Equinox is an astronomically significant day when the duration of the day and the night on the Earth is equal. Every year, this occurs twice on the 20th or the 21st of March and September.
On the day of March Equinox, the Sun moves from the south to the north of the celestial equator initiating Summer with the Spring season in the Northern Hemisphere and the Fall season in the South.
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.
Sun’s Position
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.
On the day of the Equinox, at the equator, the Sun’s geometric center is above the horizon for 12 hours and below the horizon for 12 hours.
Determine Latitude – Experiment
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 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. Any straight object can be used. The Taller the Object, the better the results.
The line formed by the base of the object and the tip of the shadow (when shortest) will lie along the north-south direction, and the shadow will be pointing exactly towards the North.
Significance and Names
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. As the Sun passes through this point, it will appear to move towards the North with each passing day until June.
In India, we call March Equinox as “Vishuvat Sankranti” as ‘the Sun Enters’ (Sankranti) the Equinox (Vishuvat) on this day. It is also called “Vasantha Vishuva”, as it brings with it – Vasantha Ruthu (Spring season) in the Northern Hemisphere. The word Equinox means Equal Night in Latin.
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More Info:
- Equinox : https://www.wikiwand.com/en/Equinox
- March Equinox : https://www.wikiwand.com/en/March_equinox