Watch This Space

Amazing who you meet on the beach! Chap from Oxford who is involved with the beam lines at their synchrotron. Fascinating. :-)

Poor weather this morning, alot of cloud and smattering of rain - from the East - so I do hope the Pacific sorts itself out in the next 24.

Found the use of the hook below the tripod centre ie. hang a bag of sand from it so it jams the legs firmly into the sand. Also discovered the high tide line.

Now there's something we can thank the Moon for. Tides. Ancient understanding did attribute it to the Moon, but with only one cycle a day. The real trick, which Newton shone rational light upon, was that in an orbiting system of two bodies the centre about which they orbit is not the centre of either body : it is somewhere b/w the two. With the Earth being the more massive of the two that point lies closer to the Earth.

So in a simple sense the water nearer the Moon would be attracted and 'rise' toward it. However on the other side of the Earth away from the Moon the water is pulled less by the Moon than the nearside, as that farside water is in an 'orbit' further from the common orbital centre and thus the Moon too. It gets less of a tug from the Moon while having an arc of wider radius to traverse. It is 'thrown outwards' so to speak and hence a tidal bulge of water there also. The Sun also gives an effect depending on it's position. The upshot is that around a New Moon ( which is when solar eclipses occur! ) these effects compound to give a larger tidal variation than otherwise.

A key point is that there is a lag in this. A line passing through the high point of each tidal bulge and the Earth's centre will not be parallel to the line from Earth to Moon. In fact it 'leads' in the sense of pointing to where the Moon will be a short time later. The nett effect here is that the Moon is pulled with a component in the direction along it's instantaneous velocity, which wants to make the Moon go faster, so to speak. But it doesn't, as what actually happens is that the Moon goes into a 'higher' orbit and that is slower. So the energy boost from tidal effects goes into pushing it uphill in the gravity well of the Earth.

Back on Earth the water pushes against the continents to slow the Earth's rotation much like a brake shoe on a brake disc. This is because most of the water's tidal motion is not 'up and down' ie. along a radius from the centre of the Earth, but transverse to that ie. going 'around' along the curvature of the Earth. That hits continental edges and applies a braking effect as the Earth is rotating around it's own axis in the same sense that the Moon is orbiting the Earth.

So the Moon retards the Earth's rotation, the Moon moves into a higher orbit and slows. Angular momentum is conserved, outside of other influences, but energy of nett motion is lost to other forms : heat generated by distorting things, eroding continental edges etc. Interestingly the LIGO inteferometers have to account for distortions in the Earth's shape as it's solid components distort as well as the liquid bits.

The Moon's own rotation around it's own axis is currently a month, this is because over a long period of time it too suffered tidal braking or acceleration such that they match. Which is why we now see only one face of the Moon. While 'The Dark Side of The Moon' is a great work by Pink Floyd, it is a common myth. If we were on the side not visible from Earth it would get recurrent sunshine with it's sunlit period being one half of one month. So a day on the Moon is one month long as it were. All this is known as 'tidal locking', which is a whole other story ... ;-)

'Tidal forces' have a more general meaning in the context of a body which has a variation in the strength of gravity across it. A star coming in towards a close shave with a black hole, say, will be dismembered into a distinctly non spherical smear as it's bits go their own ways without much 'cohesion' to prevent it. If I were to fall inwards to a black hole ( or other highly dense body ) feet first then my feet would have a greater acceleration than my head. My left arm would be following a non-parallel course with respect to the right one - both converging radially toward the centre of the hole. I will become long and thin, and eventually so on smaller scales too : my individual molecules, then atoms, then nuclei etc. This is called 'spaghettification'. I'm serious, it's an actual technical term, though some think of noodles instead! You could demonstrate this at home by whirling around a balloon full of water, as you go faster the balloon will progressively distort outwards until, well, I suppose you get told off for doing that. There's Einstein's Principle of Equivalence right there, as any acceleration ( at least for short intervals of space and time ) mimics gravity. :-)

Cheers, Mike ( FNQ Correspondent )

( edit ) Sharp punters will note that the Moon gets a pull from both tidal bulges. The nearside gives a pull component in the direction of the Moon's instantaneous velocity, the farside in a direction against that vector. But the farside retardation is less, that bulge being further away. So the nett in the direction of the Moon's motion is still a plus.

Not at all! :-)

If you see an eclipse and are not moved to profound thought then you have no heart/soul. Such perspectives ought be felt more frequently. :-)

Actually that made me think of what any of us would think if we didn't know the 'correct technical' explanation ( assuming that reassures ). Probably scared for starters. I'm sure that all cultures have experienced eclipses enough to make them memorable across generations. Even if eclipses over any given area are decades or more apart, it would be viewed by many and certainly remembered to become part of at least oral tradition.

Now the special reason I wanted catch some stars in the frame was on account of the famous 1919 eclipse that confirmed Einstein's general relativity and propelled him to worldwide fame. Starlight grazing nearby the Sun will deflect inwards ( just like any massive body ) and thus appear to be at a wider angle from a point in the sky that the centre of the Sun overlies, than it would be in the absence of the Sun. Now I won't be doing any measurements myself, plenty will be up to that I expect. I just wanted to see for myself if you could see stars nearby the Sun during an eclipse. You can !! :-)

Cheers, Mike ( FNQ Correspondent )