Why do perihelion and aphelion dates change so much?

[Phil]

Have a look at this web page:
http://aa.usno.navy.mil/data/docs/EarthSeasons.html

It gives the dates for the equinoxes, solstices, perihelion, and aphelion
for the next 20 years. The equinoxes and solstices behave as I would
expect, advancing about six hours each year, and resetting in leap years.
But the perihelion and aphelion jump back and forth by as much as three
days from year to year. Can someone explain this in terms an educated
layman can understand? Thank you.

[Paul Schlyter]

In article <Xns930482CD...@38.112.160.10>,

This is mostly because of our Moon: the Moon doesn't really orbit the
Earth, instead both the Earth and the Moon orbit their common center
of mass (which resides inside the Earth but closer to the Earth's
surface than to its center).

If the perihelion and aphelion times of the Earth+Moon center of mass
had been listed instead, there would have been much less variation.
But now the perihelion and aphelion times of the Earth's center is
listed, and then the lunar phase does matter: when the Moon is near
the first quarter ("waxing half" and visible at and after dusk), the
Earth's perhihelion happens later and its aphelion earlier. And when
the Moon is near the third (last) quarter ("waning half" and visible
before and at dawn), the Earth's aphelion happens later and its
perihelion earlier.

So re-examine the Earth's perihelion and aphelion times, and correlate
them with the lunar phase at perihelion and aphelion, and I think
you'll notice a relation.

--
----------------------------------------------------------------
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pausch at saaf dot se
WWW: http://hem.passagen.se/pausch/index.html
http://home.tiscali.se/~pausch/

[Phil]

Paul Schlyter wrote on 15 Jan 10:20:

> So re-examine the Earth's perihelion and aphelion times, and correlate
> them with the lunar phase at perihelion and aphelion, and I think
> you'll notice a relation.
>

Thanks, that makes perfect sense.

[Greg Neill]

Paul, how can one get an e-mail through to you that won't
bounce for various reasons?

[Paul Schlyter]

In article <eGEV9.14126$An1....@wagner.videotron.net>,

Greg Neill <gnei...@OVE.netcom.ca> wrote:

> Paul, how can one get an e-mail through to you that won't
> bounce for various reasons?

You could try the email address in my .sig below -- it usually works
fine.

However, I alreday received your email, and I just responded to it.