{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

ay45c2-page5

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

This preview shows page 1. Sign up to view the full content.

This is the end of the preview. Sign up to access the rest of the document.

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 4r 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 = 4D : 1 2 14 ...
View Full Document

{[ snackBarMessage ]}