Celestial Distances (Astronomy)

Recall cards on how astronomers measure distances in space. Fundamental units: the history of the meter, the light-second, and the astronomical unit (the average Earth-Sun distance), pinned down in modern times by radar ranging. Surveying the stars: parallax and triangulation, the parsec, the relation D = 1/p, the first stellar parallax measured by Bessel in 1838, and the Hipparcos and Gaia satellites that extended the method across the Galaxy. Variable stars as standard candles: pulsating cepheids and RR Lyrae stars, the light curve and period, and Henrietta Leavitt's period-luminosity relation that let cepheids reach tens of millions of light-years. And the H-R diagram method (spectroscopic parallax), the luminosity classes from supergiants to dwarfs, and the cosmic distance ladder that chains all these techniques together.