Limb — the visible edge of a disc
The limb is the apparent edge of the Sun's or the Moon's disc. Every contact is defined against it, and the Moon's is rough enough to matter.
The limb is the visible edge of a disc as seen from here — the Sun's limb, the Moon's limb. The word is Latin limbus, a border or hem, and has nothing to do with the limbs of a body; it is a technical term for an entirely ordinary idea.
Why it is worth naming
Because all four contacts are defined against it, and because "the edge of the Sun" is ambiguous in a way "the Sun's limb" is not — the Sun has no surface, and what looks like an edge is the depth at which its gas stops being transparent.
That depth is also why the Sun's limb looks different from its middle. Near the edge you are looking obliquely and therefore into shallower, cooler gas, so the disc is noticeably dimmer and redder around its rim than at its centre — limb darkening. It is the reason the last thin crescent before totality is a duller orange than the Sun an hour earlier, and the reason obscuration, which counts area and not light, slightly overstates how much illumination the Moon has actually taken away early in a partial eclipse.
Which limb is which matters too. The Moon travels eastwards against the Sun, so it arrives at the Sun's western limb: first contact happens there, and fourth on the eastern one.
The Moon's limb is not smooth
This is where the term earns its keep. The Moon's edge is a mountain range in profile: relief of one to three kilometres on a disc 3,474 km across. Negligible at any other moment, and decisive at second contact, where it is all that is left of the Sun — the effect it produces has its own page, Baily's beads.
The consequence for timings is the part that belongs here. Because the profile decides which part of the limb does the covering, the exact instants of C2 and C3 depend on where you are standing, and predictions built from the real profile can differ from smooth-disc ones by a second or more — much more at the limits. This app assumes a smooth disc. That assumption is not mentioned in the accuracy notes, which cover the position and time budget rather than the lunar profile, so it is worth stating here: no bead, and no limb-corrected contact, is predicted anywhere on this site.
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Updated 13 August 2026.