First of all, surely you know the two main types of eclipse, lunar and solar . In lunar, the Earth sits between the Sun and the Moon , casting its shadow on the surface of the satellite and obscuring it for a few hours . In solar eclipses, the opposite happens: it is the Moon that is located between the Earth and the Sun and that obscures part of its surface (because it is smaller than the planet).
As a consequence of all this, lunar eclipses can be seen from half the planet, that is , from any place from which the Moon can be seen at the time it occurs. Solar eclipses, however, can only be seen in a strip that runs across the planet’s surface. The strip in which the eclipse will be seen as partial is usually very wide, thousands of kilometers thick, while the band of totality, where the Moon completely covers the Sun’s disk, is usually around 200 kilometers wide.
But these are not all types of possible eclipses, because not only the Moon is capable of covering the Sun, nor are all lunar or solar eclipses the same . Let’s start by understanding the more conventional eclipses. Within solar eclipses we have several types. When the Moon is located exactly between the line that joins the Sun and the Earth , or the Sun and the part of the Earth’s surface where we are, not only the Moon is able to cover the Sun. Which type exactly will depend on the relative size in the sky between star and satellite.
The orbit of the Earth around the Sun and the Moon around the Earth are not perfectly circular , but slightly elliptical. This means that there will be areas of the orbit where the two bodies are closer and others where they are further away. The Moon has more variability in this regard , so it will have more influence, but the Sun also varies in apparent size , being larger in January and smaller in July.
When the Moon looks bigger than the Sun in the sky, it will be able to completely hide it, as will happen in the solar eclipse of August 12, 2026 , which will be visible from some areas of Spain. When the Moon looks smaller than the Sun, it will not be able to hide the star completely , and there will be a ring of light around the silhouette of the satellite, resulting in an annular eclipse. It can also happen, although it is very rare, that during the course of the eclipse we go from a total eclipse to an annular one or vice versa . This could happen for two reasons, either because the Moon was at the right distance to, in the hours that the eclipse lasts, move away or get close enough to go from one type to another, or because the route of the The Moon’s shadow spans widely different latitudes, causing parts of the eclipse to fall within the full umbra zone and others to lie outside it. This happened during the eclipse of November 3, 2013, which started as annular, but ended up being total .
In addition we can also witness a partial solar eclipse , in which the Moon is not exactly on the line that joins the Sun with the observation position and therefore the Moon’s disk only blocks a part of the Sun. Some eclipses are purely partial, since totality cannot be seen from any point on the planet, but all total and annular eclipses are partial from parts of the earth’s surface sufficiently far away .
With lunar eclipses we have a similar situation, but with the simplification that the Earth is too big to be able to create annular eclipses from the Moon. Therefore we will have total lunar eclipses when the Moon completely enters the Earth’s shadow region and partial eclipses when it only partially enters and not all of its surface is shaded. Lunar eclipses of penumbra may also occur. As the Sun is considerably larger than the Earth, the shadow it projects will not be a cylinder that extends to infinity, but will be a very long cone. The penumbra region, in which the Earth blocks part of the Sun’s light, but not all, will be a cylinder. In these eclipses the brightness of the Moon will decrease, but not as markedly as in total or partial ones.
But as we advanced higher, not only the Moon will be able to hide the Sun. Mercury and Venus will also block part of the sunlight that reaches us in what are known as “transits” . We do not usually think of these events as eclipses and without a doubt they are not as spectacular and visible to the naked eye as the types of eclipses already mentioned, but technically they are the same type of phenomenon: a body is located on the line that joins the Sun with Earth and blocks some of the star’s light. Since Mercury and Venus are considerably farther away than the Moon and considerably smaller than the Sun, all of their eclipses will be annular , with the difference that the “ring” of light will be many times larger than the darkened region.
These eclipses are much more difficult to observe than those already mentioned. Mercury transits the Sun about 13 or 14 times a century , with the last time it transited in 2019 and the next in 2032. Venus is even more elusive, last transiting in 2012 and not transiting again until December 2117 .
In addition and of course, eclipses can occur in other bodies of the solar system , produced by one of the dozens of different moons that orbit the giant planets and the eclipses or transits of exoplanets in front of their respective stars . These transits have in fact turned out to be the most important detection mechanism for exoplanets in recent years.
References:
NASA Eclipse Web Site, NASA Goddard Space Flight Center, Last Updated: 2016 August 27, Consultation: 2022 7 April, https://eclipse.gsfc.nasa.gov/eclipse.html
