and the Sun’s rays are directly overhead at the Tropic of Cancer. The December or winter solstice marks the start of winter. At this point the South Pole is closest to the Sun, and the Sun’s rays are directly overhead at the Tropic of Capricorn.

Quick facts about Equinox

At the North Pole, over the next days, the Sun will sink below the horizon for a kind of twilight from now until sometime in October when it will be completely dark. Spring twilight at the North Pole begins a few weeks before the vernal, or spring equinox in March, when the Sun rises above the horizon again. That’s about six months till people in the North Pole will see the sun rise again!

The people of ancient cultures used the sky as a clock and calendar. They knew that the Sun’s path across the sky, length of daylight, and location of celestial bodies, sunrise and sunset all moved in a particular way throughout the year indicating various phenomena. Additionally, earlier civilizations built the first observatories for these celestial events, like Stonehenge in Wiltshire, England, and the Intihuatana stone in Machu Picchu, Peru, to follow the bodies’ annual progress.

We are already in the next season of the year now and we hope this season will be filled with learning. If you enjoyed this blogpost in the Celestial Saga series, and would like to know more information on planets and space, with the help of fun, life-like 3D videos, simulations and AR, download the Practically app now and Bring Learning Alive!

#equinox #autumnequinox #3Dvideos #AR #simulations #practically #learnpractically #learningapp

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