The 23rd of September this year is the day of September Equinox. The day of equal day and night they say. But is it really? Let’s find out.
What is Equinox and What’s so special?
Equinox is an astronomically significant day when the duration of the day and the night on the Earth is equal. Typically, this occurs twice – on the 21st or the 22nd of March and September, every year. On the day of September Equinox, the Sun moves from the north to the south of the Celestial Equator, initiating Winter in the Northern and Summer in the Southern Hemisphere, through Autumn and Spring respectively. On this day, the Sun shines directly on the equator which leads to equally lit and unlit surfaces on Earth. Also, the Sun rises exactly from the east and sets exactly in the west.
Equal Day and Night?
Also, on this day, the Sun’s geometric centre is above the horizon for 12 hours and below the horizon for 12 hours. However, on Equinox, the day is slightly longer than the night at the equator, as ‘a day’ is the duration between the appearance and disappearance of the upper edge of the Sun’s disc at the horizon.
Find out more from this post by timeanddate.com.
Astronomy Behind the September Equinox
Astronomically, September Equinox is that instance when the Sun shines exactly from the point where the Ecliptic and the Celestial Equator intersect, as it makes its way below the Celestial Equator . This point on the celestial sphere is called the First Point of Libra. Due to the precession of the Earth’s axis, this point has now drifted into the constellation of Virgo.
September Equinox also marks the beginning of the Fall Equinox – An astronomical season between September Equinox and the December Solstice.
How to Celebrate / Observe?
The Latitude of a place can be calculated on Equinox by measuring the length of the shortest shadow of an object, say a long candle set vertically on the ground, around noon. The shadow forms the Opposite and the candle forms the Adjacent of the right-angled triangle, θ will give the latitude.
Since the ratio of the opposite and adjacent gives a trigonometric tangent of the triangle, an inverse tan of the ratio will reveal the latitude of the location from where the observations are made.
However, you’d have to wait for Local Noon, which varies place to place depending upon the time difference between your longitude and the longitude that determines the time zone for your place.
Local Name for the Equinox.
In Indian Astronomy, this day is called Vishuvat Sankranti as ‘the Sun Enters’ (Sankranti) the Equinox (Vishuvat) on this day. It is also called Sharad Vishuva, as it brings with it – Sharad Ruthu (Autumn season).
Exact Date and Time
This year, September Equinox is on the 23rd Sept 2021 at 12:44AM IST 22nd Sept 07:21PM UTC
