An eclipse of the sun – or a solar eclipse – happens when the view of the Sun
from Earth is blocked by the Moon. There are two main types of solar eclipse: a
partial eclipse and a total eclipse. A partial eclipse is when the Moon covers
part of the Sun, the effects of which aren’t really that noticeable. A
total eclipse is when the Moon covers all of the Sun and is an event which turns
day into night and, for a few minutes, produces an amazingly unique and very
special view of the Sun.
Although solar eclipses are relatively common – total eclipses occur about six times in every ten years – they aren’t particularly easy to view. This is because each eclipse is visible from different places on Earth. For example, a total eclipse in 1999 could be seen in certain parts of Europe, whereas one in 2006 could only be seen in parts of India, China and Nepal. And when you consider that 70% of Earth is covered by water, and much of the land on Earth is uninhabited, getting to see most eclipses simply isn’t practical or possible for most people. Unless you happen to be in the right place at the right time, you would have to be an extremely dedicated eclipse spotter to want to travel the world to see one. Fortunately, astronomers are very clever people and are able to predict the exact time and place that eclipses will occur, making travel arrangements all the more convenient. This is because they understand what causes eclipses. And if you too want to find out what causes them, read on! Exciting stuff, isn’t it??
What causes Eclipses?
Eclipses are caused by the basic movement of Earth around the Sun and the
movement of the Moon around Earth . As you may (hopefully!) know, at the centre
of the Solar System is the Sun. The Sun is a star, a massive ball of gas pumping
out heat and light. Travelling around the Sun are eight planets. Earth is the
third planet from the Sun, and it takes about 365 days for Earth to make one
journey around the Sun. This is why a year is 365 days long. While Earth is
orbiting the Sun, the Moon is orbiting Earth, and it takes about 27 or 28 days
for the Moon to complete one orbit, or cycle. This means that once every 27/28
days, the Moon is positioned in between Earth and the Sun. When it is in this
position, it is in its New Moon phase. Phases of the Moon are explained in more
detail here, but to summarise, the Moon’s appearance changes depending where
about it is during its orbit around Earth. As the Moon reflects light from the
Sun, you can only see the portion of it that is receiving sunlight. When the
Moon is in its New Moon phase, the light from the Sun is on the side of the Moon
that isn’t facing Earth. This means that, from Earth, the Moon cannot be seen.
The only time it can be seen during its New Moon phase is if it happens to pass
in front of the Sun. When it does, we have an eclipse!
Eclipses are a natural result of the fact that planets orbit stars and moons
orbit planets. For every planet that has a moon, there will be regular times
when that moon is positioned in between the planet and the star, therefore
causing an eclipse. This is true across the universe. However, the types of
eclipses that are visible from Earth are extremely rare, and are the result of
what can only be described as an amazing cosmic coincidence.
Here are some statistics. The Moon is 3,476 kilometres (2,159 miles) wide and it is about 384,400 kilometres (238,855 miles) away from Earth. The Sun is 1,391,000 kilometres (864,300 miles) wide but is about 149,597,890 kilometres (92,955,820) away from Moon. If you’re still awake, you can see from these figures that the Moon is much smaller than the Sun, but is much closer to Earth. In fact, the Moon is about 400 times smaller than the Sun, but is 400 times closer to Earth. This means that, from Earth, both objects appear to be the same size even though they are clearly not. And it means that, during a total eclipse, the Moon is just about able to cover the entire surface of the Sun and cause the visual spectacle to be described later. If the Moon was any smaller or bigger, or orbiting closer or further away from Earth, the effect of a total eclipse of the Sun would be nowhere near as spectacular.
Although the Moon is positioned between Earth and the Sun once in every 28 days, it doesn’t always pass in front of the Sun. There are a large number of reasons why this is the case. One of them is that, in comparison to the rest of the sky, the Sun and Moon are relatively small objects in it and they don’t stay in one place. As Earth spins, this gives the illusion that the two objects are moving across the sky. The Moon is also moving as it travels around Earth, which also affects its position in the Sky. There are further complications which prevent the Moon from always passing in front of the Sun. Amongst them is the fact that the Moon’s orbit around Earth is inclined (tilted) as in the image below. So getting the Moon to be directly in front of the Sun isn’t that easy, and most of the time, the Moon only covers part of the Sun which causes a partial solar eclipse. Fortunately, astronomers know precisely the movement of Earth and the Moon and accurately predict where eclipses, partial or total, will be visible.
Partial and Solar Eclipses
During a partial eclipse, there will be a slight darkening of the sky, which
will be hardly noticeable to most people. Even if the Moon covers most, but not
all, of the Sun, the effect will be to make the day seem slightly overcast.
However, it is easy to see the Moon in front of the Sun if you are able to look
at it. Unfortunately, looking at the Sun directly, even during an eclipse, is
very dangerous, and should only be done if you are wearing special glasses with
filters that block most of the dangerous light (not sunglasses or 3D glasses!)
or by projecting the image of the Sun onto a piece of paper or card using
binoculars or a telescope, making sure you don’t look through the lenses!
During a total solar eclipse, the whole disc of the Sun is covered by the Moon. As the Moon begins to cover the surface of the Sun, there is a gradual darkening of the sky. Once the Moon is exactly in front of the Sun, the sky will suddenly go much darker, and the Sun will take on a completely different appearance. This moment is known as totality, and usually lasts between five to seven minutes. Instead of seeing the Sun, you will see a ring around where it should be. This ring is the Sun’s chromosphere and corona, the Sun’s outer atmospheres. The chromosphere appears as a narrow line (often red) around the Sun. It separates the Sun’s visible surface – its photosphere – from its corona. The Sun’s corona is a region of extremely hot gasses (called plasma) that extends many millions of miles outwards from the Sun. It is actually hotter than the Sun’s surface! If you are lucky, you may even be able to spot solar flares.
Just before the Sun goes behind the Moon or just before it reappears (before and after totality), you may also be able to see the Diamond Ring Effect. This is caused by mountains and valleys on the surface of the Moon. As the Moon is just about to cover the entire surface of the Sun, some beams of light pass through these gaps. When there are several beams of light, they resemble beads and are known as Baily’s Beads. When there is just one, it gives the illusion of a bright diamond on a ring.
Witnessing an Eclipse
Getting to see a total eclipse is simply a case of being in the right place at the right time, and hoping that the weather is good too. For information about future eclipses, including the places you should be in to spot them, have a look at this page: http://eclipse.gsfc.nasa.gov/SEdecade/SEdecade2011.html (link opens in a new window as takes you to an external website)