RE: [asa] physics questions (surf. brightness)

From: George Cooper <>
Date: Thu Sep 18 2008 - 13:00:18 EDT

I should have qualified “surface brightness”: it is the amount of light
being observed that Is coming from a unit area. It is a measure of flux per
square arc minute, for example. [The human eye has a resolution of about 1
square arc minute, so objects that appear smaller than this are seen as
point sources only (until magnified with telescopes, of course).]


As a bonus question: How much brighter (total flux seen or avg. surf.
brightness) is a full Moon vs. a quarter Moon (when of the Moon’s disc is





From: [] On
Behalf Of Jim Armstrong
Sent: Thursday, September 18, 2008 10:57 AM
Subject: Re: [asa] physics questions (surf. brightness)


Are we perhaps actually speaking of luminance, brightness being a
physiological response? :-) JimA

George Cooper wrote:

Here is one that may surprise some….


If an astronomer is using a 1 meter (aperture dia.) telescope to observe
surface features on Jupiter and is using an eyepiece that yields the best
surface brightness, how much larger a telescope (aperture) will she need in
order to double the surface brightness (assuming she also uses the
appropriate eyepiece for max. brightness)?


[Geoerge wrote] This differs from the previous result by having the square
of the sum of the speeds instead of the sum of the squared speeds, and will
thus be greater because of the cross term 2vV.


On the balloon problem: Answers previously given are OK but there's a
broader viewpoint - perhaps surprisingly, general relativity. According to
Einstein's equivalence principle, an observer in an accelerated frame can
always treat the effects of acceleration as an artificial gravitational
field in his/her vicinity. In an accelerating car this gravity is toward
the rear and we all know that balloons go up - opposite to the gravitational
field - so in this case it will go forward.


Very nice on both.



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Received on Thu Sep 18 13:01:50 2008

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