Sorry this is late. We just got home from visiting family in Denver. -pd

STARGAZER #500 for Mar. 7, 2009

Vernal Equinox


In the earliest years of the TV western "Gunsmoke," do you recall the name
of Marshall Dillon's deputy? Nope, Festus came later. It was Chester (played
by Dennis Weaver), and perhaps you'll remember that Chester walked with a
limp, the origin of which I never learned.

Now let's go much further back to when our 5 billion year-old solar system
was young--perhaps no more than several hundred million years old. It's a good
thing we weren't around yet as the yet as the planet that would eventually
become our home was whacked by a smaller planet producing a cataclysmic
collision that destroyed the other planet and left Earth, like old Chester,
with a
limp.

Ever since, as Earth orbits the Sun, its North and South Poles aren't
straight up and down but rather are tilted at a 23 1/2 degree angle. And as a
result, each hemisphere spends half a year tilted toward the Sun and the other
half tilted away. When tilted toward the Sun, its days are longer than its
nights, and when tilted away, its nights are longer than its days.

Twice each year Earth reaches a point in its orbit when the hemispheres
reverse positions, and one of those times comes Mar. 20, the first day of spring
in the Northern Hemisphere (and fall south of the equator). Called the vernal
equinox--Latin for "spring equal night"--it's the day when for an instant
neither pole is tiled toward or away from the Sun, and the day and night are of
equal length...at least theoretically.

But a look at sunrise and sunset times shows it's not so. For my home in
Waco, Texas, Mar. 20 sunrise is at 7:32 a.m. and sunset at 7:40 p.m., thus on
this day of the "equal night," day is 8 minutes longer than night. So what's
going on here? Well, actually two things account for this apparent
discrepancy--one a human factor, the other natural.

The first has to do with how we the define day and night. Since the Sun has
a discernible diameter, in theory day should start when half the Sun has
risen and end when half has set. However, by definition, day officially begins
the instant the first part of the Sun peaks over the eastern horizon and
doesn't end until the entire Sun sinks below the western horizon, and this adds
about 2 minutes to each day.

The second factor, which lengthens day even more, comes from the bending of
light called refraction. As the Sun nears the horizon, its light rays are
bent upward as they pass through Earth's atmosphere. Acting like a
strategically
placed mirror, the atmosphere lets us see a little below the horizon--enough
that we can actually see the Sun some 2-3 minutes before it physically rises
above the horizon, and we see it 2-3 minutes after it sets.

So just as in George Orwell's Animal Farm, where "some animals are more
equal than others," days are "more equal" than nights, even at the equinox.

Daylight Time. Tonight before retiring, set your clocks forward ("spring
forward") to Daylight Saving Time.

Next Two Weeks. Avg. sunrise: 7:40 a.m.; avg. sunset: 7:36 p.m. Tomorrow
night Saturn is at opposition--opposite the Sun as seen from Earth--when it
rises at sunset and is up all night. Wednesday's full Moon is called Lenten
Moon,
Sap Moon, Crow Moon, and Worm Moon. The Moon is at 3rd quarter Mar. 18.

Naked-eye Planets. Evening: Venus, ending its run as the "evening star," is
lost in Sun by month's end, but Saturn is now at its best all night. Morning:
Jupiter and Mars are low in the east at dawn with Saturn the brightest
object in the west.

Comet Lulin Postscript. My apologies to those of you who looked
unsuccessfully for Comet Lulin. It simply didn't get as bright as I hoped.
Under dark
skies it did approach naked-eye visibility, but for those of us who live with
light pollution, it remained quite difficult. Maybe we'll have better luck next
time.

=======================================================

Stargazer appears every other week in the Waco Tribune-Herald and other
newspapers. Paul Derrick is an amateur astronomer who lives in Waco. Write
him
at 918 N. 30th St., Waco, TX 76707, call or fax at (254) 753-6920, or e-mail
at paulderrickwaco@....

Copyright 2009 by Paul Derrick. Permission is granted for free electronic
distribution as long as these paragraphs are included. Please obtain
permission from the author for publication in any other form. To be added to
(or
removed from) the free e-mail distribution list, send your e-mail address (and
name) to _paulderrickwaco@..._ (mailto:paulderrickwaco@...) .


* * See the Stargazer Web site at http://www.stargazerpaul.com. * *


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