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jkepler
2011-Mar-07, 07:54 AM
Hi,

I'm a little bit confused... What is the "exact" definition of tropical year, and why is its value (lesser) different from the days between two March equinoxes?

Kind regards,

JKepler

grapes
2011-Mar-07, 08:04 AM
It's less than the time between March equinoxes, but it's greater than the time between September equinoxes. It's a mean tropical year.

Ivan Viehoff
2011-Mar-07, 11:28 AM
Is there anything this doesn't tell you? http://en.wikipedia.org/wiki/Tropical_year

As Grapes implies, the actual time from March equinox to March equinox varies from year to year by surprisingly large amounts. In the Wikipedia article under the heading "mean equinox tropical year", one can see that the actual current tropical year can vary by as much as 5 minutes from one year to the next, and that is just a few examples from recent years. Which is why one has to have a "mean tropical year".

Unfortunately this is all very messy. The solstices and equinoxes are not equally distributed around the year - the northern summer (march equinox to september equinox) is currently longer than the southern summer by several days. The displacement from equal quarters is of the order of a day or two. But these relations are not fixed, and gradually change. The reason for the uneven distribution is because the earth's orbit is elliptical, the earth travels more slowly when it is further from the sun, and so the earth does not rotate around the sun at a constant angular velocity. The northern summer currently occurs when the earth is further from the sun. The northern summer solstice is currently close to, but not the same as, the aphelion. Because of the "precession of the equinoxes" (the axial precession of the earth), these things gradually rearrange, in a roughly 26,000 year cycle. So in about 13,000 years, you would expect the northern winter solstice which will be close to the aphelion - if the orbit of the earth around the sun stayed fixed, which it doesn't. Because the elliptical orbit of the earth around the sun also has cycles of eccentricity, and precesses. And the angle of the axis of the earth to the ecliptic also wobbles to steeper and shallower angles.... So everything is wobbling, which means that averages are rather tricky to pin down.

Hornblower
2011-Mar-07, 01:41 PM
Short answer: The tropical year is the mean value of the interval between returns to the March equinox, averaged over a very long time to allow for the short term variations.