Solar Eclipse Timing

This calculator determines what kind of solar eclipse occurs — and how long totality or annularity lasts — purely from the current Earth-Moon and Earth-Sun distances, since both vary meaningfully over each body's elliptical orbit. It first computes the angular size (apparent diameter as seen from Earth) of both the Moon and Sun using their true diameters divided by distance, converting from radians to degrees and arcminutes. The ratio of the Moon's angular size to the Sun's — the magnitude ratio — is the deciding factor: at or above 1.0 the Moon's disk fully covers the Sun's, producing a Total Solar Eclipse; below that, a ring of sun remains visible around the Moon, producing an Annular Solar Eclipse (the calculator flags ratios of 0.95 to 1.0 as "near-total" annular events, since the ring is very thin).

The umbral shadow's width on Earth's surface is estimated from the difference between the Moon's diameter and the Sun's diameter scaled by the distance ratio, and maximum eclipse duration is derived by dividing that shadow width by an assumed average shadow ground-speed of 3,400 km/hr — the typical rate at which the Moon's shadow sweeps across Earth's surface during an eclipse. Obscuration (percentage of the Sun's disk covered) uses the square of the magnitude ratio for total/annular cases, following the area relationship between angular sizes. Because real eclipse duration also depends on the Moon's position across Earth's curved surface and the observer's exact location within the shadow path, treat the duration and shadow-width figures here as maximums at the eclipse's central line, not universal constants for any given location.