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 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, initiating Winter in the Northern and Summer in the Southern Hemisphere, through Autumn and Spring respectively.
Astronomy
Astronomically, an Equinox is that instance when the Sun shines exactly from the point where the Ecliptic and the Celestial Equator intersect.
Imagine that the Equator extends into the space which we refer to as the Celestial Equator. The Sun lies to the North of the celestial equator from March to September causing Summer in the Northern Hemisphere and Winter in the Southern Hemisphere. The Sun moves to the South of the Celestial Equator from September to March causing the seasons to reverse in both the hemispheres. The Equinox marks the day when the Sun crosses the Celestial Equator.
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.
Measuring Latitude
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.
On this day, the Sun shines directly on the equator which leads to equally lit and unlit surfaces on Earth.
However, on equinox, the day is slightly longer than the night at the equator, and no day and night at .the equator is of equal duration.
Sun’s Position
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.
Vishu
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).
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More Info:
- Equinox : https://www.wikiwand.com/en/Equinox
- September Equinox : https://www.wikiwand.com/en/September_equinox