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Re: [Phys-l] celestial motion



Of course this estimate is not for St. Petersburg where the sun sets
diagonally and very slowly in summer. So light emitted 8 min ago will still
arrive after the start of sunset in that high latitude. I was just there
and can attest to this effect.

Incidentally a quick survey of the meaning of sunset on the web is either
the time when the sun begins to dip below the horizon, or when the last
sliver of the sun ceases to be visible on the horizon. The latter
definition is slightly more prevalent. The latter definition is blessed by
the Farmers Almanac, so presumably that is the definition for the commonly
reported times. Sundown is also listed as a synonym.

I really don't think it is a misconception to mathematically treat the
situation as if the sun moved. One can understand the Earth's rotation and
still use a stationary Earth model for the calculations. After all we call
it sundown, not apparent disappearance due to the Earth's rotation. After
all we talk of current flowing from + to - even though it is really electron
flow in the other direction.

Whether there is value depends on how it is used to help students understand
the models used in astronomy.

John M. Clement
Houston, TX


You have a misconception here. The cause of sunset is Earth, not the
Sun. Tracking photons leaving the Sun while discussing Earth's rotation
is a bit like apples and oranges.

True, the light that came from the sun was emitted 8 minutes ago and it
is also true that when we see those particular photons at sunset the
photons currently leaving the surface of the Sun will be arriving at
Earth when the Sun is below the local horizon. But, I don't think there
is much value in this line of thinking. (Your numbers and results ARE
correct though.)

Perhaps more useful is to think about the diameter of the sun being
about 0.5 degrees and that at a rate of 0.25 deg/min the result is that
it takes about 2 minutes for the Sun to set.