[Info-vax] CRTL and RMS vs SSIO

[Phillip Helbig undress to reply]

In article <5ee8674d-1925-4e3e-9905-bde643265e37n at googlegroups.com>,
=?UTF-8?Q?Lawrence_D'Oliveiro?= <lawrencedo99 at gmail.com> writes:

> On Thursday, October 14, 2021 at 7:35:20 PM UTC+13, Phillip Helbig (undress
>  to reply) wrote:
> > The leap seconds are added because the rotation rate of the Earth is
> > slowing down ...
> 
> Actually it is slowing down on average, but sometimes does speed up. The le=
> ap-second system also has provision for _subtracting_ a whole second (so 23=
> :59:58 gets followed by 00:00:00, skipping 23:59:59), if that is ever neces=
> sary, which it hasn't been so far.

Yes, on average.  I mentioned that it slows down because of leaves on 
the trees in northern spring, and speeds up when they fall off in 
northern autumn.

> > Basically, the tides slow the Earth down. As a result, the 
> > Moon recedes from the Earth.
> 
> It is the Moon that causes the tides. And the interaction between the tidal=
>  pull and the non-uniform density of the Earth that slows down the rotation=
> ..

Right, though the Sun is responsible to a lesser degree.

> Tides work both ways (as does gravity in general). The Earth's pull=
>  on the Moon caused the same thing to happen to the Moon, only it happened =
> much sooner.

Right, which is why the Moon always shows the same face to the Earth.

> Why does the Moon recede from the Earth? Probably nothing to do with the ti=
> des, everything to do with the fact that the Moon does not actually orbit t=
> he Earth.

It recedes to conserve angular momentum, which is necessary because the 
Earth's rotation is slowing down because of the tides.  Two bodies in 
orbit about each other with no tides would not slow down (except due to 
gravitational radiation, which in the Earth-Moon system is orders of 
magnitude smaller than the measured effects).

> > Thus, the fact that it appears the same 
> > size as the Sun is not only a coincidence, but also one that holds now 
> > but not in general.
> 
> Yes. And also the fact that the Moon is so large compared to the Earth. We =
> are really in a binary-planet system, though for some reason astronomers do=
> n't like that description ...

That depends on the definition of planet.