Monday, March 20, 2017
Vernal Equinox: first day of Spring
Today, at precisely 6:28AM Eastern Daylight Time, we mark the Vernal Equinox. At that moment, the Sun crossed the celestial equator on its path along the ecliptic. Of course, this date is critical in determining the date of Easter -- Easter is the first Sunday after the first Full Moon after today's Vernal Equinox.
If the Earth sat directly upright on its axis, the Sun would always be directly overhead at noon on the equator, it would appear to move through the stars along the celestial equator, days would always remain the same length, and every day would be like the two equinox days in Spring and Fall -- every day like today. As it happens, the Earth is tilted at about 23.5 degrees on its axis. Thus, the sun appears to diverge as much as 23.5 degrees from the celestial equator in its apparent path through the stars (the ecliptic), and ends up being directly overhead up to 23.5 degrees north or south of the equator (the Tropics of Cancer and Capricorn) on the two solstice days. This is the reason, of course, for our seasons.
So, if we start at the vernal equinox in March, today, the sun is directly overhead at the equator at noon, and night and day are the same length. [Of course, thanks to Daylight savings time, this happens at approximately 1PM, and not Noon. DST is a fictional time in which we pretend to live in the next time zone to the east.] From the Vernal equinox until the summer solstice, the Sun appears to move slowly to the north both in the sky, setting a bit further north of west each day, and in its apparent path through the constellations of the zodiac. This continues until the Sun reaches the solstice, where it stops, being directly overhead at 23.5 degrees north latitude (the Tropic of Cancer) at noon, and sitting about 23.5 degrees north of the celestial equator. From that point, the sun drifts back south until reaching the equator once more at the autumnal equinox in September, on this day, going all the way to 23.5 degrees south at the Winter Solstice.
This chart show the analemma for Earth, showing the relative locations of the Sun at noon at the Greenwich Observatory in England. Notice the change in both altitude and azimuth at the different points of the year.
The reason for all of this is that as the Earth orbits the Sun the two hemispheres of the Earth take turns being tilted toward the Sun. The following diagrams might help to illustrate what I am trying to articulate:
In this diagram, the sun appears to move against the background of the stars along the red line, the ecliptic, while the white line marks the celestial equator -- the imaginary line through space that is merely the extension of the Earth's equator. The two points where the red and white lines are at greatest divergence are the solstices, while the two points where the red and white lines cross are the two equinoxes. The yellow line shows what the sun would appear to be in front of from the Earth, the Constellation Pisces, at the vernal equinox.
This diagram shows the positions of the Earth in relation to the sun at those four points -- the Summer solstice on the left, with the Northern Hemisphere tilted toward the Sun, and the Winter solstice on the right, with the Southern Hemisphere facing the Sun more directly, with the two equinoxes between, with the Hemispheres equally oriented to the Sun.
So today, at the Vernal Equinox, the sun shines directly overhead at the equator, and both hemispheres are equally lit! Welcome to astronomical Spring!