Expert: Tom Whiting - 5/26/2008

Question
QUESTION: If the earth had a zero tilt, how would that affect the precession of the equinox.  
Is there any way to calculate the time of the "great year" currently approx.  
26800 years?

ANSWER: Hi Wayne,
Since the Earth Axis' precession is conical around a circle, while
maintaining it's current 23.5 degree tilt, there technically would
be no precession if the Earth's axis was zero tilt with respect
to the vertical. (How could there be any if we were stright up
and down?....there would be no "circle" to go around.)

As far as a "precessional orbit" or as you say, the Great Year,
the number I have memorized is 25,800 years, but I've recently
read that that number has been refined to 25,765 years.
To calculate it, I just have memorized that we (our axis) moves one degree about every 71.6 years, so 71.6  x  360 degrees gives 25,776 years.  Close enough, since no one lives that long anyway. See
http://en.wikipedia.org/wiki/Precession_%28astronomy%29

Hope this helps,
Clear skies,
Tom Whiting
Erie, PA



---------- FOLLOW-UP ----------

QUESTION: IF THE EARTH HAD NO AXIAL TILT, WOULDN'T THE GRAVITATIONAL PULL OF
THE MOON AND SUN STILL CAUSE A SLIGHT WOBBLE AND SLIGHT
PRECESSION?
IF SO, HOW WOULD ONE CALCULATE THAT PRECESSION, I.E. THE GREAT YEAR
MIGHT MOVE FOR EXAMPLE, 10 DEGREES IN 100 YEARS.

THANK YOU

Answer
Well yes, of course, but YOU yourself strictly specified a perfect
zero axial tilt forever, under hypothetically perfect conditions, independent of the moon and sun.

You can't have it both ways...even a slight "wobble and precession"
due to the moon/sun mass implies also a slight tilt, even if it's only 1/100th of a degree.
(Otherwise, there is no circle to circle 'around'...)
And I have no idea how one would calculate that because it's all
mathematically and hypothetical anyway. (I only cover real astronomy
not mathematics or physics).
If, as you say, 10 degrees in 100 years, then 10 degrees x 36 would equal 3600 years for a full precessional
orbit.

You seem to be taking an astronomy course under a "physics" department where the instuctor hates real astronomy, so he 'converts'
it to a math/physics course instead of an astronomy course, so he 'invents' hypothetical conditions for the students to solve a math/physics problems instead of...teaching you real astronomy.  Always take the course under the Earth Science department, not the Physics Department, then you won't have to figure out hypothetical problems which are totally meaningless in the teaching of the real subject of....astronomy.  I'm an astronomy major, not a math/physics expert, and I think all these hypothetical questions are a waste of time because they...ain't real to begin with. Tell
your instructor you want to learn some REAL astronomy, and not waste
time figuring out meaningless, hypothetical mathematical questions,
which you won't remember anyway, because they don't exist in nature.
And you can tell your instuctor I said so.
FWIW,
Clear Skies,
Tom Whiting
Erie, PA