it [the weather] gets through more business in spring than in any other
season. In the spring I have counted one hundred and thirty-six different
kinds of weather inside of four and twenty hours.
Mark Twain on New England weather
The Vernal Equinox is one of the two points where the Ecliptic crosses
the Celestial Equator. At the Vernal Equinox the sun appears to be moving
across the equator from the Southern Celestial Hemisphere to the Northern
Celestial Hemisphere. The other point where the sun crosses the Celestial
Equator is the Autumnal Equinox.
Recall that it is the apparent motion of the sun eastwards around the
Ecliptic that creates the seasons that we experience at different times
of the year. The arrival of the sun at the Vernal Equinox on or about
March 21 marks the beginning of Spring.
Because the number of days in the year is not a whole number, the sun
does not arrive at the Vernal Equinox at the same date every year. The
date of the equinox may vary by more than two days, occurring as early
as 8 AM CST on March 19 in the year 2096 or as late as 1 PM CST on March
21 (1903). There is a similar variation in the date of all the solstices
and equinoxes. For the purpose of ASTR106, we will assume that the solstices
and equinoxes occur on the 21st of the month - with the exception of
the Autumnal Equinox, which we will take to fall on Sept. 23.
Since the Vernal Equinox represents the beginning of the spring season,
we can also refer to it as the Spring Equinox.
Since the sun is on the Celestial Equator at the Vernal Equinox, it
has a declination there of 0°. Since position of the Vernal Equinox
is the reference point from which we measure right ascension, at the
Equinox the sun has a right ascension of 0 hours. You should then remember
the equatorial coordinates of the Vernal Equinox as 0°, 0 h. Note that
the Vernal Equinox is the center point of the Equatorial Sky Chart.