Astronomy & Celestial Mechanics
the sky, computed
Concept

Saros — where it appears

The 6,585.32-day interval after which the Sun, Moon and nodes return to nearly the same configuration, so that a similar eclipse recurs. The third of a day in the period means the next eclipse falls a third of the way round the world.

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

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

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 Three periods that nearly share a multiple. How far the draconic and anomalistic months are from a whole number, after a whole number of synodic months, in hours. At 223 synodic months — 6585.321 days — both are within an hour of closing, which is what makes an eclipse repeat. The draconic residual is 0.87 hours and the anomalistic 5.19 hours.

The eclipse that repeats a third of a world away

Three lunar periods nearly share a multiple after 6,585 days. The word "nearly" is what makes eclipses predictable, and the leftover third of a day is what moves each repeat a third of the way round the Earth.

sky · Phases and eclipses

Named alongside it

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

Lunar phaseOrbital inclinationUmbraAscending nodeEccentricityLibrationSynodic periodTerminator

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