'Round & 'Round
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Since the launching of the world's first satellite, almost 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 Dave Thurlow 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 moves 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. You can become a member. Find out how at mountwashington.org or call 1-888-rain-001.