Astronomy & Celestial Mechanics
the sky, computed
Concept

Standard atmosphere — where it appears

A reference model of the air's temperature, pressure and density against height, falling at 6.5 K per kilometre to a tropopause near 11 km. Refraction tables, airmass formulae and orbital-decay models assume it when the real profile is not known.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Also named here as zenith angle — the same set of essays touches all of them, so they are one junction rather than several.

Refraction against apparent altitude. How far the air lifts an object, in arcminutes, against where the object appears to be. The lift is 34.5′ at the horizon — larger than the Sun's own diameter — and falls below one arcminute above 45°. At the horizon it is also the least reliable number in positional astronomy, because it depends on the temperature profile of the air the sight line crosses.

The Sun sets before it sets

At the moment the Sun's lower edge appears to touch the horizon, the whole of it is already below. Refraction lifts it by more than its own diameter, squashes it while it is there, and makes every sunrise and sunset time in every almanac a statement about the air rather than about the sky.

sky · Refraction Four atmospheres, one refraction, until the last few degrees. Refraction against zenith angle, traced through the four atmospheres of the first figure. The curves lie on top of each other from the zenith to beyond 80 degrees, and separate only near the horizon, where they reach 37.3 arcminutes (isothermal), 34.0 arcminutes (standard lapse), 50.8 arcminutes (inversion), 28.7 arcminutes (hot ground). Everything above the first few hundred metres of air is different between them, and almost none of it shows.

A bend fixed by the air at the telescope

Starlight crosses fifty kilometres of air whose temperature nobody at the telescope knows, and yet the refraction it suffers can be computed to a hundredth of an arcsecond from a thermometer and a barometer on the dome. The structure of the air overhead cancels out of the first term exactly, and out of the second because of hydrostatics. It returns only within a few degrees of the horizon, and there it takes over completely.

sky · Refraction

Named alongside it

The objects these essays reach for when they reach for this one.

Atmospheric refractionZenith angleAirmassApparent altitudeAtmospheric extinctionGreen flashHorizonHydrostatic equilibriumRefractive indexScale heightSemidiameterSunrise

All concepts