Re: An astronomer's view of mechanics

Quadibloc <jsavard@xxxxxxxxx> wrote:
Because the Sun takes 220,000 years to orbit the center of the galaxy,
in describing the motions of the Solar System, we neglect the motions
of the Sun just as we would neglect the motions of the Earth when
describing a game of billiards. We are not asserting, in a geocentric,
astrological, or creationist fashion that the Sun and stars do not
move.

Hi, John and all.

Here I'm confident that you meant 220,000,000 years or so. Geologically,
that would mean that our Solar System is just finishing the galactic
revolution started around the Triassic Period of the Mesozoic Era,
I guess. This would be a bit after one of the greatest mass extinction
events known, about 250,000,000 years ago (or 250 Ma for short), which
was once hypothesized to have resulted from an impact event like that
evidently responsible for the Cretaceous-Tertiary event around 65 Ma,
but is now often viewed as having other environmental causes.

Anyway, the "Galactic Year" captured my imagination in childhood, and
has kept it since. At Griffith Planetarium in Los Angeles, there was
a kind of circular chart of geological time with an audiovisual show
you could start by pressing a button, as I recall. The colors were
purple for the Archeozoic (now Archean) Era, and cyan for the
Proterozoic -- very attractive to my youthful eyes! That place helped
get me fascinated with astronomy, geology, and Precambrian paleontology.

Given the Earth moving around the Sun, what is more natural - once
people are accustomed to the idea that, yes, the Earth does move -
than to begin one's explanation with that which does not move, the
surroundings in which the movements of the Earth, Mars, and the other
planets take place? And so we begin with the Sun at the center - and
the stars, which cannot move around the Sun appreciably in the course
of a year, as they would need, at their vast distances, to move at an
impossible rate to do so?

This perspective, of course, would be even more natural possibly for
heliocentric theorists in the age of Flamsteed and Newton, when stellar
parallax had not yet been measured (as it wouldn't be until 1838).

However, then or now, it seems to me quite acceptable to regard the
stars or galaxies as "virtually fixed" for the purpose of recognizing
the empirical reality of the sidereal day.

One car overtakes another on a circular racetrack. Does that disprove
that the pavement stands still? Or, at least, since the pavement
*does* rest on a moving Earth, that it is simplest to understand the
moving cars by looking at both of them from a seemingly still vantage
point, as cars turn their wheels to move themselves in relation to the
ground beneath them, than from the viewpoint of someone within one of
the cars?

This is for me a very neat analogy.

Most appreciatively,

Margo Schulter
mschulter@xxxxxxxxxx
Lat. 38.566 Long. -121.430

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