MadSci Network: Astronomy

Re: Do planets cause tidal bulges in the sun?

Date: Thu May 20 12:15:39 2004
Posted By: Jeff Robertson, Associate Professor of Astrophysics
Area of science: Astronomy
ID: 1084854982.As

It is not at all likely that Neptune has moved significantly due to the
tidal influence of the sun affecting its orbit.  The forces are just not
that great between the sun and Neptune even acting over millenia.  
The gravitational tidal influence of celestial objects on one another can be
significant however.  

Imagine a moon looking 2X as big as it does now with solar
eclipses lasting much longer than the seconds to minutes they do now.
You are right in that the tides have caused a distance increase between
the earth and moon over time.  This effect is because of the loss
of angular momentum.  As the tidal pulling and pushing slows the rotation
of the earth (the moon has long since stopped rotating so fast and now
presents the same face towards us always for the same reason) the earth
moon distance increases to compensate for the loss of angular momentum
from the slower rotation.

Angular momentum depends upon the mass (m), rotational velocity (v) and the
distribution of the mass about the rotational axis (r) expressed 
very simply as L = mvr.  Rotating objects or systems have and want to keep
it (in physics this means conserve it). What happens to an ice skater that
begins a spin with arms and legs outstretched but brings them close in?  As
they bring their arms and legs in (changing the mass distribution 'r'), 
they spin faster (velocity increases) to compensate and keep the value of L
the same.

So as the tides cause the rotation of the earth to slow, something's gotta 
give to keep L of the earth-moon system the same.  A smaller 'v' means
either a larger 'm' or a larger 'r'.  The moon and earth are not going to
accumulate significant amounts of mass magically from anywhere, 
so their distance gets bigger. Many "moons" from now the earth-moon
distance will be larger, the earth's rotation slower and the moon will 
look smaller so their will be NO total solar eclipses ;{

Planets can be affected by stars (and vica versa) if they are BIG enough
(many times Jupiter's mass compared to the sun) and have a large enough
tidal influence either by having a great mass or being extremely close
to their parent star.

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