Astronomy & Celestial Mechanics
the sky, computed
Theme

Light is the only messenger

One thin stream of photons carries distance, temperature, composition, motion and age — and everything visible is already old.
sunlightnewwaxing crescentfirst quarterwaxing gibbousfullwaning gibbouslast quarterwaning crescentas seen from the centrehalf lit, always The observed sky

Phases are not shadows, and eclipses are

Half the Moon is lit at every instant of every month. The phase is which part of the lit half faces the Earth — and confusing that with a shadow is the commonest error in astronomy.

 -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 Starlight

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 Starlight

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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