How is it possible to predict lunar and solar eclipses with such amazing accuracy? It almost seems like wizardy that astronomers are able to say when the next eclipse will occur, so far into the future, and right down to the second.


The ancient Babylonians knew enough about the Moon’s path and the saros cycle (the 18-year and 11-day cycle of eclipse recurrence) to be able to predict lunar eclipses well in advance.

But solar eclipses are visible from a far smaller area than lunar eclipses, and the track of any totality produced is very narrow.

A map showing the path of totality for the 2 July 2019 eclipse. To witness totality, you need to be standing between the two blue lines. Totality will last longest along the red line. Credit: ESO
A map showing the extremely narrow path of totality for the 2 July 2019 eclipse in Chile. Credit: ESO

Ancient civilisations did not have a precise enough knowledge of the Moon’s path to accurately predict solar eclipses, nor did they have the mathematical tools with which to predict them.

It is only in the last 300 years that astronomers have been able to predict solar eclipses accurately.

In the early 18th century, Edmond Halley was responsible for a big leap forward when he greatly improved upon the mathematical tables for the motion of the Sun and the Moon.

These models had essentially stayed the same since the work of the 10th century Arab astronomer Abu Batani (Latinised as Albategnius); work that was itself based upon the tables of Ptolemy from the second century.

Stellarium set date and time. Credit: Stellarium
Planetarium programs like Stellarium allow users to whizz through time to check when future celestial events will occur. Credit: Stellarium

But nowadays things are so much easier and accessible to ordinary people.

Anyone using a good planetarium program on their computer can accurately call up data and graphics for eclipses visible from any location on Earth.

The clockwork nature of the Solar System and Earth's location with respect to the Sun and Moon can be predicted well in advance.

As a result, free software such as Stellarium can be used by anyone to track future eclipses, lunar occultations, conjunctions or where a certain planet will appear at a certain time.


This article originally appeared in the July 2006 issue of BBC Sky at Night Magazine.


Pete Grego astronomer author
Peter GregoAstronomy writer

Peter Grego was a popular amateur astronomer and the author of several books on practical astronomy. He passed away in 2016. Minor planet 95935 Grego was named in his honour.