Re: AU other than Astronomical Units?
- From: Stewart Robert Hinsley <{$news$}@meden.demon.co.uk>
- Date: Mon, 28 Apr 2008 13:31:10 +0100
In message <Je-dnXXpMbkkQonVnZ2dnUVZ8h-dnZ2d@plusnet>, Neil <neil@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> writes
"Robert" <guyinct17@xxxxxxxxxxxxxxx> wrote in message news:4814c5f7$0$11638$607ed4bc@xxxxxxxxxCan AU mean something other than Astronomical Units? Given this formula:
(m1 + m2)T^2 = a^3
T=years
m=solar masses
a is in units of AU
Can anyone guess what this equation means? Is there another meaning of AU related to acceleration?
Thanks.
Very often arbitrary units.
In the above AU means astronomical unit. The equation above represents a generalisation of Kepler's 3rd law, and 'a' denotes the semi-major axis of the orbit.
The selection of units makes the constant of proportionality equal to 1; for a different set of units the equation becomes (m1+m2).T^2 = k.a^3
Consider the Earth. Then m1+m2 is negligibly differ from the mass of the Sun, so m1+m2 = 1 solar mass. The orbital period is 1 year, to T^2 = 1. The semi-major axis is 1 A.U., so a^3 = 1.
--
Stewart Robert Hinsley
.
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