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?
Basic Facts
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.
Solar Eclipse
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.
Tides
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.
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