Physics Today November 2003- Article: The Growth of Astrophysi... http://www.physicstoday.org/vol-56/iss-11/p38.html1 of 12 12/05/03 04:59 PM Welcome! Searchadvanced search Table of contentsArticlesPhysics Today November 2003- Article: The Growth of Astrophysi... http://www.physicstoday.org/vol-56/iss-11/p38.html2 of 12 12/05/03 04:59 PMThe Growth of Astrophysical UnderstandingA stroll through three millennia of astronomical speculation and discovery reminds us that inspired guesses are not enough. Progress comes primarily from the introduction of new observational and theoretical tools.Martin HarwitPerhaps the most remarkable aspect of the growth in our understanding of the universe is that we understand anything at all. Beyond the obvious regularities of the seasons, the Assyrians noted, as early as 700 BC, that the planets appeared to move in a complex semiregular pattern and that solar eclipses were possible only at the new moon, whereas lunar eclipses occurred only at the full moon. But what did all that tell the ancients about the structure of the universe?Around 250 BC, the Greek natural philosopher Aristarchus of Samos worked out the distance of the Moon and its size. He proposed a method for determining the Sun’s distance, but he was able to conclude only that the Sun was much farther away than the Moon and much larger than Earth. That led him to postulate, 18 centuries before Nicolaus Copernicus, that Earth revolves around the Sun.1Aristarchus’s theory was largely discredited, especially by Claudius Ptolemaeus of Alexandria. Ptolemy’s Almagest, which appeared in about 150 AD, dominated Western astronomical thought for a millennium and a half. Ptolemy argued that Earth could not be rotating. Rotation, he thought, would throw anything not firmly attached off the surface, and "animals and other weights would be left hanging in the air." Moreover, Earth’s rotation would be so fast that "never would a cloud be seen to move toward the east."2That sounds quaint today, but it wasn’t illogical. Ptolemy was a great scientist. The first lesson in astrophysics, however, is that every cosmic phenomenon is governed by competing effects--in this case, gravity, centrifugal forces, and friction. Unless we know the order of magnitude of each, we are likely to draw wrong conclusions.The observersWhen Copernicus revived the notion of a heliocentric system in 1543, he could offer no observational confirmation. The ground for a final resolution had to be prepared by Tycho Brahe (1546-1601), the greatest of the pre-telescope observers. Tycho constructed astronomical instruments more precise than any previously known. Over a 20-year period, he assembled the most accurate, systematic data that had ever been compiled on the positions of the planets.The young Johannes Kepler, a theorist if ever there was one, dogged Tycho, intent on getting his hands on the data, which the great observer was jealously guarding so he could deduce the orbits of the planets himself. When Tycho was banished in 1597 from his island observatory in Denmark and sought political refuge in Prague, Kepler followed him. But it was not until after Tycho’s death that Kepler inherited and began analyzing the Free this monthNuclear Bunker Busters,Mini-Nukes, and the US Nuclear StockpileThe Growth ofAstrophysical UnderstandingThe Business ofAcademic PhysicsNo Time to BeBrief: A Scientific Biography of Wolfgang PauliLetters Sponsored linksFor your conferenceStepping MotorControls Step-Pak modular system for control of several steppingmotors, different types and sizes.www.acsmotion.comPhysics Today November 2003- Article: The Growth of Astrophysi... http://www.physicstoday.org/vol-56/iss-11/p38.html3 of 12 12/05/03 04:59 PMdata.3One sees parallels to today’s theorists impatiently seeking to get an early look at the data from the Wilkinson Microwave Anisotropy Probe’s mapping of the cosmic microwave background. The WMAP data were, until just a few months ago, embargoed pending the publication of a full year’s set of observations.4 (See Physics Today, April 2003, page 21*.) As soon as the data were released, new theoretical analyses began to appear within days on the World Wide Web.Kepler reduced Tycho’s data and arrived at his three laws of planetary motion:The planets move in elliptical orbits--rather than in circles and epicycles.The rate at which a planet sweeps out area within its orbital ellipse is constant.The periods of the planetary orbits increase as the 3/2 power of their semimajor axes.The last of these findings was the first quantitative relationship between two observational parameters in astronomy. It constituted what one would call a well-posed question: Why does Kepler’s third law hold?With the advent of the astronomical spyglass in 1609 (the word telescope was not coined until the following year), Galileo Galilei quickly discovered an extraordinary new set of phenomena: mountains on the Moon, moons orbiting Jupiter, and the moonlike phases of Venus. To Galileo, those three observations meant that Earth is just one of the planets, all of them orbiting the Sun. For him, that clinched the Copernican theory. The Church, however, forbade Galileo to teach the theory and eventually confined him to house arrest until his death in 1642.Why did it take until the 17th century for the great discoveries of Kepler and Galileo to come about? Today the answer is clear. Tycho’s precision instruments and the spyglass, invented in Holland in 1608 and, a year later, improved by Galileo and pointed at the heavens, provided observational data that had simply been unavailable before.Although Tycho’s instruments gave the best positional data ever assembled, they were still limited by the abilities of the unaided eye, which cannot discern the moons of Jupiter. Galileo’s telescopes provided a breakthrough in angular resolution and light-gathering power, a path astronomers are still treading as they build ever larger telescopes and interferometers.A brief history of instrumentation and its successes illustrates the ability of new instruments to promoteastronomical discovery.5 (See also my article in PhysicsToday November 1981, page 172.*) There is a vastrange of wavelengths, from the radio domain to the very highest gamma-ray energies, about which Tycho couldknow nothing. He had only his eyes to rely on, and theymerely covered the minuscule visual portion of the electromagnetic spectrum. The naked eye provides