The Vernal Equinox:

What  do you get in Springtime (or thereabouts; that's how we define
it), when the Earth rolls around in its orbit to a place  where,  if
we Earthlings  were  to  gaze  up,  the  Sun  lines  up  against the
intersection  of  two  imaginary  rings,  and  the stars behind that
intersection are the stars of Pisces?  What if it just  happened  to
be 1988 at 4:39 a.m. EST  on  March  20th, well, just what would you
have?

A  little  cat's cradle, and some pinball, that's what.  Oh, and the
Vernal Equinox of course.  Every year I try my best to  explain  the
Vernal Equinox,  but  I  still  get  dozens  of  calls  about it, so
obviously  I don't quite have the hang of it.  I probably won't this
time either, but allow me to  try  again.   Here  goes:  the  Vernal
Equinox is both an event  and  a  direction  in  the sky, and is the
fundamental reckoning point for all other astronomical objects.

Wait!   I  don't  expect  you  to be overcome with emotion upon this
realization, but I don't want you to feel  that  an  explanation  of
the Equinox is as bad as  a  trip to the dentist either.  The Vernal
Equinox,  event  and  direction, is terrifically important to all of
us.  We use it to measure how long it takes the Earth the go  around
the Sun, for instance.  We  use  it  to measure the positions of all
the  other  planets,  the  Moon,  the stars, other galaxies, all the
directions  in  the  Universe.   If  you  can't  quite  grasp  that,
consider this: since  ancient  times  we've  used  it  to  mark  the
passage of the seasons.  The  Vernal  Equinox  was the date when our
ancestors  saw  the  Sun rise precisely in the East. This meant that
the harsh days of Winter were  finally  over.   They  had  survived!
Because of  the  unseen,  and  therefore  mysterious  tilting of the
Earth to the Sun, Spring had arrived.

But  how do we figure it?  What if I told you to direct your line of
sight straight through the center of a  (transparent)  Sun  and  see
the stars behind  it?   And  what  if  I  told  you that those stars
belonged  to  a  particular  place in the constellation Pisces?  And
what if I told you that this would only work  on  a  particular  day
during the year, at  a  particular  time  of  the day, that date and
time  (this year) being March 20, 4:39 a.m.?  Are you still with me?
That particular direction and that particular date is  that  of  the
Vernal Equinox.

At this point you may  be  wondering  about the point.  (In the sky,
that  is.)  If  you're  not,  you  should  be.   Look  at  this in 3
dimensions: imagine the Earth's orbit around the Sun.  (We  call  it
the ecliptic...) And imagine the  Earth's equator stretched out into
space.   (We call this the celestial equator.) Now keep in mind that
the Earth's celestial equator is skewed from the ecliptic by 23  1/2
degrees (the unseen tilt).   In  space,  the two planes intersect in
two  places,  and  these  intersections  are  called  the equinoxes.
(See, we're getting there.) The stars seen in the background of  one
of those points, if you  look  up  from  the  Earth on March 20th at
4:39 a.m.EST, through a transparent Sun, are the stars of Pisces.

Years  ago, of course, they were the stars of Aries, hence the other
name for the Vernal Equinox: The First Point of Aries.  But,  that's
another story.


BY:

Gail S. Cleere 202/653-1541
NAVOBSY WASH

This  news  release  may  be  reproduced, with credit please, to the
U.S. Naval Observatory.
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