Posted: Jan 02, 2015 2:58 pm
Veida wrote:DavidMcC wrote:Elsewhere you said the following, which seems to be related to what you say above.
Were it not for the earth's spin about its own axis, the night time effect would be to increase our weight, but, as it is, conservation of angular momentum changes that, as with the tides.
Care to explain that? How would our weight increase at night if the Earth didn't spin about its axis?
(It wouldn't. I'm asking only in the hope that you'll explain yourself so I/we can pinpoint where you went wrong.)
I don't think I did go wrong, Veida. Without the earth's rotaion, all that would matter would be the combined pulls of earth, sun and moon, which would combine differently at night (when the sun's pull would add to that of the earth, but the moon's pull would depend on where the moon was (it can be above the horizon, or below)).
Ah. I think I see where you went wrong. You seem to think that the weight difference due to tidal forces is a first order effect from gravitation.
It isn't. It is a second order effect, that is due to the gravitation from e.g. the sun not being uniform - it varies slightly in strength between dayside and nightside, and it varies slightly in direction between dawnside and duskside.
All first-order effects are cancelled out due to the Earth being in free fall.
"The sun isn't uniform"??
An important thing I missed out in this recent "debate" is that there is an equatorial bulge in the oceans, caused by the earth's rotation about its own axis, and that this bulge is tilted up by the moon's gravitional pull. That tilt is the reason why any one point on the earth's surface passes through two maxima and minima every 24 hours. I explained this a long time ago, in an old thread, but you might have missed it.