Expert: Jayendra Upadhye - 11/17/2004

Question
Hi!
I have 2 statements, I need to know if they make sense, and if they don't I need to know why.  Thanks for any input

1) If we could magically replace the sun with a giant rock that has precisely the same mass, Earth's orbit would not change.
2) the fact that the moon rotates once in precisely the time it takes to orbit Earth once is such an astonishing coincidence that scientists probably never will be able to explain it.

Answer
Hello johnny,
1 - The first statement is completely true.
The reason is that the earth's orbital parameters are governed simply by its
a) - orbital speed (tangential linear velocity)
b) - Its orbital radius
c) - Its mass and the sun's mass.
replacing the sun with a rock of same mass does not change any of the above and so the earth's orbit remains unchanged.

2 - That is bunk!
Scientists DO KNOW why this has happened.
This happens because of tidal gravitational interaction between two closely orbiting bodies. (reasonably closely as in the earth moon system).
Also this is not unique to earth-moon.
Other satellites in the solar system also show this behaviour too, in fact mercury also shows this type of behaviour.
The moon slows the earth and the earth speeds up the moon in its orbit. As a consequence, we have to introduce the concept of LEAP SECOND (look it up in google as "leap second"). The moon as a consequence recedes from earth approx 4 cm a year.
Also over many millions of years (it is a slow process), the moon gets phase-locked keeping one face continuously pointed to the earth.
So it is all fully explained.
look up "lunar recession rate" in google.

In both cases, there are hundreds of sites on google explaining both.
Pls do rate this answer.
Jayen