Besselian elements

The Besselian elements are a set of values used to calculate and predict the local circumstances of occultations for an observer on Earth.

This method is particularly used for solar eclipses, but is also applied for occultations of stars or planets by the Moon and transits of Venus or Mercury.

In addition, for lunar eclipses a similar method is used, in which the shadow is cast on the Moon instead of the Earth.

This method was developed in the 1820s by the German mathematician and astronomer, Friedrich Bessel, and later improved by William Chauvenet.

A variable called ΔT measures how much that rotation has slowed over time and must also be taken into account when predicting local eclipse circumstances.