Astronomy questions

Hi,

I am trying to improve my limited knowledge of astronomy and would
appreciate your help with a couple of questions.

I am finding out that there is a difference between anomalistic year
and sideral year. How long ago was that discovered ? I assume it's
because the ellipse the earth is describing around the sun is
precessing (or progresssing ?). Do we know how long it takes for it to
describe a full circle. Does it progress at a constant rate ?

What is the angle between the galactic plane and the ecliptic plane ?
Does it change over time ? Do we know why they do not match ?

Thank you,
Claude

"Claude" ) asked:

I am finding out that there is a difference between anomalistic year
and sideral year. How long ago was that discovered ? I assume it's
because the ellipse the earth is describing around the sun is
precessing (or progresssing ?). Do we know how long it takes for it
to describe a full circle. Does it progress at a constant rate ?

I'm not an expert on those questions, although I'm sure I could
find the answers to all of them (except possibly the first) in
my general astronomy textbooks. I suggest you search the web
for "anomalistic year" AND "sidereal year". You should find
what you want.

A couple of years ago I converted these figures from decimal
days to seconds and wrote them down in my Observer's Handbook:

Anomalistic year 2380 713.120 seconds
Sidereal year 2360 591.424 seconds
---------------------------------------
Difference 20 121.696 seconds = 5 hr 35 min 21.696 s

From which you can easily calculate the time for a full cycle.
I have no reason to think these values are changing, so the
rate would be constant. Venus, Mars, and Jupiter must be the
largest influences perturbing Earth's orbit. I'd think their
effects would average out over the years, so the rate wouldn't
change.

What is the angle between the galactic plane and the ecliptic plane ?

Earth's rotation axis is, as everyone knows, 23.44 deg away
from perpendicular to the ecliptic, but I'm not immediately
finding the right ascension of the north pole of the ecliptic.
A small map shows it in Draco, roughly 18 hr R.A.

My Observer's Handbook shows the galactic north pole at right
ascension 12 hr 49 min, declination 27 deg 24' (1950). That's
between Coma Berenices and Arcturus.

Does it change over time ?

No, not that that I'm aware of. It would take quite a large
perturbing force! If Venus orbited in an elliptical orbit
inclined at a high angle to the plane of Earth's orbit, that
might do it.

Do we know why they do not match ?

There's no reason why they should. The protosolar cloud
collapsed independantly of everything else going on in the
galaxy at the time, so there was no reason for it to align
with anything. That would be true of all star systems.
Rotation of stars is measured by the Doppler change in the
light from the approaching and receeding limbs, showing the
stars to have random orientations.

It isn't like on Star Trek where the Enterprise is always
"right side up" (when it isn't in deep trouble), and the pole
of a planet is always at the top. :-)

-- Jeff, in Minneapolis

..

I fouled up! I read the numbers off the wrong lines, giving
periods relating to the Moon's orbit instead of the Earth's!
Thanks to Stuart Levy for pointing this out to me!

I erroneously wrote:

Anomalistic year 2380 713.120 seconds
Sidereal year 2360 591.424 seconds
---------------------------------------
Difference 20 121.696 seconds = 5 hr 35 min 21.696 s

I should have written:

Anomalistic year 31 558 429.44 seconds
Sidereal year 31 558 152.96 seconds
----------------------------------------
Difference 276.48 seconds

Which is a lot more like the difference I expected, but my
expectation wasn't certain enough to tell me that something
was wrong. Sorry about that! Please check my new numbers
to see whether I've made some other mistake.

-- Jeff, in Minneapolis

..