|How does Earth keep its orbit around the Sun and
not come closer to the Sun?
|Question Date: 2005-01-08|
The Earth is always being pulled towards the Sun
by gravity. If the Earth were stationary compared
to the Sun, it would fall into the sun under the
force of gravity. However the Earth is actually
moving sideways compared to the center of the Sun
at 3 km/second (~2 miles/second). The Earth is not
moving fast enough to "escape" the Sun's gravity
and leave the solar system, but it is going too
fast to be pulled into the Sun. Therefore, it
keeps going around and around - orbiting the Sun.
It is rather like a tether ball. Think of the top
of the post as the Sun and the ball as the Earth.
The string between them is like the force of
gravity keeping them the same distance apart. When
you hit the tether ball it spins around the post.
If there were no air or rope friction, the ball
would spin forever without getting any closer to
the post. That is essentially what the Earth is
doing when it orbits the Sun - in the vacuum of
outer space, it does not loose speed to air
friction, so it just keeps going around the Sun.
Well, that's a good question, and Newton worried
about the same thing! Actually, due to conservation
of angular momentum, all the planets are in fairly
stable orbits, with minor changes over millions of
years, but no chance that they will fly off or
anything! The earth's orbit is in the shape of an
ellipse, which means that we get a little bit
closer and farther from the sun over the course of
a year. We also wobble in the tilt of our axis, so
that the North Pole does not always point to the
star Polaris, which is currently our north star.
But, the orbits are pretty stable, because there
is a fairly constant gravitational force between
the sun and the earth keeping the earth in its
orbit. The strength of this force changes slightly
over the course of an orbit,being a bit stronger
when the earth is a bit closer - at those
times(currently, when the northern hemisphere is
having winter) the earth actually orbits a bit
faster. (Not to be confused with spin!)
The Earth is "falling" around the Sun. The Earth
has some initial momentum - it is moving in a
direction,which is perpendicular to the direction
of the Sun from the Earth. The Sun's gravity is
enough to keep the Earth from flying off in a
straight line, away from the Sun, but not enough
to bring the Earth closer in - the Earth is
continually changing its direction of movement,
but in such a way that it follows a nearly
circular path around the Sun.
If the Sun's
gravity were stronger, it would pull the Earth in
closer, but then the angle between the Earth's
motion would also be changing more rapidly, so it
would continue revolving around the Sun.
concept is called the conservation of angular
momentum, which is one of the basic principles of
Kepler's law and Newtons laws explains this very
well. If we could just STOP the Earth for a moment
relative to the sun and then allow it to freely
move it WOULD fall into the sun. But the earth was
born from a ring of material that was MOVING
around the sun on a stable orbit. So after the
debris coagulated to form the earth, this initial
orbital energy was retained. Hence the earth is
moving on a stable orbit of fixed radius. Look up
Keplers laws the period of revolution of a body
squared is proportional to the distance between
the sun and the body cubed.
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