| WHAT IS AN ECLIPSE OF THE SUN? |
An eclipse is
the partial or total hiding of light from a celestial body (the Sun) as
it passes through the shadow cast by another body (the Moon).
(Adapted from the Philip's Astronomy Dictionary)
It's All About Shadows
If you walk down the street on a sunny day into the shadow of a building, you can say that the Sun is being 'eclipsed' by the building. The same thing happens with much larger objects, like the Moon, but of course the shadow is much larger! In fact, the Moon's shadow is nearly 3500 kilometres in diameter. The time when this eclipse happens is when the Sun, Moon and the Earth are perfectly lined up - the time of New Moon.
However, because the Moon is much smaller than the Sun, the shadow is actually a cone shape (see diagram below).
Diagram showing the solar system alignment during a Total Solar Eclipse with the Sun (left), Moon (middle) and the Earth (right)
When the Moon is close enough to us in its orbit we find the shadow-cone of the Moon can just touch the Earth. This is the time, and place, of a TOTAL SOLAR ECLIPSE, or a TOTAL ECLIPSE OF THE SUN. As you can see from the diagram above, this type of eclipse is only seen on certain parts of the Earth - the bits at the end of the shadow-cone.
As the Moon moves along its orbit the shadow moves across the Earth at a rate of about 2,000 miles per hour. The areas covered are called the Path of Totality, and only when you are standing in this path will you see a total eclipse. Further north or south of this line you will see a Partial Eclipse where only a part of the Sun is covered.
Why don't we see an eclipse every month? Well, unfortunately the Moon's orbit around us is tilted about 5 degrees to Earth's orbit around the Sun. This means the Moon's shadow usually passes above or below the Sun at New Moon. But at least twice a year everything lines up so that some part of the Moon's shadow falls on Earth's surface and a partial, annular or total eclipse of the Sun is seen from somewhere on the Earth.
Sometimes the Moon is a little too far away from us in its orbit, and then it cannot completely cover the Sun. This time we get an ANNULAR ECLIPSE. As you can see from the diagram below, the darkest part of the Moon's shadow (the umbra) is not long enough to reach the Earth, unlike during a Total Eclipse.
The view of an Annular Eclipse (right), and what's going on (left)
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Total Solar Eclipse Events
FIRST CONTACT: The instant the Moon starts to move in front of the Sun. Very slowly you will notice an increasing 'bite' being taken out of the Sun.
Darkening Skies: For about half an hour you will not really notice anything happening unless you know the eclipse is happening. This is because light levels are falling very gradually and the Sun's disc is bright enough to overcome the encroaching Moon.
Trees!: Try and find a tree to watch the dappling of sunlight. Usually the pinhole effect of the leaves causes circles of light on the ground. During an eclipse these dapples are transformed into hundreds of crescents.
Stars and Planets: The sky just before totality can be dark enough for the brighter stars and planets to be visible.
Plants and Animals: Birds begin to fly back to their nest, and nocturnal animals may appear. Listen out for any owls, and watch as some flowers begin to close their petals. The temperature is also dropping, and it can be quite chilly by the time of Totality.
SECOND CONTACT: This is it! But just before the Sun is covered you can see the Moon's shadow rushing towards you from the west. During the final moments watch as the Sun shines through all the valleys mountains of the Moon causing an effect known as 'Bailey's Beads'. The final view as the Sun disappears can take on the appearance of a 'Diamond Ring'.
TOTALITY: You may have from one second to seven and a half minutes to enjoy this spectacle. Totality is the only time when the Sun's outer atmosphere, the Corona, is visible. This is a pearly-white delicate structure of streamers weaving away from the Sun. Then, after too short a time, it's all over...
THIRD CONTACT: The moment the Sun reappears from behind the Moon. Due to the fact that your eyes have become slightly dark-adapted during Totality this looks more like a 'Diamond Ring' than Second Contact. Over the next hour and twenty minutes everything slowly returns to normal.
FOURTH CONTACT: The final 'bite' of the Moon disappears, and the Sun is full once again.
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Viewing Solar Eclipses SAFELY
The Sun can only be viewed safely with the naked eye during the few brief seconds or minutes of Totality. The partial eclipse and partial phases of the total eclipse are never safe to watch without taking special precautions. Even when 99% of the Sun's surface is obscured during the partial phases of a total eclipse, the remaining crescent is intensely bright and cannot be viewed safely without eye protection.
Do not attempt to observe a partial phase with the naked eye. Failure to use appropriate filtration may result in permanent eye damage or blindness!
Click here for my guide to Safe Sun Viewing.
The only way to safely see an eclipse directly as it happens is to use specially designed safety viewers that filter out all of the harmful Ultra-Violet and Infra-Red radiation from the Sun. Only buy viewers that have been Certified by the European Union (CE marked) and by the British Standards Institute.
When using Safe Eclipse Viewers please make sure you follow all instructions printed on them carefully.