1. Evolving perspectives - a historical prologue
Discovery of new worlds
Galileo, the telescope, and the unseen cosmos
One of the most fascinating and lively books in astronomy, Sidereus Nuncius or Starry Messenger, was published in 1610. In it, the Italian astronomer and physicist Galileo Galilei (1564-1642) described how he turned the newly devised telescope toward the heavens, bringing the sky down to Earth and the Earth into the sky. In 1609 he found craters, rugged mountains and valleys on the Moon, perceiving another Earth-like world hanging unattached in space. Galileo next used his rudimentary telescope to show that the Earth is not the only object with a satellite, our Moon, accompanying its motion through space. In 1610 he discovered four satellites that circle Jupiter. This meant that there was more than one center of motion in the Universe, and it contradicted Ptolemy's theory in which all astronomical objects move around the central Earth.
Early telescopic observations by Galileo were also used to show that Venus goes through a complete sequence of Moon-like phases, from new to full, appearing at times as a thin crescent and thickening at other times into a round disk. This meant to Galileo that Venus had to circle the Sun. If nearby Venus orbited the Earth inside the Sun’s orbit, then it could never appear completely illuminated, but Venus could appear in all its phases if it orbited the Sun. Of course, Venus might orbit the Sun while the Earth remained at rest, so Galileo’s persuasive evidence did not provide definite proof of the complete Copernican model.
Telescopes to extend our vision
Astronomical telescopes, which have been in use for about four hundred years, have enabled us to detect previously unknown objects, or to see known ones in greater detail, transforming our perception of the planets and their satellites.
There are two kinds of telescopes, the refractor, used by Galileo, and the reflector, initiated by Isaac Newton. As the names suggest, the refractor uses a lens to focus light, employing the principle of refraction, while the reflector uses a mirror to reflect and focus light. Modern refractors consist of a lens and a detector. The parallel light rays from a distant object are bent by refraction at the curved surface of a convex lens, known as the objective or the object-glass. The objective lens brings the incoming light to a focus, where the light rays meet and an image is formed. A detector placed at the focal plane, parallel to the objective lens, is used to record the image.
Unfortunately, the objective lens in a refractor does not bring all parallel rays of light to a unique focus. As shown by Isaac Newton, sunlight is a mixture of all the colors seen in the prismatic display of a rainbow or in sunlight reflected by the crystals of new-fallen snow. Each color has a definite wavelength, from the long red waves to the short violet ones. When sunlight passes through a glass lens, each wavelength or color is bent or refracted through a slightly different angle. This unequal refraction, known as chromatic aberration, produces a blurred image. In 1668, Newton got around the problem by building an entirely different kind of telescope, the reflector, that uses a primary, concave mirror with a parabolic shape to gather the parallel light rays of a distant object and focus them to a point. Light does not pass though a mirror, as it does through a lens, and the mirror concentrates light of all colors to the same focus, producing a sharp image.
The serendipitous discovery of Uranus
The first planet to be discovered since the dawn of history was found accidentally, by a professional musician and self-taught amateur astronomer, William Herschel (1738-1822), using a home-made reflecting telescope in a systematic study of the stars from his home in Bath, England. While surveying the heavens on the night of 13 March 1781, Herschel came across an unusual object that was definitely not a star. It showed a disk, which no star can do, and it moved slowly from one night to another across the background of distant stars. This meant that it belonged to our solar system. After some controversy, the new planet was instead named Uranus, after the Greek personification of the sky.
When he found Uranus, Herschel was apparently unaware of a numerical sequence that predicted its relative distance from the Sun. Known as the Titius-Bode law, after the last names of the first persons to state it, the sequence describes the regular spacing of the planets, suggesting that the next planet beyond Saturn would be located at 19.6 AU, or at about twice Saturn’s distance. The so-called “law” also indicated a missing planet at 2.8 AU, in the gap between Mars and Jupiter, and suggested that another unknown planet would be located at 39 AU, or about twice the distance of Uranus. As it turned out, the asteroids were next discovered in the gap, and Neptune was eventually found close to the most distant location.
The ubiquitous asteroids
Most of the asteroids with well-determined orbits lie in a great asteroid belt between the orbits of Mars and Jupiter, at distances of 2.2 to 3.3 AU and with orbital periods of 3 to 6 Earth years. The asteroids are so little, and distributed across such a large range of distances, that the asteroid belt is largely empty space. This leaves plenty of room for spacecraft to pass though to the giant planets, undamaged by collision with any asteroid.
The asteroids are scattered around their orbits in a haphazard fashion, much like runners near the end of a long race on a small track. So, at first glance, the asteroids appear to fill the belt quite uniformly. But, if the asteroids could be arranged along a line outward from the Sun – as though they had been placed on the starting line – a different pattern would emerge. Not all distances from the Sun are equally well represented. There are a few prominent gaps, and these are named the Kirkwood gaps after the American astronomer, Daniel Kirkwood (1814-1895), who discovered them in 1866. A careful comparison with the period of Jupiter, 11.56 Earth years, shows that the missing periods are rational fractions of it. Asteroids with these orbits always come nearest Jupiter in the same point of their orbit, so Jovian gravitational perturbations recur repeatedly at the same orbital position. The recurring gravitational jolts dislodge the asteroids from their orbits, but the details depend on modern concepts of chaos.
Neptune’s discovery, triumph of Newtonian gravitational theory
Neptune's discovery was no accident, in contrast to those of Uranus and the first asteroid. It was a direct consequence of precise mathematical calculations of Uranus' motion. A large, unknown world, located far beyond Uranus, was evidently producing a gravitational tug on Uranus, causing it to deviate from the expected location. Two astronomer-mathematicians, John Couch Adams (1819-92) in England and Urbain Jean Joseph Le Verrier (1811-77) in France, independently took on the challenge of locating that planet by a mathematical analysis of the wanderings of Uranus. Neptune is located at a mean distance of 30.07 AU, at 1.6 times the distance of Uranus and fairly close to the 38.8 AU predicted by the Titius-Bode law. Remote Neptune takes so long to travel around the Sun, about 165 Earth years, that it has not made a full orbit since it was discovered in 1846.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University