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PREFACE
All sciences are making an advance, but Astronomy is moving
at high
speed. Since the principles of this science were settled
by Copernicus,
four hundred years ago, it has never had to beat a retreat.
It is
rewritten not to correct material errors, but to incorporate
new
discoveries.
At one time, Astronomy studied mostly tides, seasons, and
telescopic
aspects of the planets; now these are only primary matters.
Once it
considered stars as mere fixed points of light; now it studies
them as
suns, determines their age, size, color, movements, chemical
constitution, and the revolution of their planets. Once
it considered
space as empty; now it knows that every cubic inch of it
quivers with
greater intensity of force than that which is visible in
Niagara. Every
inch of surface that can be conceived of between suns is
more wave-
tossed than the ocean in a storm.
The invention of the telescope constituted one era in Astronomy;
its
perfection in our day, another; and the discoveries of the
spectroscope
a third--no less important than either of the others. New
discoveries
are made every day with the advancement of telescopes. The
Hubble
space telescope has let man see further into the universe
then ever
before. Astronomy and space science is an ever changing
study, and
possibly the most exciting of the sciences. It is for one
reason that this
book was written, to hopefully interest more people in the
exciting
study of the universe around us.
CONTENTS
1.Why Study Light?
2.Introduction To Light
3.Introduction To Color
4.Astronomical Instruments
5.Our Solar System
6.The Sun
7.Earth's Moon
8.Planets
9.Asteroids
10.Meteoroids
11.Comets
12.Kuiper Belt
13.Beyond Our Solar System
14.Stars
15.Constellations
16.Find The Stars In The Sky
Why
Study Light?
For most of history, humans have used visible light to explore
the
skies. With basic tools and the human eye, we developed
sophisticated
methods of time keeping and calendars. Telescopes were invented
in
the 17th century. Astronomers then mapped the sky in greater
detailstill with visible light. They learned about the
temperature,
constituents, distribution, and the motions of stars.
There are two main techniques for analyzing starlight. One
is called
spectroscopy and the other photometry. Spectroscopy spreads
out the
different wavelengths of light into a spectrum for study.
Photometry
measures the quantity of light in specific wavelengths or
by combining
all wavelengths. Astronomers use many filters in their work.
Filters
help astronomers analyze particular components of the spectrum.
For
example, a red filter blocks out all visible light wavelengths
except
those that fall around 600 nanometers (it lets through red
light).
Introduction
to Light
Light is a form of radiant energy or energy that travels
in waves. Since
Greek times, scientists have debated the nature of light.
Physicists now
recognize that light sometimes behaves like waves and, at
other times,
like particles. When moving from place to place, light acts
like a
system of waves. In empty space, light has a fixed speed
and the
wavelength can be measured. In the past 300 years, scientists
have
improved the way they measure the speed of light, and they
have
determined that it travels at nearly 299,792 kilometers,
or 186,281
miles, per second.
When we talk about light, we usually mean any radiation
that we can
see. These wavelengths range from about 16/1,000,000 of
an inch
to 32/1,000,000 of an inch. There are other kinds of radiation
such as
ultraviolet light and infrared light, but their wavelengths
are shorter
or longer than the visible light wavelengths. When light
hits some form
of matter, it behaves in different ways. When it strikes
an opaque
object, it makes a shadow, but light does bend around obstacles.
The
bending of light around edges or around small slits is called
diffraction
and makes patterns of bands or fringes.
All light can be traced to certain energy sources, like
the Sun, an
electric bulb, or a match, but most of what hits the eye
is reflected
light. When light strikes some materials, it is bounced
off or reflected.
If the material is not opaque, the light goes through it
at a slower
speed, and it is bent or refracted. Some light is absorbed
into the
material and changed into other forms of energy, usually
heat energy.
The light waves make the electrons in the materials vibrate
and this
kinetic energy or movement energy makes heat. Friction of
the moving
electrons makes heat.
Experiments With Light
A light set in a room is seen from every place; hence light
streams in
every possible direction. If put in the centre of a hollow
sphere, every
point of the surface will be equally illumined. If put in
a sphere of twice
the diameter, the same light will fall on all the larger
surface. The
surfaces of spheres are as the squares of their diameters;
hence, in
the larger sphere the surface is illumined only one-quarter
as much as
the smaller. The same is true of large and small rooms.
In Fig. 7 it is
apparent that the light that falls on the first square is
spread, at twice
the distance, over the second square, which is four times
as large, and
at three times the distance over nine times the surface.
The varying amount
of light received by each planet is also shown in fractions
above each world, the amount received by the earth being
1.
Fig. 7.
Fig. 8.--Measuring Intensities of Light.
The intensity of light is easily measured. Let two lights
of different
brightness, as in Fig. 8, cast shadows on the same screen.
Arrange
them as to distance so that both shadows shall be equally
dark. Let
them fall side by side, and study them carefully.
Measure
the respective distances. Suppose one is twenty inches,
the other forty. Light varies as the square of the distance:
the square of 20 is 400, of
40 is 1600. Divide 1600 by 400, and the result is that one
light is four
times as bright as the other.
Fig.
9.--Reflection and Diffusion of Light.
Light can be handled, directed, and bent, as well as iron
bars. Darken
a room and admit a beam of sunlight through a shutter, or
a ray of
lamp-light through the key-hole. If there is dust in the
room it will be
observed that light goes in straight lines. Because of this
men are able
to arrange houses and trees in rows, the hunter aims his
rifle
correctly, and the astronomer projects straight lines to
infinity. Take a
hand-mirror, or better, a piece of glass coated on one side
with black
varnish, and you can send your ray anywhere. By using two
mirrors,
or having an assistant and using several, you can cause
a ray of light
to turn as many corners as you please.
Set a small light near one edge of a mirror; then, by putting
the eye
near the opposite edge, you see almost as many flames as
you please
from the multiplied reflections. How can this be accounted
for?
Into your beam of sunlight, admitted through a half-inch
hole, put the
mirror at an oblique angle; you can arrange it so as to
throw half a
dozen bright spots on the opposite wall.
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