Don’t be left in the dark: eclipse facts.

A solar eclipse is one of the few astronomical events that actually gets people on mass to look up and think about what happens in the heavens. In this article, I want to indulge your astronomical interests with five interesting eclipse facts alongside a few of my own images of todays eclipse.

1. We can all agree that an eclipse is a pretty rare event, but you may not know that todays eclipse is especially unusual! This is because it occurred on the spring equinox and also reached the North Pole. This is the first time the North Pole has seen the sun for over 6 months; meaning that anyone, or anything, up at the Pole would have had to endure an extra few minutes of darkness because of the eclipse.

2. The movements of celestial objects are amazingly regular and cycles of movement can be predicted long in advance. Solar eclipses, like the one we saw today, are the result of ongoing cycles which repeat every 18 years, these are called Saros cycles. Many Saros cycles run simultaneously and todays eclipse was part of cycle #120. That means that, although the next eclipse in cycle #120 will not occur for another 18 years, similar eclipses will occur as part of other cycles. Therefore, there will be another total eclipse next year, this will be part of another Saros cycle (#130) and will be visible over Indonesia and the Pacific ocean. These cycles do not continue repeating forever actually, after about 1300 years, each Saros cycle stops, and a new one takes its place. Sadly, despite the wealth of Saros cycles running right now, we wont actually see another total eclipse in the UK until 2090.

3. Solar eclipses occur because of an amazing coincidence. The Sun is about 400 times larger than the Moon but the Moon is about 400 times closer. Therefore both appear to have the same size in the sky (about 0.5 degrees of arc, which is approximately the size your thumb when you extend your arm). – As explained perfectly here by Father Ted Crilly.

4. Eventually we will no longer witness any Solar eclipses. This is because the Moon is slowly moving away from the Earth; but why? The Moon exerts a gravitational force on the Earth that ‘deforms’ the oceans. We notice this affect in the form of tides. Over time, this deformation of the oceans exerts a small gravitational force back on the Moon which accelerates it, pushing it away from the Earth whilst also slowing down the Earth’s rotation. Eventually the Moon will be too far from the Earth to fully cover the face of the Sun and the solar eclipse will become history.

5. Solar eclipses allowed physicists to test general relativity. When light travels close to a massive object (such as a galaxy) its huge gravity actually bends the light slightly towards it. When you calculate this bending affect, using old Newtonian physics and newer general relativity, you get different answers. What was needed was a test. In 1919, Sir Arthur Eddington found that if you could measure the position of a star whose light past near the Sun then you could calculate the true light bending affect of the Sun’s gravity. He could only perform this measurement during a solar eclipse because most of the Sun’s glare is blocked out by the passing Moon, meaning that he could observe stars appearing near the Sun. He found that their positions, relative to other night sky objects, changed very slightly when they were influenced by the Sun’s light-bending gravity. Indeed, the amount of bending was found to agree with…you guessed it…general relativity.

Guest post by: Dr. Daniel Elijah