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all planets, stars, galaxies, etc. are moving

measured by a series of pictures in arcseconds/yr

Because the Earth goes around the Sun, we change our position in space

The reflex motion of a star is relative to how close it is

distance (in parces) = 1/parallax (in arcsec)

M₁+M₂=a³/P²

a: semi-major axis

P: period

Brightness = directly measured (magnitudes)

Flux = number of phons received per second

Luminosity = intrinsic, drived if you know the distance