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Round and Round
Mon Jan 13, 2003
Listen in RealAudio 
Since the launching of the world's first satellite, half a century ago, the word "orbit" has
been common in our everyday vocabulary, but we seldom talk about our most
important orbit: the
Earth around the Sun. Hi, I'm Bryan Yeaton and this is The Weather Notebook. Let's
look at
few facts about Earth's orbit.
First, we consider 365 days to make one year, the time required for the Earth to orbit
once
around the Sun. But it's really 365-and-a-quarter days. Also, the path of the Earth's
orbit
is not quite circular. It is a slightly eccentric ellipse. The eccentricity of the orbit, a
measure of the degree of its departure from a true circle, is currently about 1.67
percent,
but this varies from near zero to 5 percent over a span of 100 thousand years.
And, the mean Earth-Sun distance is 93 million miles. We are closest to the sun, 91.4
million
miles, around January 1st -- ironically, the start of our Northern Hemisphere winter. Six
months later, we are furthest away, 94.5 million miles. However, the main cause of our
seasons
is not the variation in distance from the sun but the tilt of the planet's axis.
Finally, Earth has a mean orbital velocity of nearly 67,000 mph, but Kepler's Second
Law of
Planetary Motion tells us that a planet sweeps at different speeds over different parts of
its
orbit. For the Northern Hemisphere, these variations result in the summer actually
being five
days longer than winter. Sometimes, that's hard to believe.
Thanks today go to Sean Doucette, Keith Heidorn, Subaru, and the National Science
Foundation.
The Weather Notebook is a production of the Mount Washington Observatory. Find out
how you can
become a member; go to www.mountwashington.org.
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