MadSci Network: Astronomy

Re: Why doesn't one side of the earth face the sun continuously?

Date: Fri Dec 8 09:57:10 2000
Posted By: Meghan Gray, Grad student, Astronomy, Cambridge University
Area of science: Astronomy
ID: 973694042.As

Dear Bill,

You're right, the Earth's gravity does have a big effect on the Moon...and
the other way round.  We only see one side of the Moon because the
gravitational forces of the Earth have gradually slowed it down and locked
it into a `synchronous' orbit,  so that its rotational period matches its
orbital period around the Earth (incidentally, this is quite common for
other satellites in the solar system).  It rotates exactly once for each
time it orbits the Earth, meaning the same side of the Moon always faces

But the Moon's gravity is also acting on the Earth...the most obvious
effects being the tides.  The bulges raised in the surface of the ocean
get dragged along by the Earth's rotation, meaning that the force between
the Earth and the Moon is no longer pointing directly along the line
connecting them.  This results in a torque which causes the Earth's
rotation to slow slightly and also moves the Moon farther away (by a few
centimetres a year).  Eventually, the Earth's rotation will match the
Moon's period, and only one side of the Earth will be visible from the
Moon. The timescale for this, however, is likely to be several tens of
billions of years (by which time the Sun will long since have died!)

But the Sun's gravity influences the Earth as well, of course...this is
why the tides are strongest when the Sun, Moon, and Earth are aligned. But
to bring some math into, tidal forces decrease as the cube of the
distance between the objects.  That's why although the Sun is much more
massive than the Moon, the tides raised by the Moon are much stronger,
because it's so much closer.

So while the Sun may produce a similar torque on the Earth to try to
force it into synchronous rotataion, my guess is that the timescale for
this is much, much too long to make much of a difference!

A nice website with lots of information on the solar system is


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