Re: [Phys-l] celestial motion

[John Denker <jsd@av8n.com>]

On 08/22/2008 10:26 AM, Anthony Lapinski wrote:
> Thanks for all the interesting/detailed responses so far. I simply wanted
> to know the Sun's approximate angular speed across the sky. How many of
> its own diameters does it move in 8 minutes. I read somewhere years ago
> that it was two diameters, but my calculations showed four, which seemed
> too large.

There's several questions tangled together there.

Asking about diameters is one thing;  sunsets is another.
Note that in temperate latitudes, which is where most phys-l subscribers
hang out, the sun does not set vertically ... so the sunset process takes
noticeably longer that what you would guess by dividing "the" diameter by
"the" rate.

Not to mention mountains.

Not to mention refraction in the atmosphere.

Why do we care how many diameters the sun moves in 8 minutes anyway?

I find that it is equal to

             R^2
      2 π --------
            φ cD

where R is the distance from earth to sun, φ is the diameter of the
sun, and cD is the distance light travels in a day.

> From the point of view of dimensional analysis, we may remark that this
is a dimensionless group ... but there is more to physics than dimensional
analysis.  Is there any physical significance to this group?