Why there isn’t a solar eclipse every month
The Sunday Nation
Nairobi,
24 January 2010
Last weeks Solar Eclipse rekindled people’s curiosity about issues of outer space. Jeremiah, for example, wants to know why it is dangerous to look at the eclipse with the naked eye.
Well, simply because you can go blind; but contrary to popular belief, it is not the eclipse itself that is dangerous! The presence of the moon in front of the sun does NOT make it more harmful. The reason people are advised not to look at the eclipse without protection is that, even though almost fully covered, the sun is still too bright for the eyes.
A seemingly simple question came from three readers: “Why isn’t there an eclipse every month?” Now this question is based on the knowledge that the moon goes around the Earth once every month (27 and a quarter days), thus every cycle it must cross paths with the sun.
If you draw the three objects at play (Sun, Moon and Earth) with their orbits on a piece of paper, you will easily see that we should expect to get a solar eclipse somewhere on Earth once every month. Unfortunately, however, this drawing which is common in school textbooks is not accurate.
This is because the orbits or the Earth (around the Sun) and the Moon (around the Earth) are not on the same plane. A more accurate representation can be drawn using two pieces of paper.
On the first paper, we draw the orbit of the Earth around the sun and on the second, the path of the Moon around the Earth. Then we cut out the papers along these orbits.
Now we have two (almost) circular pieces of paper – one with the Sun at the centre and the Earth at the edge; the other has the Earth at the centre and the Moon at the edge.
Next, we bring the two “Earths” together and place the two pieces of paper on a flat table. This produces the same inaccurate representation as before. To get a more realistic view, we have to lift the two from the table and tilt the moon’s orbit about 5 degrees from that of the Earth.
Now we see why there isn’t an eclipse every month. In fact, if we leave it that way, we might conclude that there should never be an eclipse at any time! So there is one final correction: The Moon’s orbit (paper with earth at the centre) rotates slowly completing one cycle in about 18 and a half years.
Due to this rotation of the orbit (known as precession), the Moon will fall between the Earth and the Sun every once in a while. As a result of these complex motions, there is an eclipse somewhere on Earth twice every year. However, a repeat identical one occurs at the same place only once in about 18 and a half years.
OK: There was a solar eclipse in
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