What is a solar eclipse?

A solar eclipse occurs when the Moon passes between the sun and Earth, and the Moon fully or partially blocks ("occults") the sun. The degree of occultation depending on where on the planet you are watching from, so you will only see the event as a total eclipse from specific places on a specific day. In some places, the shadow of the Moon will fully block out the sun, but in other nearby places it will only appear to partly block the sun – and most of the world won't have an eclipse at all.

Why are eclipses rare?

As well as being an amazing spectacle, total solar eclipses gain in excitement by their rarity. There are only occasional days a year when the Moon's orbit swings it in front of the sun – with a total eclipse only occurring along a very narrow path on Earth's surface traced by the Moon's shadow (umbra).

If the Moon were in a perfectly circular orbit, a little closer to the Earth, and in the same orbital plane, there would be total solar eclipses every single month. However, the Moon's orbit is tilted at more than 5 degrees to Earth's elliptical orbit around the sun, so its shadow only rarely falls on the Earth as it gets between our planet and the sun. Only when the heavens align just so does the Moon cross the Earth's orbit at exactly the right point in terms of distance and alignment to cause either a partial or total eclipse. The geometry means there can never be more than two total solar eclipses in any year.

Are some eclipses better than others?

The total eclipse on March 20, 2015 was one of the best in purely astronomical terms (if not weather, for some parts of Europe...), as the Moon was at one of its closest points to the Earth, therefore appearing extra large - a so-called supermoon. This proximity to the Earth explains why this eclipse was visible from a much larger swathe of the planet than many others, with varying degrees of eclipse visible in Greenland, Iceland (almost total), Europe, northern Africa, the Middle East and NW Asia.

Are there different types of solar eclipse?

While a total solar eclipse occurs when the Moon is close enough to the Earth to completely cover the sun as it traverses in front of our star, annular eclipses are a related phenomenon where the Moon traverses in front of the sun but at a distance from the Earth that means it only partially covers the star – it will appear as a dark circle crossing the sun's dazzling orb.

Annular vs. total eclipses

During a total solar eclipse, the Moon is just the right distance from the Earth that it totally covers the sun when it moves in between. In an annular eclipse, however, the Moon's position means it passes across the face of the sun but doesn't totally cover it.

This is by no means a lesser spectacle! Annular eclipses are just as beautiful and spectacular as total eclipses because they offer the unique phenomenon you don't get in a total eclipse, known as the "ring of fire". This is when the outer edges of the blazing sun completely encircle the silhouette of the Moon. The last annular eclipse was visible from southern Chile, Argentina and Angola in February 2017, with the next scheduled for December 2019 which will be visible from swathes of the Middle East and southeast Asia.

How long does an eclipse last?

How long does an eclipse last? The length of time the eclipsing sun will 'draw' a path across the globe will also vary from eclipse to eclipse. So every eclipse will last a different length of time, and the period of totality – when the sun is totally covered by the Moon – will vary. Please bear in mind that totality never lasts more than a few minutes, so you need to be ready at the big moment, not 'just popping off the loo' or anything like that! The brevity of total eclipses is part of their unique appeal – truly a rare moment. But it also emphasises a key point that any 'eclipse holiday' involves grabbing the chance to explore the area you're in on the days before and after the cosmic event.

Just to give you some figures, the total eclipse due on July 16, 2186 will be the longest for the next 1,000 years - with a maximum duration of 7 minutes 29 seconds of 'totality'. The briefest total eclipse of this current millennium happened in February 2003, with a maximum of just 9 seconds of totality! For more information on eclipses generally, including the maximum duration of any specific eclipse coming up, check this NASA eclipse page.

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Tell me some dates!

Because humans have become pretty good at astronomy, we can now predict when eclipses will occur with great accuracy a long way into the future - barring some cosmic cataclysm that moves the Earth, Moon or Sun. If you're curious – or want something to tell your children or grand-children to put in their diaries – NASA publish a list of dates for eclipses up to the year 3000!

Between 2001 and the year 2100 there will be 224 eclipses, with 68 of them total. See a list of dates.
Written by Norman Miller
Photo credits: [Page banner: James Niland] [Why are eclipses rare?: Jongsun Lee] [Annular vs. total eclipses: Kevin Baird] [Tell me some dates!: COD Newsroom]