The Vernal Equinox, the Astronomical Beginning of spring, occurs this year on Saturday, March 20, 2021 at 9:37 UTC (5:37 AM EDT).
What’s so special about this event?
12 equal hours of daylight and darkness
The beginning of Astronomical Autumn for our friends in the Southern Hemisphere
The sun rises due east and sets due west at any point on the planet.
The sun’s elevation above the nearest horizon (southern horizon for Northern Hemisphere observers and northern horizon for Southern Hemisphere observers) is exactly equal to the compliment of their latitude; for example, if your latitude is +40 (40 degrees north latitude), the elevation of the sun would be 50 degrees above the southern horizon.
What does the “Astronomical Beginning of Spring” mean?
The Earth orbits the sun in a specific equatorial plane known as the “Plane of the Ecliptic” and the sun traces an imaginary line on the sky as it appears to move from east to west. This apparent motion of the sun is caused by the Earth’s west to east (counterclockwise) rotation. As inhabitants of the surface, this translates into an apparent motion of the sun across the sky. The imaginary line traced out by the sun is known simply as the “Ecliptic”.
Another aspect of the Celestial Coordinate system that plays a key role is the “Celestial Equator”, the projection of the Earth’s Equator onto the sky. This imaginary line has a Declination (celestial latitude) of 0, just like the actual equator on the earth, and this is the key factor that provides for due east-west rising and setting of the sun and equal hours of daylight and darkness.
The Astronomical Beginning of Spring is really a convention, marking the first point since the beginning of the New Year (January 1st) when the sun, seemingly traveling along the Ecliptic, crosses the Celestial equator.
Likewise, like the Vernal Equinox, the Astronomical Beginning of Autumn occurs on or near September 21 of each year and marks the 2nd and last time after the New Year there are equal hours of daylight and darkness, in addition to all the other aspects of these 2 singular seasonal occurrences listed above.
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