Astronomy & Celestial Mechanics
the sky, computed
Field

Starlight

Distance, brightness and colour: what can be measured when nothing can be visited.
distant starsJanuaryJuly2 AU1.3 pc2p = 1.54″distance = 1 ÷ parallax in arcsecondsangles hugely exaggerated: the true angle here is a five-thousandth of a degree

The triangle that reaches the stars, and stops

Parallax is the only distance measurement in astronomy that assumes nothing. It is also the only one with a hard ceiling, and everything beyond that ceiling rests on it.

 -20-100102030apparent magnitudethe Sunfull MoonVenus at its bestSiriusVega, by definitionthe naked-eye limita good amateur telescopea large ground-based surveythe deepest exposuresbrighter ←→ fainterfive magnitudes = ×100 in light

A scale that runs backwards, multiplies, and works

Brighter stars have smaller magnitudes, and five steps is a factor of a hundred. A scale invented by eye in the second century BC turned out to be logarithmic, because eyes are.

 visible050010001500200000.20.40.60.811.2wavelength (nm)3000 K, peak 966 nm5800 K, peak 500 nm10000 K, peak 290 nmeach curve scaled to its own peak

Colour is a thermometer, and it reads across the galaxy

A star's colour gives its surface temperature, from two brightness measurements and no other information. It is the cheapest useful measurement in astronomy.

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