Re: 62 million year extinction cycle

Odysseus <odysseus1479-at@xxxxxxxxxxxx> writes:

In article <1181510789.855895.92020@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
boson boss <junkerade@xxxxxxxxx> wrote:
<snip> What kind of a cycle is it? I know about 26000 years around the
galactic plane...

You're probably thinking of lunisolar precession, which has a period of
about 26 ka -- and has to do with the 'wobble' of the Earth's equatorial
plane, not the solar system's position WRT the galactic plane. The Sun's
orbit around the Galaxy is thought to take more than two hundred mlllion
years, so we cross the galactic plane at intervals of somewhat over 100
Ma.

The article said 62 million years was the period of the extinction cycle and
the period of the motion above and below the galactic plane. I think that
means that, assuming your figure of about 200 million years is correct for
the galactic orbit of the sun, that the sun crosses the galactic plane 6
times in each orbit around the Milky Way.

I guess there is no reason to assume that the period of the sun around the
Milky Way and the period of the sun above and below the galactic plane have
to be commensurate, nor that the sun has to return to the place in the
Milky Way where it started (even ignoring the fact that the Milky Way is
changing too). So, it is conceivable that in some orbits around the Milky
Way, the sun could actually cross the galactic plane 7 times. If we take
the figures 200 million and 62 million literally, that would happen roughly
every 600 million years.
--
Ignorantly,
Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx>
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.
.

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