Predicting Eclipses

A partial solar eclipse from 2021 coloured a stunning orange-red, photographed from Virginia in America. Featured Image Credits: Bill Ingalls, NASA

When does a solar eclipse occur?

A solar eclipse takes place when the Moon passes between the Sun and the Earth. The moon casts a shadow upon the Earth which makes day turn to night, but what is really happening?

The Moon orbits the Earth every 27.32 days. Perhaps counter-intuitively, it takes closer to 29.5 days for a complete cycle of the phases of the Moon (from new Moon to new Moon). This is due to the Earth and Moon also orbiting around the Sun, which changes their positions relative to the incoming sunlight. A solar eclipse can only occur when the Moon is in its ‘new’ phase, sitting directly between the Sun and Earth, when all the light the moon receives is on the side facing away from the Earth.

Diagram of space with the Earth at the center and the Sun and Moon appear along two circular paths. The cirles represent the orbits of the Sun and Moon, with the circles intersecting at two "nodes".
This diagrams showing where the locations (nodes) the Sun and Moon would be respect to the Earth, where a solar eclipse would be occur. Credits: Jack Arnott.

The Moon also makes an elliptic orbit around the Earth – only this orbit is tilted by roughly 5 degrees. This creates two ‘nodes’ – points where the Moon’s orbit around the Earth crosses the Earth’s orbit around the Sun. This is visualised in the diagram.

An eclipse will happen when a new moon and the Sun are close to each other at these nodes. This is called an ‘eclipse season’ and lasts for about 32 days. Any new moon in this period will result in a solar eclipse. These numbers are averages though, not exact figures – so the length of an eclipse season can vary. Apart from the Moon orbiting the Earth, there are several factors that impact the occurrence of solar eclipses.

The most significant one is the gravitational forces that the Earth, the Moon and the Sun exert on each other. But the Earth isn’t a perfect, rigid, spherical object, so observational differences arise between predictions and actual trajectories. Small deviations from the path of an eclipse and the length are frequently observed.


Annular solar eclipseA solar eclipse where the Moon doesn’t fully block out the Sun because the Moon appears smaller compared to the Sun. This is due to the Moon being near or at its furthest point away from Earth.
Lunar eclipseWhere the Earth lines up between the Sun and Moon, such that it blocks the Sun’s light from reaching the Moon.
NodesPoints where the orbit of the Earth around the Sun and the orbit of the Moon around the Earth cross. When the Moon and Sun are near or at these points, a solar eclipse can occur.
Partial solar eclipseA solar eclipse where the Moon doesn’t fully block out the Sun, due to the Moon not fully lining up with the Sun.
Phase (of the Moon)A phase is the shape of the Moon visible at a specific time of the month. It takes about a month for the Moon to go through each of its eight Moon phases.
Saros cycleA period of around 18 years, 11 days, and 8 hours. This slow repetitive rhythm can be used to predict when a solar or lunar eclipse can occur.
Solar eclipseWhere the Moon passes between the Earth and Sun and ends up blocking out some or all the Sun from view on Earth.
Total solar eclipseA solar eclipse where the Moon completely blocks out the Sun’s light from view, from the Moon fully lining up with the Sun.

Further reading

Bellstedt, S. (2023) Humans have been predicting eclipses for thousands of years, but it’s harder than you might think, Astronomy Magazine, [Accessed: 30 June 2023].

Seeds, M.A. (2007) ‘Chapter 3 – Section 4: Predicting Eclipses’, in Foundations of Astronomy. Belmont, CA: Thomson Brooks/Cole.

Stand-up Maths (2015) Solar Eclipse Maths and the Cosmic Coincidence of the Saros Cycle, YouTube. YouTube, [Accessed: 30 June 2023].

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