[Geezer]

I know what your trying to do. You're trying to get me to say that Pluto is not a planet so we have to debate that for the next ten years!

Ho no! I'm not falling for that one.  []

[LeeE]

Actually, a solar orbit clock wouldn't be much good for measuring durations less than the orbital period because the orbits are all slightly eccentric.

[syhprum]

Before the development of the marine chronometer by Harrison et al the chief source of timekeeping for navigation was the passage of the Moon thru the star field.
As Lee points out the eccentricity of the orbit causes a lot of problems and prediction tables had to be prepared so that adjustments could be made.
When I suggested that large bodies made good time keepers I had in mind their rotational periods not of course their orbital time about other bodies.   

[Geezer]

Before the development of the marine chronometer by Harrison et al the chief source of timekeeping for navigation was the passage of the Moon thru the star field.
As Lee points out the eccentricity of the orbit causes a lot of problems and prediction tables had to be prepared so that adjustments could be made.
When I suggested that large bodies made good time keepers I had in mind their rotational periods not of course their orbital time about other bodies.   

So here's the thing. Does the second define the period of the "Earth pendulum", or does the period of the "Earth pendulum" define the second?

(I think it's really the latter, but I suspect I'll get some heat for saying that!)

[LeeE]

It's the former: as I thought we'd already agreed, the second is defined by the "transition between the two hyperfine levels of the ground state of the caesium 133 atom" and most definitely not by the Earth's rotational and/or orbital periods (which are subject to stuff like earthquakes etc.).

The Earth's rotational and orbital periods are defined in terms of the second.

[Geezer]

It's the former: as I thought we'd already agreed, the second is defined by the "transition between the two hyperfine levels of the ground state of the caesium 133 atom" and most definitely not by the Earth's rotational and/or orbital periods (which are subject to stuff like earthquakes etc.).

The Earth's rotational and orbital periods are defined in terms of the second.

As I said, I'll probably get some heat  []

As you say, today, the second is defined in terms of atomic activity. However, originally the second was a subdivision of the period of Earth's orbit. We still have to make adjustments to reconcile the two.

[syhprum]

Geezer

No the second was never defined with reference to the Earths orbital time it was defined with reference to the Earths rotational period relative to the distant stars.

[Geezer]

Geezer

No the second was never defined with reference to the Earths orbital time it was defined with reference to the Earths rotational period relative to the distant stars.

Thanks Syhprum. If I understand correctly, that means the second is a fraction of the time it takes for the Earth to rotate 360 degrees on its axis.

Who came up with the concept of the second first?