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ay45c2-page5 - analogy with the de nition of radians the...

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Unformatted text preview: analogy with the de nition of radians, the unit of solid angle, the steradian, measures the area on the unit sphere that the object covers. Since a sphere has area 4r there are 4 steradians in the whole sky. The corresponding number of square degrees is 2 360 = 41252:96  40000 4 2 since  is the \radius of a unit circle in degrees." The Sun and Moon are both very close to 0:5 diameter, so their areas are each (0:5) =4 = 0:20 square degrees, or  1=200000 of the celestial sphere. (If you think hard, you will see why this tells you that full moonlight is about 10 times dimmer than full sunlight!) 2 360 2 2 5 2.2 Brightness measurements 2.2.1 Flux and UBV system Flux is the energy arriving from a particular object (star, galaxy,...) per unit area of detector per unit time, so it has units erg cm 2s 1 . 1 cm 2 distance D You can see that if the source emits isotropically (same in all directions), the relation between ux F and total luminosity L (erg s ) is L F = 4D : 1 2 14 ...
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