(no subject)

Sat Dec 16 14:59:42 EST 2006

Jason Stephenson wrote:

> Tom Buskey wrote:
> > On 12/15/06, Bayard Coolidge <n1ho at yahoo.com> wrote:
> >> I haven't got a clue as to what Solstice is, but Bill Sconce's response
> > 
> > When the earth is furthest/nearest the sun.  Winter solstice is Dec
> > 21st.  Summer is June 21st.
> > 
> 
> It's actually the other way 'round. The earth is closer to the sun on 
> the winter solstice.

This is wrong too.  You're almost right, but in a few thousand more
years you'll be very wrong.  The solstices are when the the Sun is
directly overhead at some point on the Tropic of Cancer or the Tropic of
Capricorn.  The equinoxes are when that "subsolar" point crosses the
equator.

The Earth is closest to the sun at the perihelion, which occurs in early
January.  In July we we'll be the furthest away, a point called the
aphelion.

> The Winter Solstice is the shortest day* of the year in most of the 
> Northern Hemishpere. The Summer Solstice is the longest.

Right.  Both hemispheres actually.  The winter solstice in the southern
hemisphere is in June.  BTW, we've already passed the earliest sunset.  The
latest sunrise won't occur until early January.  Both the Earth's tilt and
eccentricity are involved in that phenomenon, often referred to as the
equation of time.  Applied to a well aligned sundial, you can tell time to a
few minutes.

These are astronomical data for 2007:

2007 Penacook, Latitude   43.29  Longitude   71.60
The time of rising and setting will change one minute for
each 12.6 miles traveled east or west.
Time zone 5, daylight time begins on Apr  1 and ends on Oct 28
Spring: Mar 20  7:07P, Summer: Jun 21  2:12P  Perihelion: Jan  3  4:21P
Autumn: Sep 23  5:58A, Winter: Dec 22  1:17A    Aphelion: Jul  5  8:16A

Caveat: I computed the above applying limited physics and a 25 year-old
reference point.  The U.S. Naval Observatory is the better reference, the
position of the moon seems able to perturb perihelion and aphelion by a couple
days (the Earth/Moon barycenter is above the surface of the Earth, so no real
surprise).  I suspect the planets have an observable effect too.