Unformatted text preview: iii) parallax: (using Earth's orbit) p
1 AU to distant objects
(providing reference
frame) p
p
p Angles p are small (< 100) and the distance D = R=p. If R = 1 AU,
and p is measured in seconds of arc, then D comes out in parsecs. 1 pc
= distance at which 1 AU subtends 1 arcsec = 3:086 10 cm.
18 iv) inverse square law and \standard candles"
If you know that a certain object has luminosity L (e.g., by looking at similar objects near you) [L in erg/sec or watts], then the
distance of that object is given from its observed ux L
F = 4D :
2 (Remember F is in energy/sec/unit area.)
Sphere D
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 Fall '09
 dion
 Astronomical unit, Inversesquare law, inverse square law, parsecs

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