What is an eclipse of the Sun?

Eclipses come in two flavours – eclipses of the Sun and eclipses of the Moon. Both occur when the Earth, Sun and Moon line up and both can be spectacular events. This article is about the first type, an eclipse of the Sun, or solar eclipse.

You can read more about an eclipse of the Moon here.

A total eclipse photographed in 2008 by Anthony Ayiomamitis
A total eclipse photographed in 2008 by Anthony Ayiomamitis

An eclipse of the Sun happens when the Moon passes between us and the Sun, hiding it partly or fully from view. It can therefore only occur at New Moon.

By incredible chance, the Moon appears almost exactly the same size as the Sun in the sky, despite really being considerably smaller. This is because it lies so much closer. It means that from parts of the Earth, the Moon can completely cover up the Sun, briefly turning day into night.

When the Moon completely covers the Sun, there is what is termed a Total Eclipse. When it occurs, the Sun’s atmosphere, called the corona, comes into view as a ghostly band – the only time it can be seen from Earth. Any prominences leaping from the Sun’s edge will also become visible.

The shape of the corona changes depending on where the Sun is in its 11-year cycle of activity. Around solar minimum, this glow stretches mainly from the equatorial regions and is much less evident towards its poles. Around solar maximum, the corona is more evenly spread all around the Sun’s disk.

A splendid photo of a solar corona is shown here, taken by legendary astro-imager Anthony Ayiomamitis. See more of his pictures at his website.

Because the Moon just covers the Sun, the area from which a total eclipse is visible is very small, perhaps 160km wide, on the Earth. This stretches into a lengthy eclipse track that can run for thousands of kilometers across continents as the Moon travels in its orbit and the Earth rotates below. But it means that from any one point on Earth, total eclipses are extremely rare.

More common are partial eclipses which may be seen for hundreds of kilometers around the track of a total eclipse. From these areas, the Moon is seen only partly to hide the Sun. The further you are from the zone of totality, the smaller the amount of Sun is blotted out. Some eclipse are only seen as partial anywhere in the world when the Moon’s main shadow just misses hitting the Earth.

Key moments during an eclipse are termed contacts.

  • First Contact occurs when the Moon’s limb first encroaches on the Sun.
  • Second Contact happens when the Moon completely covers the Sun and totality begins.
  • At Third Contact, the sun’s light breaks through a lunar valley, producing the famous Diamond Ring Effect and marking the end of totality.
  • Fourth Contact is when the trailing edge of the Moon moves completely off the solar disk.

Only during the period of totality is is safe to look directly at the eclipse but be sure to do so because it is an amazing spectacle. Look away, or use your eclipse specs again, the moment the Sun reappears.

animation of annular eclipse
Animation of annular eclipse

There is a further type of solar eclipse, called an annular eclipse. This happens when the Moon is around its furthest distance from Earth in its orbit and appears too small to cover the Sun’s disk completely. Instead, when they line up, a ring of sunlight is left around the Moon’s edge, a phenomenon that has been dubbed the Ring of Fire. Although you do not get the subtle effects seen in a total eclipse, they are still spectacular. Because sunlight remains brilliant, it is vital to protect your eyes throughout such an eclipse.

Eclipses of the Sun do not happen every New Moon because the plane of the Moon’s orbit is tilted by around five degrees to that of the Earth around the Sun so that it normally passes above or below it. They occur at the points where the two orbits cross, called the nodes.

Solar eclipses may be observed using the usual safe techniques you must always use to observe the Sun, such as projecting the Sun’s disk through a telescope or using a proper dense solar filter. Alternatively, you can project a tiny image of the eclipse without a telescope by making a pinhole in a piece of card and allowing the Sun’s light to shine through it onto a second piece of card.


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By Paul Sutherland

Paul Sutherland has been a professional journalist for nearly 40 years. He writes regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy, plus he has authored three books on astronomy and contributed to others.

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