1. Evolving perspectives - a historical prologue

    • The wandering planets move in a narrow track against the unchanging background stars, and some of these vagabonds can suddenly turn around, apparently moving in the opposite direction before continuing on their usual course.

    • The ancient Greeks noticed that the Earth always casts a curved shadow on the Moon during a lunar eclipse, demonstrating that our planet is a sphere.

    • For centuries, astronomers tried to describe the observed planetary motions using uniform, circular motions with the stationary Earth at the center and with the distant celestial sphere revolving about the Earth once a day.

    • Around 145 AD, Claudius Ptolemy devised an intricate system of uniform motion around small and large circles to model the motions of the Sun, Moon and planets around a stationary Earth; his model was used to predict their location in the sky for more than a thousand years.

    • The stars seem to be revolving around the Earth each night, but the Earth is instead spinning beneath the stars. This rotation also causes the Sun to move across the sky each day.

    • Mikolaj Kopernik, better known as Nicolaus Copernicus, argued in 1543 that the Earth is just one of several planets that are whirling endlessly about the Sun, all moving in the same direction but at different distances from the Sun and with speeds that decrease with increasing distance.

    • Almost four centuries ago, Johannes Kepler used accurate observations, obtained by Tycho Brahe, to conclude that the planets move in ellipses, or ovals, with the Sun at one focus, and to infer a precise mathematical relation between the mean orbital distance and period of each planet.

    • More distant planets take longer to move once around the Sun and they move with slower speeds; their orbital periods are in proportion to the cubes of their distances.

    • Astronomy is an instrument-driven science in which novel telescopes and new technology enable us to discover cosmic objects that are otherwise invisible and hitherto unknown.

    • Many major astronomical discoveries have been unanticipated and serendipitous, made while new telescopes were used to study other, known cosmic objects; the earliest of these accidental discoveries include the four large moons of Jupiter, the planet Uranus, and the first known asteroid, Ceres, discovered respectively by Galileo Galilei in 1610, William Herschel in 1781, and Giuseppe Piazzi in 1801.

    • The asteroid belt between the orbits of Mars and Jupiter contains more than 500,000 asteroids, but it is largely empty space and has a total mass that is much less than that of the Earthís Moon.

    • Two kinds of telescopes, the refractor and the reflector, enable astronomers to detect faint objects that cannot be seen with the unaided eye, and to resolve fine details on luminous planets that otherwise remain blurred.

    • Jupiter, Saturn and Uranus have a retinue of large satellites, and Neptune has only one really large moon that moves in the opposite direction to all the other large satellites. Mercury and Venus have no moons, the Earth has one satellite, our Moon, and Mars has two very small ones.

    • Christiaan Huygens discovered Saturnís rings in 1659; they are completely detached from the planet and consist of innumerable tiny satellites each with an independent orbit about Saturn.

    • In his Principia, published in 1686, Isaac Newton showed how the laws of motion and universal gravitation describe the movements of the planets and everything else in the Universe.

    • The solar system is held together by the Sunís gravitational attraction, which keeps the planets in their orbits; they move at precisely the right speed required to just overcome the pull of solar gravity.

    • The gravitational attraction between two objects increases in proportion to the product of their masses and in inverse proportion to the square of the distance between them.

    • The planet Neptune was discovered in 1846, near the location predicted by mathematical calculations under the assumption that the gravitational pull of a large, unknown world, located far beyond Uranus, was causing Uranusí observed positions to deviate from its predicted ones.

    • Estimates for the mean Earth-Sun distance, known as the astronomical unit or AU, were gradually refined over the centuries, eventually setting the scale of the solar system at 1AU = 149.6 million kilometers. At this distance, it takes 499 seconds for light to travel from the Sun to the Earth.

    • The nearest star other than the Sun is located at a distance of 4.24 light-years; it is about 270 thousand times further away from the Earth than the Sun.

    • The Sun is the most massive and largest object in our solar system. The Sunís mass, which is 333,000 times the Earthís mass, can be inferred from Keplerís third law using the Earthís orbital period of one year and the Earthís mean distance from the Sun, the AU.

    • The Sunís size, at 109 times the diameter of the Earth, can be inferred from the Sunís distance and angular extent.

    • The temperature of the Sunís visible disk is 5780 kelvin; it can be determined from the Sunís total irradiance of the Earth, the Earth-Sun distance or the AU, and the radius of the Sun.

    • The temperature at the center of the Sun is 15.6 million kelvin, estimated from the speed a proton must be moving to counteract the gravitational compression of the massive Sun.

    • The composition of the Sun is encoded in absorption lines that appear in the visible spectrum of sunlight.

    • The lightest element, hydrogen, is the most abundant element in the Sun, and the next most abundant solar element, helium, was first discovered in the Sun.

    • The regular spacing of hydrogenís spectral lines can be explained by quantum theory, in which the angular momentum and energy of an orbiting electron are quantized, depending on an integer quantum number.

    • The eight major planets can be divided into two groups – the four rocky, dense terrestrial planets, Mercury, Venus, Earth and Mars, located relatively near the Sun, and the four giant, low-density planets, Jupiter, Saturn, Uranus and Neptune, that are further from the Sun.

    • The temperature and density increase systematically with depth in the giant planets, owing to the greater compression by overlying material.

    • As the result of differentiation in their originally molten interiors, the rocky terrestrial planets contain dense iron cores surrounded by less dense, silicate mantles.

    • The terrestrial planets contain partially molten, liquid cores, but their internal temperatures cool as time goes on due to the depletion of radioactive elements and the emission of internal heat.

Copyright 2010, Professor Kenneth R. Lang, Tufts University