Although astronomy events are those that bring awe during the night, some events from astronomy can also be enjoyed during daylight. One such event is the Equinox. The September Equinox will occur on the 23rd of September 2021.
Equinox is an astronomically significant day when the duration of the day and the night on the Earth is equal. Typically, this occurs twice –in March and in September, every year.
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.
On the day of September Equinox, the Sun moves from the north to the south of the Celestial Equator (Earth’s equator extended into space), initiating Winter in the Northern Hemisphere, and hence, is also known as Autumnal Equinox.
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.
The best way to observe equinox is through a simple experiment which will help us determine the latitude of the place we live. The Latitude of a place can be calculated on this day by measuring the length of the shortest shadow (measured between 12pm to 1pm) of an object observed, say a candle set vertically on the ground, around noon. The shadow forms the Opposite and the pen forms the Adjacent of the right-angled triangle, θ will give the latitude.
Astronomically, Equinox is that instance when the Sun shines exactly from the point where the Ecliptic (Path of the Sun) and the Celestial Equator (Earth’s equator extended into space) intersect. This point for September Equinox 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. As the Sun passes through this point, it will appear to move towards the South with each passing day until December.
In India, we call this day “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) in the Northern Hemisphere. The word Equinox means Equal Night in Latin.
Poornaprajna Amateur Astronomers’ Club wishes everyone a clear, cloudless sky on this afternoon to perform the beautiful experiment and observe the day of Autumnal Equinox.
-Mr. Atul Bhat
Asst. Professor,
Dept of physics Poornaprajna College, Udupi
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