A total solar eclipse is one of the rarest of rare events, ever witnessed on Earth and also in the universe. One may say that such an eclipse is all 'shadowy' business. It occurs when the Moon is placed exactly between the Sun and the Earth. If you happen to be in the Moon's shadow which falls on Earth, you see the Moon completely covering the solar disk and then moving away. Not only is it one of the most beautiful sights you could ever see, it's also a once-in-a-lifetime experience. Let me explain, why it's such a rare experience and what are the circumstances which make it possible. We will see how the distances between the Sun, Moon and Earth, their orbital plane angles and their sizes, are just so rightly proportioned that they conspire to make a solar eclipse possible! Then we will go further and explore if the occurrence of this rare phenomenon on Earth; the only planet with life, is more than just a coincidence.
How is a Total Solar Eclipse Made Possible?
The total solar eclipse occurs on a new Moon, when Sun, Moon and Earth are so exactly aligned, that the dense part of Moon's shadow, that is the cone of the umbra touches the earth. It passes over, caressing the surface of the Earth at a speed of more than 1700 km/hr. The penumbra is the less denser part of the shadow that falls on the Earth. If you are standing in the penumbra part of the shadow, what you will see is a 'partial' solar eclipse. That is, you will see the Moon covering only a part of the Sun. The 'total' in total solar eclipse indicates that the whole surface of the Sun is exactly covered by the Moon.
One wonders what makes this possible! The Sun is so huge in its extent, that the Moon is smaller than a pygmy in its comparison. It's very likely that the Moon's angular size could not have matched the Sun's size. Still, the whole of Sun is covered by the Moon during a total solar eclipse. That's because their angular sizes at that particular moment match exactly.
Now consider these facts. The diameter of the Sun is roughly 400 times the diameter of the Moon and the distance between the Earth and the Sun is approximately 400 times the distance between the Earth and the Moon. This proportionality of distance contributes to the matching of the angular sizes of the Sun and the Moon. The distance is so well synchronized, that when the Moon is close to its perigee (the point at which the Moon gets closest to the Earth), the total solar eclipse occurs. At least two solar eclipses occur every year with a maximum of five. It is estimated that 40% of the time, they are total solar eclipses.
Thus, the distance between the Earth and the Moon plays a vital role in making a solar eclipse possible. If the Moon is not near perigee and the distance is not right, the tip of the umbra shadow cone will not touch the Earth. In such a case, people on the Earth positioned right opposite the tip of the umbra will see an annular solar eclipse. That is, the Moon will cover the Sun concentrically from inside but a thin border ring of the Sun will still be visible.
The Moon's plane of orbit is tilted with respect to the plane of the Sun's equator (called the 'ecliptic'), at an angle of 5.145 degrees. In addition to this, Moon has an elliptic orbit and the Sun's gravity affects the Moon's orbit. In spite of all these reasons for misalignment, sometimes, the trio of Earth, Moon and Sun do align themselves in a line to make a 'Total Solar Eclipse' possible. Then, if you consider all these facts in totality, you can understand how unlikely it is that such an event may occur.
One wonders, whether such a beautiful phenomenon like an eclipse, also occurs on other planets like Mars. What would a total solar eclipse caused by a Martian Moon look like? Is it even possible there? Only scientists can tell us.
What Makes Life on Earth Possible?
Well, if you start listing the reasons that make life on Earth possible, the list will be endless. Let us think about the effect of the Moon's proximity and the distance from the Sun, on the possibility of life on the Earth.
The Moon causes sea tides. The tides play a vital role in the regulation of life cycles on the Earth, setting the rhythm of life for several sea life forms. The strength of the tidal forces is influenced by the distance of the Moon from Earth. Therefore, as the first life forms emerged in the sea, the Moon's role in subtly influencing evolution of life is unquestionable.
Second thing to be noted is the distance of Earth from the Sun. If our planet would have been closer to the Sun like Mercury or Venus, the high temperature would not have sustained life and would have made it almost impossible. Similarly, if the planet would have been too far from the Sun, the lower temperatures would not have made life possible. The point to be noted is that the current distance is just right for sustaining life.
The Earth and the Moon share a geological history and the current hypothesis about their origin, says that a massive object collided with the hot and molten proto-earth, to create the Moon. The Earth-Sun distance is such that the Earth's atmosphere is protected from the solar wind, blowing at high velocity. It consists of high energy particles, that could have made a stable atmosphere on the Earth impossible, if Earth would have been too close!
The odds were very largely propped against emergence of life on Earth. If you really consider all the factors, you will realize that emergence of life on Earth is nothing less than a miracle. Thus, the alignment of the Earth-Moon-Sun system has played a vital role in the formation of life on Earth, just like it makes a beautiful event like total solar eclipse possible. The similarity between the circumstances and reasons contributing towards the two events should be taken note of. There is something special about the combination of factors that lead to making life on Earth possible and an eclipse too. When such extremely rare events occur at the same place, one wonders whether serendipity is at work!
A Hint to Find Extra Terrestrial Life!
The fact that two such extremely rare events occur at the same place makes one wonder about a connection between the two. If serendipity works here, why couldn't it work elsewhere? Why can't we consider the conditions which make an eclipse possible, as indirect indicators of existence of extraterrestrial life? What I mean by that is, we should look for extra solar planetary systems with distances and sizes matching our own solar system. Planets which have a similar alignment like the Earth-Moon-Sun system should be searched. This might be an important clue to start looking for life beyond the Earth, on other planets or other extra-solar planets!