Thought Experiments

Partial eclipse visible to local residents

By Tim Philp For The Expositor

This March 9, 2016, file photo shows a total solar eclipse in Belitung, Indonesia. A solar eclipse on Monday is set to star in several special broadcasts on TV and online. (The Associated Press)

This March 9, 2016, file photo shows a total solar eclipse in Belitung, Indonesia. A solar eclipse on Monday is set to star in several special broadcasts on TV and online. (The Associated Press)

In just three days, the sky will put on one of the most dramatic shows that you are likely to see.

The dynamics of the orbits of the Earth, moon and sun will align on Monday to cause the moon to pass between the Earth and sun, casting a shadow on the surface of the Earth.

In times past, solar eclipses were harbingers of evil and disaster and there is at least one case where an eclipse stopped a war. According to Herodotus, fighting between the Lydians and the Medes was halted when the sun was blotted out by the moon. Both sides took this as a sign from the gods and ended their war.

There are two kinds of eclipses, solar and lunar. Lunar eclipses happen when the moon passes through the shadow of the Earth. This only happens during the full moon and the event can be seen over the entire hemisphere of the Earth. Solar eclipses, however, happen when the moon passes between the sun and Earth. As the moon is much smaller than the Earth, solar eclipses are visible only over a relatively restricted path.

Sadly, for those of us in Canada, the path of totality passes several hundred kilometres south of us. However, while we may not get to see the full glory of a total eclipse, we will be able to see the moon take a pretty big bite out of the solar disk.

In Brantford, the eclipse will cover about 73 per cent of the sun at the time of maximum eclipse. This is called a partial eclipse. The next time we will see a total solar eclipse in Brantford will be July 1, 2057. Even that one will not be exactly total as it happens when the moon is at its closest point to the Earth and, at maximum, there will still be a ring of the sun visible around the moon. This is called an annular eclipse.

We will, however, see many partial solar eclipses like the one we will see on Monday. There will be many opportunities to see the more common lunar eclipses in the coming years

You might wonder: Why don't we have an eclipse every month? After all, the moon orbits the Earth every month, why doesn't it pass in front of the sun each time? The answer is that the orbit of the moon is slightly tilted with respect to the orbit of the Earth. Most times the moon will pass either above or below the sun during the new moon period. Eclipses can only happen when the moon is crossing the orbit of the Earth. It is at these nodal points when the moon can block the sun's light or pass into the Earth's shadow.

To add to the complications, the moon's orbit is elliptical. This means that sometimes it is closer to the Earth than at other times. If the moon is farthest away from the Earth and the orbit coincides with a nodal point in the orbit, you can have an eclipse where the moon is too small to completely cover the sun. This leaves a dark shadow surrounded by a bright ring - an annular eclipse.

While an eclipse can be a magnificent sight, it also can be dangerous. Most people have enough common sense not to look directly at the sun with unshielded eyes as the light is bright enough to damage your eyes. Even using some filter material can be dangerous as the sun also emits dangerous ultra-violet radiation. This light is extremely powerful and is responsible for causing sunburn. Our eyes cannot see ultra-violet light, but its effects can be devastating. Staring at the sun can cause the eyes to become sunburned even through the wrong filter material. This can lead to a condition that welders know as "arc eye," and it is an extremely painful condition. If the damage is more extensive, blindness can result. While this is happening, you might not feel any pain in your eyes until it is too late.

The difficulty is that many so called "eclipse glasses" are not made to exclude ultra-violet radiation. To guard against this happening, you can get some welder's glasses or a piece of welding glass that is designed to block ultra-violet rays. You will still get a good view of the eclipse, but you should also take your looks sparingly and don't stare at the sun for long periods.

Perhaps the safest way to view the eclipse is on television or the Internet.

You can find instructions on the internet on how to make a pinhole eclipse viewer to allow you to see the show without risking your eyes. Looking at a projected image is certainly safer, but it is also wise to wear eye protection. Never use ordinary sunglasses to observe the sun. You will damage your eyes!

Solar eclipses are spectacular events, but you need to use care and common sense to see them. Your eyes are more important than any eclipse.

Tim Philp has enjoyed science since he was old enough to read. Having worked in technical fields all his life, he shares his love of science with readers weekly. He can be reached by e-mail at: tphilp@bfree.on.ca or via snail mail c/o The Expositor.

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