Today 21st March, 2016 is Vernal Equinox day – Equinox is an event when Day and Night are of equal length. This is standard definition.
However it is not so in real practice. Those into mathematical part of Astrology and Astronomy will know that for any given day, Sunrise timing differs with change in Latitude. The Sunrise on equator and on an equinox will be earlier than on the poles – that is farther a location is from equator, Sunrise will be late by several minutes.
The date of Vernal Equinox will wary too – this can be March 19, 20 or 21 – surprised?? Remember when Spring Equinox was celebrated on March 21? Well, remember again, because the last time that happened was in 2007 and the next time it will happen is in 2101!
Contrary to popular wisdom, the March Equinox can take place on March 19, 20 or 21. In the 21st century, the March Equinox has only occurred twice on March 21 – 2003 and 2007. A March 19 equinox will be more frequent during the last decades of the century.
All of this has to do with definition of Sunrise – On the equator, the day and night stay approximately the same length all year round, but the day will always appear a little longer than 12 hours, due to the reasons below.
On the equinoxes, the geometric center of the sun is above the horizon for 12 hours, and you might think that the length of the day (hours of daylight) would be 12 hours too.
However, ‘sunrise’ is defined as the moment the upper edge of the sun’s disk becomes visible above the horizon – not when the center of the sun is visible. In the same sense, ‘sunset’ refers to the moment the Sun’s upper edge, not the center, disappears below the horizon. The time it takes for the sun to fully rise and set, which is several minutes, is added to the day and subtracted from the night, and therefore the equinox day lasts a little longer than 12 hours.
Pic Courtesy – timeanddate.com