Archive for tides

The Moon is Distant from the Sea

Posted in Poetry, Uncategorized with tags , , on May 19, 2016 by telescoper

The moon is distant from the sea,
And yet with amber hands
She leads him, docile as a boy,
Along appointed sands.

He never misses a degree;
Obedient to her eye,
He comes just so far toward the town,
Just so far goes away.

Oh, Signor, thine the amber hand,
And mine the distant sea, —
Obedient to the least command
Thine eyes impose on me.

by Emily Dickinson (1830-1886)


The Eclipse Coincidence Question

Posted in The Universe and Stuff with tags , , , , on March 22, 2015 by telescoper

The day before last week’s (partial) solar eclipse I posted an item in which I mentioned the apparent coincidence that makes total eclipses possible, namely that the Moon and Sun have very similar angular sizes when seen from Earth.

In the interest of balance I thought I would direct you to a paper by Steve Balbus that develops a detailed argument to the contrary along the lines I described briefly in my earlier post. I am not entirely convinced but do read it and make up your own mind:

Here is the abstract:

The nearly equal lunar and solar angular sizes as subtended at the Earth is generally regarded as a coincidence. This is, however, an incidental consequence of the tidal forces from these bodies being comparable. Comparable magnitudes implies strong temporal modulation, as the forcing frequencies are nearly but not precisely equal. We suggest that on the basis of paleogeographic reconstructions, in the Devonian period, when the first tetrapods appeared on land, a large tidal range would accompany these modulated tides. This would have been conducive to the formation of a network of isolated tidal pools, lending support to A.S. Romer’s classic idea that the evaporation of shallow pools was an evolutionary impetus for the development of chiridian limbs in aquatic tetrapodomorphs. Romer saw this as the reason for the existence of limbs, but strong selection pressure for terrestrial navigation would have been present even if the limbs were aquatic in origin. Since even a modest difference in the Moon’s angular size relative to the Sun’s would lead to a qualitatively different tidal modulation, the fact that we live on a planet with a Sun and Moon of close apparent size is not entirely coincidental: it may have an anthropic basis.

I don’t know if it’s a coincidence or not but I always follow the advice given by my role model, Agatha Christie’s Miss Marple in Nemesis: “Any coincidence is worth noticing. You can throw it away later if it is only a coincidence.”..

Supermoon Surgery

Posted in Biographical, The Universe and Stuff with tags , , , , , , , on August 11, 2014 by telescoper

I have a busy day today (including my Annual Appraisal) to kick off a very busy week dominated by the release of this years A-level results and consequent admissions business, so I’ll just post a quickie though one which is at least fairly topical.

Last night (10th August) I took a (not very good) picture of the Moon with my phone:



This is a so-called “supermoon“, a not particularly rare phenomenon which takes place when there is a Full Moon that coincides with the Moon being at the point of its orbit which is closest to the Earth, i.e. its perigee. A much better name is “Perigee Full Moon”, but that somehow doesn’t seem to have caught on in the popular media. The Moon orbits the Earth in an ellipse rather than a circle and at its closest approach it is about 14% closer than at its furthest (apogee). It therefore looks about 14% bigger and about 30% brighter during a Perigee Full Moon than during an Apogee Full Moon.

The Moon was certainly looking very bright when I took the picture last night, at least compared to a few minutes later when it disappeared behind a supercloud.

The Moon’s proximity to Earth during this Full Moon does have a noticeable effect on terrestrial tides, but not a particularly strong one; certainly not enough to trigger the end of the world. Actually, the tides have an amplitude just a few inches higher than average during a Perigee Full Moon. In any case roughly one in 14 Full Moons is a supermoon so it’s actually quite a common event, and as far as I’m aware the world didn’t come to an end during the last one or the one before that or the one before that or…

Anyway, all this supermoon malarkey reminded me of something that happened about 15 years ago,  just after I had moved to Nottingham to take up the position of Professor of Astrophysics at the University of Nottingham. I was sitting in my office, working – blogs hadn’t been invented then – when the phone rang and the voice at the other end said May I speak to Professor Coles please? When I replied that I was he, the caller went on to explain that he was a surgeon who worked at Queen’s Medical Centre, a hospital located right next to the University of Nottingham, with teaching staff working for the University.

It turned out that news of the setting up of the new Astronomy group there had made it into the University newsletter which my caller had seen. He asked if I had a few moments to answer a question about astrophysics which had been bothering him for some time and which he had just been discussing with some of his colleagues.  I said yes, and he asked: Does the Moon rotate?

I paused a bit, thinking how best to explain, and he went on to clarify his point, which was that if the Moon always has the same face towards the Earth does that mean it’s not rotating?

Understanding his question, I went on to explain that, yes, the Moon does rotate and that the reason it always shows the same face to the Earth (more-or-less, ignoring libration) is that the period of its rotation is the same as the Moon’s orbital period around the Earth. I also explained how to demonstrate this with two coffee mugs, moving one in a circle around the other and rotating the outer one so as to keep the handle pointing towards the central mug. Moreover, I explained the physics of this phenomenon, which is called tidal locking, and pointed out other examples in astrophysics.

After this spiel the caller said that was all very interesting but he had to go  now. Assuming I had bored him, as I fear I tend to do rather a lot, I apologized for going on about it for too long. He said no he wasn’t at all bored by the detail I had put in, he found it all absolutely fascinating. The reason for him needing to go was that he had to go back to tell the answer to the colleagues he had been discussing it with  just before phoning me.  They were all  in the operating theatre,  standing around a patient lying on the operating table, waiting  for him to return and complete the operation he had left in order to make the call…