The Observant Astronomer

The passing scene as observed by an observant Jew, who daylights as an astronomer.

Wednesday, March 30, 2005

Cassini at Titan: Visit 4

Cassini will be passing by Titan again tomorrow, so look for some new results in a week or two. Meanwhile, the Cassini people have released a nifty new picture of the rings and SXVI/Prometheus as well as close up shots of the co-orbiting satellites SX/Janus and SXI/Epimetheus.

Update (2005 04 05): The first image has now been released. Is that the Medititanian Sea?

The Great Divide: Objectives

Underlying many of the Torah vs. Science arguments is a mistaken assumption about what the Pre-SR science is doing. Since it is this Pre-SR viewpoint that is taken as the "Torah's" view on the subject, it is worthwhile to try and understand just what the Pre-SR thinkers were doing, and how this differs from Post-SR. Here I will discuss the objectives of the Pre-SR astronomers. Since we are discussing astronomy, we are safe in following the Rambam and making reference to Ptolomaic and Aristotelian concepts.

The first thing to recognize is that the Pre-SR astronomers were essentially working in two dimensions. Their primary concern was to be able to predict the positions of the planets* on the sky. Why? Because these predictions were needed to compute horoscopes.

Call a modern-day astronomer an astrologer and you'll get either a grimace or an ear-full. But for most of our scientific antecedants, right through the Copernican/Keplerian reveloution, the whole point of calculating planetary motions was to be able to know where the planets were at the time for which you need a horoscope. So the entire objective was to calculate where the seven planets were with respect to the fixed stars. Kepler's Rudolphine Tables were such a hit because they did this with a much greater accuracy than any previous set of tables.

Of secondary concern was the third dimension, i.e. the distances of the planets. It was well understood that these are not constant, since the "sizes" of the planets changes. By "size" what they meant was "brighness", since, with the exception of the Sun and Moon, planetary disks are effectively unresolved by the human eye. But solar eclipses, when the Moon passes in front of the Sun, provided clear evidence that the relative sizes of the Sun and Moon changed, because sometimes, even when the Moon passes across the center of the Sun's disk, a ring of light is left behind, forming an annular rather than a total eclipse.

In any event, the best they could do was to give relative rather than absolute distances for the planets, usually by nesting the spheres that contained the circles on which the planets travelled, but, even then, the resulting size variations were often not in accord with reality. This didn't bother them overmuch, since the angular positions on the sky were coming outacceptably, but it poses a big problem if you are planning on sending a physical object to one of those planets.

* In Pre-SR astronomy the planets, in order of distance from the Earth are: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn. In some schemes Mercury and Venus were switched. These were the objects that wandered in the heavens

Tuesday, March 22, 2005

Seeing Extrasolar Planets

Two groups announced today the first direct observations of light from extrasolar planets. In both cases, the telescope used was the Spitzer Space (Infrared) Telescope. Up until now we've been able to detect these objects only indirectly. Most have been found via their gravitational effects on their host stars by studying the changes in their star's radial velocity. A rare few have also been seen to cause a slight dimming in their host star when they pass in front of the star as seen from Earth. The two planets involved here are HD 209458 b and TrES-1 and were observed when they, in turn, pass behind their host star. In the infrared, the total light was seen to dip by a fraction of a percent while the planet was in behind the star. That difference is the light emitted by the planet itself.

Stranger than we can imagine (I)

The Saturnian moon SII/Enceladus is the most reflective body in the solar system. Now we know why.

Cassini, in its recent passages past the moon, has detected a tenous, water-vapour atmosphere around the moon. The moon is the smallest object known to have an atmosphere and its gravity is too weak to retain it for long. Hence, it must be continuously replenished, possibly by the sort of geyser/volcanic activity seen on JI/Io and N1/Triton. Meanwhile, some of that material is freezing back onto the surface, continuously giving Enceladus a fresh, snowy surface.

Cassini will have a return engagement with Enceladus on 14 July, after two more visits to SI/Titan on 31 March, and 16 April. Keep an eye on the Cassini home page for more results.

It could be worse?

The link is to a report that the Israeli Attorney-General, Menachem Mazuz, is once again iterfering in a civil case which found against the UN, on the grounds that it will impair Israel's relationship with the UN.

Monday, March 21, 2005

The Second Day of Spring

Yesterday was the Vernal Equinox, at 2005 March 20, 12:34 PM GMT to be precise. Thus today is the second day of spring. Or is it?

Leave aside the hemispherecentricity of calling this the second day of spring and let us include the antipodelians by calling it the second day of autumn for those south of the equator. The characterization is still wrong. The designation of March 21st (roughly) as the first day of spring, 21 June the first day of summer, 21 Sept the first of fall, and 21 Dec the first of winter reflect not just a northern viewpoint, but a viewpoint limited to quite limited geographical regions. Are there actually places where the seasons are (roughly) the same length? What of places that don't have these four seasons at all. What of places where there are only two seasons, hot and dry vs. cold and wet, for example. If 21 June is the beginning of summer, why are there traditions calling it Midsummer Day? Your challenge for today. Assess these canonical seasonal dates for your location and comment below on how well they hold up. Can we locate a place where they really apply?

Occam's Shaving Brush

I made the mistake of turning on the radio Motzai Shabbos. It was tuned to the local talk radio station, and the host was talking about the recent fireball over Washington state. He quoted someone who saw it in eastern Washington, who said that it was very bright at that (roughly) "It didn't move as quickly as a meteor usual does." From which the host went on to conclude that until they "find the rock", this was an Unidentified Flying Object. He went on that they often "don't find a rock" and that there is some "mystery" about meteors in generclick

What is it with these people that everything has to become a mystery and conspiracy. Subjective impressions are taken as proof. Arguments beg the question. (You only require a "rock" if you assume that all meteors result in meteorite falls. Evidence to the contrary is inadmissable.) For people such as these, Occam's razor does not hold. They would much rather obscure things with Occam's deservedly less-famous shaving brush.

Thursday, March 17, 2005

Unsourced II

Canada is not the only country which risks being cut off from its fundamental roots. Follow the link to read a transcript of some recent remarks by U.S. Supreme Court Justice Scalia on the "Living Constitution" school of judicial interpretation. He make a cogent case for the potential fatal affect on your polity when its judicial tradition comes unstuck from its fundamental sources and is left to continuous reinterpretation according to the current fashion.

Update: Rishon Rishon brings another perspective to this question. He argues that it is the very fact that the U.S. Constitution is above ideology that permits ideological diversity within the United States. From the positive, infer the negative.

A Telling Conjunction

I don't know whether this was intentional or not, but the following two advertisements appeared one after the other on a local radio station yesterday:

1. One singing the praises of the state lotteries, and encouraging people to play.
2. One giving advice on reducing teenage gambling.

If you'd never started the first, you wouldn't have a such a problem with the second one now, would you?

Monday, March 14, 2005

Sources of News

Sitting in the traffic in front of the New York Times building yesterday, Girl Child #1 (age 9) asked me where the newspapers get all the news to fill their pages every day. Answered I, they make it up.

Thursday, March 10, 2005

Rethinking Black Holes

The above-linked paper by Chapline suggests that by neglecting quantum effects, we've misunderstood the nature of the event horizon surrounding black holes. By drawing some analogies with superfluid physics, he claims that the interiors of black holes do not contain singularities, but have a normal sort of space-time with a higher vacuum energy. Weird things happen to particles as they pass through the putative event horizon ---possibly explaining the positron anhilation line seen in the galactic center---but that for particles with low enough energies, it is possible to penetrate and then escape. What he doesn't explain, at least to my reading, is what happens to the matter that collapsed to make the object in the first place. Transformed into vacuum? I'm also puzzled as to why he calls these "dark energy stars". Isn't the dark energy meant to be repulsive?

Update: What's interesting about this paper from a historical perspective is their comment at the beginning of section 2:
In the 1950s a consensus was reached, partly as a result of meetings such as (sic) famous meeting at Chapel Hill in 1957, that although quantum effects might be important blow (sic) some very small distance, on an macroscopic scale the predictions of classical general relativity (GR) should be taken seriously. In the summer of 2000 Bob Laughlin and I (i.e. Chapline) realized that this cannot possibly be correct. Indeed I am sure it will be a puzzle to future historians of science as to why it took so long to realize this.

Tefillin and Acupuncture

This is more Rebeljew's bailiwich than mine, but The Biur Chametz has uncovered a 2002 paper (Schram, 2002, Journal of Chinese Medicine, 70, 5) entitled Tefillin: An Ancient Acupuncture Point Prescription for Mental Clarity which claims all sorts of physical and mental benefits from the acupuncture pressure points stimulated by tefillin and their straps. (The helicity of the Ashkenazic minhag in winding the arm appears to somewhat inferior to the reverse helicity, which could explain certain pecularities seen in my shul. As could the personalities of those who consistently have their shel rosh out of place. Note to self: Must be more diligent in keeping people centered.) I wonder if any of his fellow practitioners have followed up on his suggestion in the penultimate paragraph, though.

Wednesday, March 09, 2005

Enceladus day for Cassini

The Cassini spacecraft continues its tour of the Saturn system with a close encounter with the SII/Enceladus today. NASA has released some new images of Titan which add more evidence in support of fluid activity on Titan's surface. (These images and more are featured in a paper in the current (10 March 2005) issue of Nature.) Those sure look like coastlines and rivers to my untrained eye. I don't think you'll be wanting to book a vacation on the Titanian coast though. The fluid is almost certainly liquid methane (CH4).

Tuesday, March 08, 2005

Hans Bethe Z"L

For those of you who like to debate the age of the universe, consider the following.

Until the late 57th century (i.e. early 20th C.E.) it was known that the Sun was billions of years old, but there was no known way to keep it shining so long. The timescale for energy released by gravitational contraction was way too short, and even if the Sun was mostly carbon, coal burning wouldn't help all that much.

It was Hans Bethe who, in 5698 (1938), showed that the fusion of 4 hydrogen nuclei into one helium nucleus could provide the required energy. As it turned out, the CNO-catalyzed reaction chain he originally proposed dominates in stars with higher masses than our Sun, but a more direct chain of reactions is the source of energy that has kept the Sun burning all this time. For this fundamental discovery, he was awarded the 1967 Nobel Prize in physics. (His Nobel Prize lecture presents a good summary of this work, even if you have to ignore the equations.)

The Great Divide: Terminology

It looks like it's going to be necessary to compare modern astronomical thought with that which proceeded it, so I want to set up some terminology. I think that what we need to distinguish is between two schools of thought which I will refer to as pre- and post- Scientific Revolution. Pre-SR and Post-SR, for short.

Pre-SR astronomy is the realm of the natural philosophers. Authority is ancient and text based. If your ultimate source is Almagest, you are doing Pre-SR science.

Post-SR astronomy is the realm of the scientists. Authority is derived from observations of the universe itself. If your theories are based on real data, you are doing Post-SR science.

Friday, March 04, 2005


As a frum astronomer, the questions I'm asked most often are about supposed discrepancies between Torah and science. These usually focus on either the age of the universe, or geocentrism vs. heliocentrism. (The third popular subject is the Rambam's Hilchot Kiddush HaChodesh, but that is another issue.)

Usually, the latter question is posed in terms of annual geocentrism: "You, Mr. Astronomer, claim that the Earth goes around the Sun, but Torah says that it is the other way around." But what is the source for the latter statement? If you look at any pre-Copernican text, Jewish or otherwise, indeed the Earth is (roughly) at the center and everything, including the Sun in its yearly cycle on the sky, is in motion around it. But this annual geocentrism is, in a sense, a corollary of diurnal geocentrism; the idea that the sky revolves around the Earth once daily. It is only for the latter geocentrism that one finds Biblical support. There we have texts where the Sun, Moon, and stars are rising and setting and standing still in the sky. So, goes the classical argument, it must be these bodies that are moving, right? After all, if the Earth were turning, we'd feel it, wouldn't we? We would be blown away by the strong winds. So the Earth must be fixed in place, and everything else moving. Q.E. D.

One of the nice things about classical physics is that it is intuitively obvious, unlike modern physics which, when it isn't being bizarre, is simply counterintuitive. After all, a soccer ball will, pace Newton's first law, come to a stop in the grass unless someone keeps kicking it. What is unrecognized is that the force of the kicks are required to overcome the opposing force of friction from the ground. Similarly with the sky. We see the Sun rise and set, and so conclude that it is the Sun that is moving. But, is it reasonable to expect the Torah to say that "the Earth rotated eastward until the Sun was hidden by the body of the Earth" every time it wanted to refer to "sunset"? No one speaks in such a way, not even the modern, educated person, who knows that the Earth rotates daily on its axis. Indeed, chazal clearly state that the Torah speaks in the language of man. So why is it seen to be a contradiction that modern science says that it is the Earth that is rotating once daily, and not the heavens?

If we hold that diurnal geocentrism is nothing more than linguistic convention, then what of annual geocentrism? To begin with, let us recognize that the scientific viewpoint has moved on considerably from the strict heliocentrism advocated by Copernicus and charactured by the opponents of modern science. Copernicus's tables for planetary positions were not that much better than those of his Ptolomaic contemporaries. The real revolutionary was not so much Copernicus, as Kepler. By substituting ellipses for the ancient dogma of strict circular motion, Kepler was able to predict planetary positions to much greater accuracy than anyone else had ever done. Within a year or so of his death, his tables permitted the first observation of a transit of Venus across the face of the Sun. The Keplerian model, however, is not strictly heliocentric. True, the Earth's orbit surrounds the Sun, but the Sun is at one focus of the ellipse, not at the center of the ellipse. Newton put Kepler's empirical laws on a physical basis by providing the explanation of why they hold. But if you compute the orbit of two bodies under Newtonian gravity you find that they both move about their mutual center of mass, which undermines the whole notion of heliocentrism. (Indeed, under most medieval solar-system models, the Earth wasn't at the center of the Sun's orbit for much the same reason that drove Kepler to his ellipses.)

Whatever the centrism, the concept is that the object in the center is fixed, and the other object moves around it. If both are moving, then neither can be considered fixed. In any case, we must ask, "Fixed with respect to what?" Newtonian dynamics permitted the concept of absolute space and time, so that at least in theory there was a reference with which to fix things, even if the Sun wasn't nailed down to it. Under Einsteinian relativity, there is no absolute reference frame. All motion is relative, and the choice of reference frames, a convention. A reference frame that makes the required equations particularly simple, or their solution straight forward, may be preferred on those grounds, but such a reference frame is not otherwise more or less "correct" than any other. For solar system problems, the center of mass of the whole system is one such frame, but that makes the Earth's motion barycentric rather than heliocentric. All that is left is a dispute about an otherwise arbitrary choice of coordinate system.