Vernal Equinox

(This column was published in the March 20, 1995 Buffalo News.)

   Tonight at 9:14 p.m. the earth in its 580 million mile tour around the sun 
will pass through its vernal equinox. More simply stated, spring will arrive. 
I invite you to explore with me what that means to astronomers.

   Consider first what scientists call a mind experiment: a thought process 
with no equipment necessary -- except your mind.

   Think of a pingpong ball following a nearly circular path around a lighted 
bulb. That model represents the earth's yearly revolution around the sun. Now 
think of that pingpong ball as held between the index finger and thumb of one 
of your hands while being turned with your other index finger. That represents 
the earth's daily rotation: half of the ball always lighted, the other half 
dark.

   That model is a familiar one, but without modification it leaves us with a 
serious problem. The ball's rotation lights all points half the time -- except 
where our supporting fingers touch, which correspond to the north and south 
poles. Yet we know that is not what happens: in winter days are short, in 
summer long.

   We must therefore adjust our model. The circle our pingpong ball earth 
follows around the light bulb sun establishes a plane, which astronomers call 
the ecliptic. (The other planets also revolve about the sun close to this same 
plane.) To represent the earth's rotation accurately we must tilt our fingers 
slightly to this ecliptic. The tilt is 23.5 degrees, about a quarter way from 
perpendicular toward the plane.

   The same tilt is maintained as the ball revolves around the sun. At all 
times half of the ball is still lighted. But with this adjustment, when the 
north pole (where our index finger touches the ball) is leaned toward the sun, 
it is lighted all the time, despite the rotation. And points near that pole 
are also lighted more than half of the time. When, half a revolution later, 
that same north pole tips away from the sun, it is dark all of the time and 
nearby points are lighted less than half of the time. The extreme positions 
are called the summer solstice (longest day) and winter solstice (shortest 
day).

   Clearly there are two times between these extremes when daytime and 
nighttime are the same length. Those are called the vernal (spring) and 
autumnal (fall) equinoxes. (Technically at those times the line of 
intersection of the ecliptic and the earth's equatorial plane points directly 
at the sun.)

   Here are data for these four events at Buffalo this year:

                               sunrise      sunset        day length
Vernal Equinox (Mar 20)       6:19 a.m.    6:27 p.m.    12 hrs. 8 min.
Summer Solstice (Jun 21)      4:36 a.m.    7:58 p.m.    15 hrs. 22 min.
Autumnal Equinox (Sep 23)     6:02 a.m.    6:13 p.m.    12 hrs. 11 min.
Winter Solstice (Dec 22)      7:44 a.m.    4:45 p.m.     9 hrs.  1 min.

   This information suggests two more problems. Day lengths for the equinoxes 
are a few minutes longer than the expected 12 hours. Dave Sage of the National 
Weather Bureau Office at Buffalo explains that those extra minutes are due to 
the sun's size -- it peeps over the horizon -- as well as atmospheric bending 
of its rays.

   The other problem: we expect the times to be from 6:00 a.m. to 6:00 p.m. 
and ours are a little late. Some of this variation is due to the equinox 
occurring so late in the evening and our location west of the 75 degree 
longitude for which our time zone is calculated.

   The story of spring arrival is complicated but we all welcome the result: 
this wonderful season of rebirth.