A maximum annular eclipse or “ring of fire” is due to be seen in Argentina and Chile on October 2 2024 at about 18:46 UTC. A partial annular eclipse will be seen in other parts of South America, the Pacific and Antarctica. The annular eclipse is the poorer, but arguably prettier, cousin to the total eclipse that gets the most attention from excited astronomers and solar enthusiasts.
These two types of eclipses happen when the Moon passes between the Earth and the Sun. In both cases, it is not safe to look at the sun without specialist eye protection designed for viewing the Sun.
In a total eclipse, the position of the Moon between the Sun and the Earth means that the surface of the Sun is completely obscured by the Moon. This allows only the corona – the outermost part of the Sun’s atmosphere – to become visible. The corona is formed from the gaseous ejections that emanate from the surface of the Sun. These can be uniquely studied during a total eclipse.
In an annular eclipse, the outer ring of the Sun’s surface, its photosphere, is also visible. This produces a ring of light that is a million times brighter than the corona. This means that an annular eclipse is less interesting to solar watchers because the corona is not visible. For many observers, however, it is a wonderful event to behold. The bright ring that becomes visible in the sky at the maximum annular eclipse is appropriately named the “ring of fire”.
Some eclipses only partially cover the Sun. The annular and total eclipses occur when the whole of the Moon passes across the centre of the Sun. To understand why annular eclipses happen, we need to consider the Moon’s orbit.
The gravitational attraction between the Moon and the Earth causes the Moon to orbit the Earth. The orbit is not circular, it is elliptical, which means the distance from the Earth to the Moon changes during its journey around the Earth.
Bienvenido Velasco / EPA Images
When the Moon is closest to the Earth, the size of the Moon appears to be the same size as the Sun, giving us the total eclipse. The closest position a planet or Moon makes to the object it is orbiting around is called the periapsis. The Moon’s furthest point from the Earth is called the apoapsis and for an annular eclipse to occur, the Moon must be near or at this furthest point.
Annular and total eclipses happen with the same frequency, however it is astonishing that we live in a time where they happen at all. Currently, the Moon is 400 times smaller than the Sun and the distance between the Earth and the Sun is 400 times bigger than the Earth-Moon distance. This happy circumstance means that the Moon appears to be the same size as the Sun. Two and half billion years ago, when the first microbes were present on Earth, recent predictions suggest the Moon was 37,000 miles closer to Earth than it is now.
This means that the apparent size of the Moon would have been too big for annular eclipses to happen. Currently, the Moon is moving away from the Earth at a rate of 3.8cm a year, so at some point, total eclipses will cease to occur.
Infrequent events
Eclipses happen infrequently because the respective orbits of the Earth and the Moon and of the Sun and the Earth are not in the same plane – an imaginary flat surface formed by the orbits of two or more objects lining up. The Earth-Moon orbit is inclined at five degrees with respect to the Sun-Earth orbit. This means that the Moon does not regularly pass directly in front of the Sun during lunar orbits. Instead, the Moon will pass above or below the Sun.
The frequency of solar eclipses is predictable, but they do not follow a regular
pattern. This produces eclipses that are unevenly spaced over time and that often occur in clusters, giving several eclipses in close succession.
The uneven pattern of eclipses is dependent on the complex gravitational field that
exists around celestial bodies such as the Sun, the planets and our Moon. Each celestial body distorts the space and time around it, which creates a gravitational field.
When several celestial bodies are close together, they interact. This means that more complex gravitational fields are produced. A further complication is that stars, planets and moons are constantly moving relative to one another, causing the gravitational field to move as well.
In the field of astrodynamics, the behaviour of these moving gravitational fields is analysed using a method called N-body dynamics. A current television series takes its name from a specific N-body problem called The Three-Body Problem. The mathematics of the Three-Body Problem is used to understand the behaviour of the solar system. In addition, computer simulations are used to create predictions about when and where the eclipses will happen.
The ecliptic path the Moon takes around the Earth has been billions of years in the
making and was influenced by other celestial bodies in our solar system. This
gravitational alchemy has created the burning spectacle that’s become the annular
eclipse we see in our night sky today.
