MadSci Network: Astronomy |
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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