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
- 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
- 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
- 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
- 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
- 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
- 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
- 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 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