Astronomy & Celestial Mechanics
the sky, computed
Concept

Terminator — where it appears

The line dividing the illuminated and unilluminated halves of a body. Its curvature as seen from elsewhere is what makes a phase, and the low illumination angle along it is what makes surface relief visible in a way full illumination does not.

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

Phases are a viewing angle, not a shadow. A satellite at eight points of its orbit. Exactly half of it is lit at every one of them; what changes is how much of the lit half faces the centre. Nothing is in shadow except at an eclipse.

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.

sky · Phases and eclipses A radius that depends on the colour it is measured in. Transit depth against wavelength for a planet of 1.38 Jupiter radii at 1400 K, whose atmosphere has a scale height of 538 km — computed from H = kT/µg, not assumed. One scale height of extra opacity adds 153 parts per million to a transit of 1.40 per cent, so the whole spectral signal is 862 ppm at its strongest: one part in 16 of the transit that carries it. The features are at real band centres — sodium at 0.589 µm, water at 1.4 µm, carbon dioxide at 4.3 µm — with the rise at the blue end the Rayleigh slope of scattering off the smallest particles.

A radius that depends on the colour it is measured in

Measure a transit in one colour and then another, and the planet is a different size. The difference is a few atmospheric scale heights, which is a few hundred parts per million of an already tiny signal.

exoplanets · Exoplanet atmospheres

Named alongside it

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

Atmospheric compositionCloudsEccentricityLunar phaseMean molecular weightOpacityOrbital inclinationRayleigh scatteringSarosScale heightSodium doubletTransmission spectroscopy

All concepts