Re: which sides are the perihelion and aphelion?

From: tadchem (thomas.davidson_at_dla.mil)
Date: 02/11/05 

Date: 11 Feb 2005 08:40:54 -0800

jameshanley39@yahoo.co.uk wrote:

<snip repost>

> And is the sun on the major axis of the ellipse

Yes.

> or on the line joining
> the points where the summer and winter solstice occur?

Not necessarily.

The sun and the earth orbit about their mutual center of gravity (their
'barycenter')
http://www.onelook.com/?w=barycenter&ls=a
IIRC, this is about 450 km from the center of the sun (well within it!)
so the sun surrounds the barycenter.

Ignoring the perturbations (periodic disturbances) of this orbital
motion by other bodies in the system, the barycenter is at one focus of
the ellipse, which itself must be on the major axis of the ellipse
geometrically.

The solstices are determined by the intersection of the plane of the
earth's equator and the plane of the earth's orbit, independently of
where the periapses are. The plane of the earth's equator shifts as
the axis of rotation precesses (an anti-clockwise rotation, as seen
looking up from the earth towards the North pole of the ecliptic, with
a period of about 25,000 years).

The sidereal year (time it takes the earth to move 360° around the
sun) is 365.256363 days (mean solar days).

The anomalistic year (time it takes earth to move from perihelion to
perihelion) is 365.259635 days. This is longer than the sidereal year
because the line of apsides (perihelion-aphelion line) rotates through
the sky in the same direction as the sun's apparent motion against the
stars.

The tropical year (time it takes earth to move from summer solstice to
summer solstice, or spring equinox to spring equinox, etc) is
365.242190 days. This is shorter because the precession of the earth's
axis of rotation moves the solstices and equinoxes in directions
opposite to that of the sun's apparent motion.

http://scienceworld.wolfram.com/astronomy/Year.html

> Either way seems suprising. Because-Given that the earth tilts
directly
> towards the sun at the summer solstice.

Roughly, not exactly...

> If the sun is on the major axis, (not the solstice line), then the
> earth - for it to tilt towards the sun at the summer solstice) is not
> tilting along either line.

Correct.

> If the sun is on the solstice line, then the sun is not just
> horizontally off, it is vertically off too(how off?).

The Analemma represents the curve that combines the effects of the
sun's apparent north-south motion during the year [due to the tilt of
the earth's axis] and the sun's apparent east-west motion [due to the
fact that the earth's orbit is an ellipse with the sun slightly
off-center, and thus the sun is sometimes to the east of the center of
the earth's orbit (the midpoint of the major axis) and sometime to the
west].

http://www.astrosurf.com/lombry/analemma4.htm
http://scienceworld.wolfram.com/astronomy/Analemma.html

> thanks

HTH

Tom Davidson
Richmond, VA

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