DK Eyewitness - Astronomy

Celestial mechanics
The Frenchman Pierre Simon
Laplace (1749–1827) was the first
scientist to make an attempt to
compute all the motions of the
Moon and the planets by
mathematical means. In his fivevolume
work, Traité de méchanique
céleste (1799–1825), Laplace treated
all motion in the solar system as a
purely mathematical problem,
using his work to support the
theory of universal gravitation
(p.21). His idea, for which he was
severely criticized during the
following century, was that the
heavens were a great celestial
machine, like a timepiece that, once
set in motion, would go on forever.
Escaping
elements
Lighter
elements
Inferior
planets
Jupiter
Sun
Photographing the planets
One of the key tasks of space missions (pp.34–35) is to send back pictures
of distant planets and moons. They do this using imaging devices very
similar to those used in digital cameras. The heart of the system is a
CCD, or charge-coupled device. This is a silicon chip with thousands
of light-sensitive pixels, or picture
elements. The amount of light falling
on each pixel produces a different
electrical signal. This is read by an
onboard computer and converted
into a stream of
digital signals that
can be radioed back
to Earth, where they
are reconstructed
into the image
by computer.
Mercury
Venus
Earth
Mars
Saturn
Uranus
Neptune
Orbiting the sun
Some of the planets,
including Earth, orbit the
Sun in ellipses (p.18) that
are close to being circles. Others
have more eccentric orbits. Comets
(p.58) have the most eccentric orbits,
which are very elongated. The distance between
the planets and the Sun is measured in terms of
“astronomical units” or AU; each unit is equal to the
average distance between Earth and the Sun, or 93 million miles
(149.6 million km). This drawing shows the orbits nearly to scale.
Mariner 9 photographs
of the surface of Mars
By increasing
the vibration,
the balls are
given more
energy
Heavier
elements
Kinetic energy
machine
Creating color
The CCDs used in astronomy rarely produce
color images directly, but use the most sensitive
black-and-white chips. To get a color image,
separate images are taken through color filters,
and the results are combined in a computer to
give a realistic color view.
Hydrogen in the solar system
Hydrogen is a common element in the solar system. Hydrogen
atoms are so energetic that lightweight planets cannot hang on to
them. This is why the heavier nitrogen makes up such a high percentage
of Earth’s atmosphere (p.42). Lighter hydrogen has escaped because
Earth’s gravity is not strong enough to hold on to it. The red balls in this
kinetic energy machine represent the heavier elements;
the tiny silver balls represent the lighter elements, such
as hydrogen. Our massive Sun is made up largely of
hydrogen. Its great mass pulls the hydrogen inward
and, at its core, hydrogen fuses into helium under the
extreme heat and pressure. It is this reaction, like a
giant hydrogen bomb, that makes the Sun shine.
Hydrogen also makes up a large part of Jupiter,
Saturn, Uranus, and Neptune (pp.50–57).
Color mosaic of mars
The detail in an individual CCD
image of a planet is limited by the
number of pixels on the chip. To get a
high-quality image, several shots are
taken of different parts of the planet,
and then a mosaic is produced, like
this one of Mars.
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