Moon, the satellite which moves around our planet Earth, is the most extensively observed celestial objects by humans. We do look at the Sun, at sunrise and sunset, but rarely do we look at it during the day (and it is not advisable to look at the Sun when it is up above in the sky). Looking at Moon conjures benign and romantic expressions. Different phases of the Moon give humans something to talk about - full moon and new moon have taken on mystical interpretations by different civilizations. Stories of werewolves are interwoven with full moon, while most Indians consider new moon to be inauspicious, except Tamilians who believe in the opposite.
So well do we know our nearest neighbor?
No other planet in Solar System has a moon so big relative to its size. It has about one-eightieth mass and one-fourth diameter. It is about 380,000 Km from Earth (on average), about 400 times closer than Sun.
The Far Side of Moon
We all know that most celestial objects such as planets and stars rotate on their own axis. Sun takes 28 days to make one rotation, and there are some extremely dense stars in the galaxy called pulsars which rotate 640 times in one second. Moon also rotates around its axis once in 28 days. So, naturally we should assume that we get to see the entire Moon’s surface over a period of few days. But that’s not what happens.
And here is the stunning fact – we get to see only one side of the Moon forever while the other side is hidden from our view forever (at least that is the case in over ten thousand years of modern civilizations).
We never get to see the other side of the Moon no matter what. Why does that happen? Coincidentally, Moon takes the same number of days to rotate and revolve – 28 days. As it rotates, it also revolves by the same amount, and therefore we get to see the same side always. We saw the other side of the Moon only when humans launched rockets into space that went around the Moon.
An eclipse is caused when Sun, Moon and Earth come in a straight line- solar eclipse, when Moon comes in the middle, and lunar eclipse when Earth comes in the middle. During solar eclipse the Moon blocks the Sun almost exactly – both of them turning out to be exactly the same size. Now, how does this happen that Moon blocks out the Sun completely and exactly? Coincidentally, Moon is about 400 times smaller than Sun (in diameter) and it is 400 times closer to Earth than the Sun. So, for an Earthman, this turns out be quite convenient, wherein Sun and Moon appear to be of the exact same size.
One of the fortunate outcomes of this event was exploited in 1919 wherein two teams set out to Brazil and West Africa to verify Theory of Relativity. Einstein stormed into the scientific world with four amazing papers in 1905 (dubbed the ‘Miracle Year’), covering the topics of Brownian motion, photoelectric effect, special relativity, and mass-energy equivalence. With his special theory of relativity he shattered the axiom set forth my Newton that space and time were absolute. Ten years later, he presented to the world his magnum opus ‘General Theory of Relativity’. Once again he challenged Newton to give a completely different interpretation to gravity. While Newton assumed that gravity is a force, Einstein said that gravity warps the space-time fabric causing dimples in it. This means that objects coming close to a heavy object are drawn towards it because of the dimple it creates. This means that light would also bend around massive objects. Now how do we test this? Because Earth’s gravity was not sufficient enough to bend light! The nearest heavy object was Sun.
So, Einstein predicted that a star appearing close to Sun would get displaced from its original position because Sun will act like a magnetic lens bending the rays slightly. However, since Sun is so bright during the day, we cannot see any stars close its edge. But Einstein also said that we could use a solar eclipse where Moon blocks out Sun to see a star displaced by certain angle – he even predicted what that angle would be.
Lo and behold, the teams came back to announce that Einstein was right. Light gets bent by Gravity. And let’s say thanks to the Moon’s ability to block out the Sun completely and exactly.
While we are able to understand and predict solar eclipses, they were quite mysterious to our ancestors. Almost all civilizations attributed them with some mystical powers. Chinese believed that a dragon was about to gobble the Sun, and to scare the dragon away, they came out onto streets to create lot of noise. The similar superstition amongst Europeans who came onto streets to make noise led to coining of the word ‘pandemonium’ (demons everywhere). Aztecs believed that Sun was under attack by monsters and hence they sacrificed the cripples to help sun warriors save the Sun. Hindus thought that the evil planets Rahu and Ketu gobbled up the Sun. Many modern Indians continue to believe in mystical powers of solar eclipse and refuse to come out onto streets. They shun food and sing devotional songs to ward of the evil.
Those who live close to the sea know about the tides which are caused by gravity of Sun and Moon. Though Moon has very small mass compared to Sun, it plays a much bigger role on the tides because it is much closer to the Earth (gravity decreases with square of distance). The effect of Moon, which is twice that of Sun, has always been playing a big role on Earth, not only on the tides, but on its life and duration of its day.
A billion years ago, Moon was much closer to Earth- it was 200,000 kilometers closer. And because of this, Earth rotated faster around its axis and the day was only 18 hours long. But most importantly, the tides varied by hundreds of meters each day inundating most of the coastline making it difficult for life to migrate from water to land. Only when Moon moved away, and tides became less varying, the life started to move to land. (And there was a time when Moon was much closer, and a day on Earth was only 4 hours long.)
Organisms like worms started to use these tides to time their reproduction. Is it a reason why human’s ovulation period is also 28 days? I don’t know that answer. Is just another coincidence?
In future, Moon will move farther away, the tidal range will be smaller, but our days will get longer.
Craters on the Moon
Any observation, even a casual one with a naked eye, would reveal the craters on the Moon. Detailed photographs show hundreds of craters. These craters are formed by flying objects that bombarded Moon.
Why is that we don’t see craters forming now on Moon? Two reasons. One, the bombardment doesn’t happen regularly on a human timescale – the last one was possibly witnessed in 12th century by British monks. Second, the Solar System was quite a violent place in the initial 500 million years, with many objects whizzing and flying by and therefore bombardment was a constant phenomenon unlike now. That was when Solar System was getting formed out of spinning gas and dust. The entire Solar System along with Sun, planets and their respective moons came about almost at the same time. That’s why all planets move around the Sun in the same direction as Sun’s rotation, because they came out of the disk of rotating mass. Sun took up all the mass, about 99.86% of the mass of solar system, leaving only about 0.04% of the mass to the planets and the moons. Out of this, four gaseous planets, Jupiter and beyond, take up the bulk – Jupiter being 318 times the mass of Earth.
Earth and Moon must have been formed at about the same time. Moon must be a chunk of Earth that got separated due to collision with another celestial object.
So, if Earth and Moon are of the same material and if moon came out of Earth, why does it show so many craters while Earth has almost no craters.
The answer lies in two things – one is atmosphere/water, and the other is the large sized hot core of Earth.
The atmosphere of earth protects the water from getting evaporated. Earth has large water mass on its surface. Though oceans are 4 kilometers deep on average, it is covers nearly 70% of the surface of the planet. The surface of Earth is constantly altered by our atmosphere which brings rains and rivers. They can create structures like Grand Canyon within few millions of years. The tectonic movement of its continental plates caused by the heat coming from center of Earth keeps changing the surface of the earth. It takes the land mass inside while it creates new stretches; it brings seabed onto mountains and plummets mountains onto the seabed. .
Moon’s small mass gives it small gravity. Because of this it could never maintain an atmosphere. Without any rains and rivers and without large sized hot core to drive tectonic movements like on Earth, Moon remains the same for millions of years, pock-marked, retaining all its blemishes since the beginning.