NYRB: Steven Weinberg: The Missions of Astronomy

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Volume 56, Number 16 · October 22, 2009

The Missions of Astronomy

By Steven Weinberg

A few years ago, I decided that I needed to know more about the history of science, so naturally I volunteered to teach the subject. In working up my lectures, I was struck with the fact that in the ancient world, astronomy reached what from a modern perspective was a much higher level of accuracy and sophistication than any other science.^[1] One obvious reason for this is that visible astronomical phenomena are much simpler and easier to study than the things we can observe on the earth's surface. The ancients did not know it, but the earth and moon and planets all spin at nearly constant rates, and they travel in their orbits under the influence of a single dominant force, that of gravitation.

In consequence, the changes in what is seen in the sky are simple and periodic: the moon regularly waxes and wanes, the sun and moon and stars seem to revolve once a day around the celestial pole, and the sun traces a path through the same constellations of stars every year, those of the zodiac.^[2] Even with crude instruments these periodic changes could be and were studied with a fair degree of mathematical precision, much greater than was possible for things on earth like the flight of a bird or the flow of water in a river.

But there was another reason why astronomy was so prominent in ancient and medieval science. It was useful in a way that the physics and biology of the time were not. Even before history began, people must have used the apparent motion of the sun as at least a crude clock, calendar, and compass. These functions became much more precise with the introduction of what may have been the first scientific instrument, the gnomon, attributed by the Greeks variously to Anaximander or to the Babylonians.

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The gnomon is simply a straight pole, set vertically in a flat, level patch of ground open to the sun's rays. When during each day the gnomon's shadow is shortest, that is noon. At noon, the gnomon's shadow anywhere in the latitude of Greece or Mesopotamia points due north, so all the points of the compass can be permanently and accurately marked out on the ground around the gnomon. Watching the shadow from day to day, one can note the days when the noon shadow is shortest or longest. That is the summer or the winter solstice. From the length of the noon shadow at the summer solstice one can calculate the latitude. The shadow at sunset points somewhat south of east in the spring and summer, and somewhat north of east in the fall and winter; when the shadow at sunset points due east, that is the spring or fall equinox.^[3]

Using the gnomon as a calendar, the Athenian astronomers Meton and Euctemon made a discovery around 430 BC that was to trouble astronomers for two thousand years: the four seasons, whose beginnings and endings are precisely marked by the solstices and equinoxes, have slightly different lengths. This ruled out the possibility that the sun travels around the earth (or the earth around the sun) with constant velocity in a circle, for in that case the equinoxes and solstices would be evenly spaced throughout the year. This was one of the reasons that Hipparchus of Nicaea, the greatest observational astronomer of the ancient world, found it necessary around 150 BC to introduce the idea of epicycles, the idea that the sun (and planets) move on circles whose centers themselves move on circles around the earth, an idea that was picked up and elaborated three centuries later by Claudius Ptolemy.

Even Copernicus, because he was committed to orbits composed of circles, retained the idea of epicycles. It was not until the early years of the seventeenth century that Johannes Kepler finally explained what Hipparchus and Ptolemy had attributed to epicycles. The earth's orbit around the sun is not a circle but an ellipse; the sun is not at the center of the ellipse but at a point called the focus, off to one side; and the speed of the earth is not constant but faster when it is near the sun and slower when farther away.

or the human uses I have been discussing, the sun has its limitations. The sun can of course be used to tell time and directions only during the day, and before the introduction of the gnomon its annual motions gave only a crude idea of the time of year. From earliest recorded times, the stars were put to use to fill these gaps. Homer knew of the stars' use at night as a compass. In the Odyssey, Calypso gives Odysseus instructions how to go from her island eastward toward Ithaca: he is told to keep the Bear on his left. The Bear, of course, is Ursa Major, aka the Big Dipper, a constellation near the North Pole of the sky (called the celestial pole) that in the latitude of the Mediterranean never sets beneath the horizon (or, as Homer says, never bathes in the ocean). With north on his left, Odysseus would be sailing east, toward home.^[4]

The stars were also put to use as a calendar. The Egyptians very early appear to have anticipated the flooding of the Nile by observing the rising of the star Sirius. Around 700 BC the Greek poet Hesiod inWorks and Days advised farmers to plow at the cosmical setting of the Pleiades constellation—that is, on the day in the year on which the Pleiades star cluster is first seen to set before the sun comes up.

Observing the stars for these reasons, it was noticed in many early civilizations that there are five "stars," called planets by the Greeks, that in the course of a year move against the background of all the other stars, staying pretty much on the same path along the zodiac as the sun, but sometimes seeming to reverse their course. The problem of understanding these motions perplexed astronomers for millennia, and finally led to the birth of modern physics with the work of Isaac Newton.

The usefulness of astronomy was important not only because it focused attention on the sun and stars and planets and thereby led to scientific discoveries. Utility was also important in the development of science because when one is actually using a scientific theory rather than just speculating about it, there is a large premium on getting things right. If Calypso had told Odysseus to keep the moon on his left, he would have gone around in circles and never reached home. In contrast, Aristotle's theory of motion could survive through the Middle Ages because it was never put to practical use in a way that could reveal how wrong it was. Astronomers did try to use Aristotle's theory of the planetary system (due originally to Plato's pupil Eudoxus and his pupil Callippus), in which the sun and moon and planets ride on coupled transparent spheres centered on the earth, a theory that (unlike the epicycle theory) was consistent with Aristotle's physics.

They found that it did not work—for instance, Aristotle's theory could not account for the changes in brightness of the planets over time, changes that Ptolemy understood to be due to the fact that each planet is not always at the same distance from the earth. Because of the prestige of Aristotle's philosophy some philosophers and physicians (but few working astronomers) continued through the ancient world and the Middle Ages to adhere to his theory of the solar system, but by the time of Galileo it was no longer taken seriously. When Galileo wrote his Dialogue Concerning the Two Chief Systems of the World, the two systems that Galileo considered were those of Ptolemy and Copernicus, not Aristotle.

There was one more reason that the usefulness of astronomy was important to the advance of science: it promoted government support of scientific research. The first great example was the Museum of Alexandria, established by the Greek kings of Egypt early in the Hellenistic era, around 300 BC. This was not a museum in the modern sense, a place where visitors can come to look at fossils and pictures, but a research institution, devoted to the Muses, including Urania, the muse of astronomy. The kings of Egypt supported studies in Alexandria of the construction of catapults and other artillery and of the flights of projectiles, probably at the Museum, but the Museum also provided salaries to Aristarchus, who measured the distances and sizes of the sun and moon, and to Eratosthenes, who measured the circumference of the earth.

The Museum was the first of a succession of government-supported centers of research, including the House of Wisdom established around 830 AD by the caliph al-Mamun in Baghdad, and Tycho Brahe's observatory Uraniborg, on an island given to Brahe by the Danish king Frederick II in 1576. The tradition of government-supported research continues in our day, at particle physics laboratories like CERN and Fermilab, and on unmanned observatories like Hubble and WMAP and Planck, put into space by NASA and the European Space Agency.

n fact, in the past astronomy benefited from an overestimate of its usefulness. The legacies of the Babylonians to the Hellenistic world included not only a large body of accurate astronomical observations (and perhaps the gnomon) but also the pseudoscience of astrology. Ptolemy was the author not only of a great astronomical treatise, the Almagest, but also of a book on astrology, theTetrabiblos. Much of the royal support for compiling tables of astronomical data in the medieval and early modern periods was motivated by the use of these tables by astrologers. This appears to contradict what I said about the importance in applications of getting the science right, but the astrologers did generally get the astronomy right, at least as to the apparent motions of the planets and stars, and they could hide their failure to account for human affairs in the obscurity of their predictions.

Not everyone has been enthusiastic about the utilitarian side of astronomy. In Plato's Republic there is a discussion of the education to be provided for future philosopher kings. Socrates suggests that astronomy ought to be included, and his stooge Glaucon hastily agrees, because "it's not only farmers and others who need to be sensitive to the seasons, months, and phases of the year; it's just as important for military commanders as well." Poor Glaucon—Socrates calls him naive, and explains that the real reason to study astronomy is that it forces the mind to look upward and think of things that are nobler than our everyday world.

Although surprises are always possible, my own main research area, elementary particle physics, has no direct applications that anyone can foresee,^[5] so it gives me little joy to note the importance of utility to the historical development of science. By now pure sciences like particle physics have developed standards of verification that make applications unnecessary in keeping us honest (or so we like to think), and their intellectual excitement incites the efforts of scientists without any thought of practical use. But research in pure science still has to compete for government support with more immediately useful sciences, like chemistry and biology.

Unfortunately for the ability of astronomy to compete for support, the uses of astronomy that I have discussed so far have largely become obsolete. We now use atomic clocks to tell time, so accurately that we can measure tiny changes in the length of the day and year. We can look up today's date on our watches or computer screens. And recently the stars have even lost their importance for navigation.

In 2005 I was on the bark Sea Cloud, cruising the Aegean Sea. One evening I fell into a discussion about navigation with the ship's captain. He showed me how to use a sextant and chronometer to find positions at sea. Measuring the angle between the horizon and the position of a given star with the sextant at a known chronometer time tells you that your ship must lie somewhere on a particular curve on the map of the earth. Doing the same with another star gives another curve, and where they intersect, there is your position. Doing the same with a third star and finding that the third curve intersects the first two at the same point tells you that you have not made a mistake. After demonstrating all this, my friend the captain of the Sea Cloud complained that the young officers coming into the merchant marine could no longer find their position with chronometer and sextant. The advent of global positioning satellites had made celestial navigation unnecessary.

One use remains to astronomy: it continues to have a crucial part in our discovery of the laws of nature. As I mentioned, it was the problem of the motion of the planets that led Newton to the discovery of his laws of motion and gravitation. The fact that atoms emit and absorb light at only certain wavelengths, which in the twentieth century led to the development of quantum mechanics, was discovered in the early nineteenth century in observations of the spectrum of the sun. Later in the nineteenth century these solar observations revealed the existence of new elements, such as helium, that were previously unknown on earth. Early in the twentieth century Einstein's General Theory of Relativity was tested astronomically, at first by comparison of his theory's predictions with the observed motion of the planet Mercury, and then through the successful prediction of the deflection of starlight by the gravitational field of the sun.

After the confirmation of General Relativity, for a while the source of the data that inspired progress in fundamental physics switched away from astronomy, first toward atomic physics and then in the 1930s toward nuclear and particle physics. But progress in particle physics has slowed since the formulation of the Standard Model of elementary particles in the 1960s and 1970s, which accounted for all the data about elementary particles that was then available. The only things discovered in recent years in particle physics that go beyond the Standard Model are the tiny masses of the various kinds of neutrinos, and these first showed up in a sort of astronomy, the search for neutrinos from the sun.

eanwhile, we are now in what it has become trite to call a golden age of cosmology. Astronomical observation and cosmological theory have invigorated each other, to the point that we can now say with a straight face that the universe in its present phase of expansion is 13.73 billion years old, give or take 0.16 billion years. This work has revealed that only about 4.5 percent of the energy of the universe is in the form of ordinary matter—electrons and atomic nuclei. Some 23 percent of the total energy is in the masses of particles of "dark matter," particles that do not interact with ordinary matter or radiation, and whose existence is so far known only through observations of effects of the gravitational forces they exert on ordinary matter and light. The greatest part of the energy budget of the universe, about 72 percent, is a "dark energy" that does not reside in the masses of any sort of particle, but in space itself, and that is causing the present expansion of the universe to accelerate. The explanation of dark energy is now the deepest problem facing elementary particle physics.

Exciting as all this is, both astronomy and particle physics have increasingly had to struggle for government support. In 1993 Congress canceled a program to build an accelerator, the Superconducting Super Collider, that would have greatly extended the range of masses of new particles that might be created, perhaps including the particles of dark matter. The European consortium CERN has picked up this task, but its new accelerator, the Large Hadron Collider, will be able to explore only about a third of the range of masses that could have been reached by the Super Collider, and support for the next accelerator after the Large Hadron Collider seems increasingly in doubt. In astronomy, NASA has cut back on the Beyond Einstein and Explorer programs, major programs of astronomical research of the sort that has made possible the great progress of recent years in cosmology.

Of course, there are many worthy calls on government funds. What particularly galls many scientists is the existence of a vastly expensive NASA program that often masquerades as science.^[6] I refer, of course, to the manned space flight program. In 2004 President Bush announced a "new vision" for NASA, a return of astronauts to the moon followed by a manned mission to Mars. A few days later the NASA Office of Space Science announced cuts in its unmanned Beyond Einstein and Explorer programs, with the explanation that they did not support the President's new vision.

Astronauts are not effective in scientific research. For the cost of taking astronauts safely to the moon or planets and bringing them back, one could send many hundreds of robots that could do far more in the way of exploration. Astronauts in orbiting astronomical observatories would create vibrations and radiate heat, which would foul up sensitive astronomical observations. All of the satellites like Hubble or COBE or WMAP or Planck that have made possible the recent progress in cosmology have been unmanned. No important science has been done at the manned International Space Station, and it is hard to imagine any significant future work that could not be done more cheaply on unmanned facilities.

It is often said that manned space flight is necessary for science because without it the public would not support any space programs,^[7] including unmanned missions like Hubble and WMAP that do real science. I doubt this. I think that there is an intrinsic excitement to astronomy in general and cosmology in particular, quite apart from the spectator sport of manned space flight. As illustration, I will close with a verse of Claudius Ptolemy:

I know that I am mortal and the creature of a day; but when I search out the massed wheeling circles of the stars, my feet no longer touch the Earth, but, side by side with Zeus himself, I take my fill of ambrosia, the food of the gods.

Notes

^[1]This article is based on a talk given on September 25, 2009, at the Harry Ransom Center for Humanistic Studies of the University of Texas at Austin, to commemorate its exhibition "Other Worlds: Rare Astronomical Works," on view September 8, 2009–January 3, 2010.

^[2]Of course the stars are not visible during the day, but some of them can be seen just after sunset, when the sun's position in the sky is still known.

^[3]A gnomon is different from a sundial, because the pole that casts a shadow in a sundial is not vertical but set at an angle chosen so that the pole's shadow follows about the same path during each day of the year. This makes the sundial more useful as a clock, but less useful as a calendar.

^[4]It may be wondered why Calypso did not tell Odysseus to keep the North Star on his left. The reason is that in Homer's time the star Polaris, which is now the North Star, was not at the North Pole of the sky. This is not because of any motion of Polaris itself, but because of a phenomenon known as the precession of the equinoxes, discovered by Hipparchus. In modern terms, the axis of the earth's rotation does not keep a fixed direction in the sky, but precesses like the axis of a spinning top, making a full circle every 25,727 years. It is a measure of the accuracy of Greek astronomy that the data of Hipparcus indicated a period of 28,000 years.

^[5]I say "direct" applications because experimental and theoretical work in particle physics that pushes technology and mathematics to their current limits occasionally spins off new technology or mathematics of great practical importance. One celebrated example is the World Wide Web. This can provide a valid argument for government support, but it is not why we do the research.

^[6]I have written about this at greater length in "The Wrong Stuff," The New York Review, April 8, 2004.

^[7]This opinion was most recently expressed by Giovanni Bignami, the head of the European Space Agency Science Advisory Committee, in "Why We Need Space Travel," Nature, July 16, 2009.