Be A Stargazer - Guide to Astronomy

Table of Contents:

Why Study Light?...4
Introduction To Light...5
Introduction To Color...10
Astronomical Instruments...14
Our Solar System...18
The Sun...20
Earth's Moon...22
Planets...27
Asteroids...66
Meteoroids...68
Comets...70
Kuiper Belt...72
Beyond Our Solar System...74
Stars...76
Constellations...94
Find The Stars In The Sky...102


ASTRONOMICAL INSTRUMENTS
THE TELESCOPE
Some astronomical instruments are of the simplest character, some
most delicate and complex. When a man smokes a piece of glass, in
order to see an eclipse of the sun, he makes a simple instrument.
Ferguson, lying on his back and slipping beads on a string at a certain
distance above his eye, measured the relative distances of the stars.
Refracting Telescope
The use of more complex instruments commenced when Galileo
applied the telescope to the heavens. He cannot be said to have
invented the telescope, but he certainly constructed his own without a
pattern, and used it to good purpose. It consists of a lens, O B (Fig.
13), which acts as a multiple prism to bend all the rays to one point at
R. Place the eye there, and it receives as much light as if it were as
large as the lens O B. The rays, however, are convergent, and the
point difficult to find. Hence there is placed at R a concave lens,
passing through which the rays emerge in parallel lines, and are
received by the eye. Binoculars are made upon precisely this principle
today, because they can be made conveniently short.
Fig. 13.—Refracting Telescope.
If, instead of a concave lens at R, converting the converging rays into
parallel ones, we place a convex or magnifying lens, the minute image
is enlarged as much as an object seems diminished when the telescope
is reversed. This is the grand principle of the refracting telescope.
Difficulties innumerable arise as we attempt to enlarge the
instruments. These have been overcome, one after another.
The Reflecting Telescope
This instrument differs radically from the refracting one already
described. It receives the light in a concave mirror, M (Fig. 14), which
reflects it to the focus F, producing the same result as the lens of the
refracting telescope. Here a mirror may be placed obliquely, reflecting
the image at right angles to the eye, outside the tube, in which case it
is called the Newtonian telescope; or a mirror at R may be placed
perpendicularly, and send the rays through an opening in the mirror at
M. This form is called the Gregorian telescope. Or the mirror M may be
slightly inclined to the coming rays, so as to bring the point F entirely
outside the tube, in which case it is called the Herschelian telescope. In
either case the image may be magnified, as in the refracting telescope.
Fig. 14.—Reflecting Telescope.
Reflecting telescopes are made of all sizes, up to the Cyclopean eye of
the Subaru telescope which is 327 inches i diameter. The form of
instrument to be preferred depends on the use to which it is to be put.
The loss of light in passing through glass lenses is about two-tenths.
The loss by reflection is often one-half. In view of this peculiarity and
many others, it is held that a twenty-six-inch refractor is fully equal to
any six-foot reflector.
The mounting of large telescopes demands the highest engineering
ability. The whole instrument, with its vast weight , with its
accompanying tube and appurtenances, must be pointed as accuratley
as a rifle, and held as steadily as the axis of the globe. To give it the
required steadiness, the foundation on which it is placed is sunk deep
in the earth, far from rail or other roads, and no part of the
observatory is allowed to touch this support.
When a star is once found, the earth swiftly rotates the telescope away
from it, and it passes out of the field. To avoid this, clock-work is so
arranged that the great telescope follows the star by the hour, if
required. It will take a star at its eastern rising, and hold it constantly
in view while it climbs to the meridian and sinks in the west. The
reflector demands still more difficult engineering.
The Spectroscope
A spectrum is a collection of the colors which are dispersed by a prism
from any given light. If it is sunlight, it is a solar spectrum; if the
source of light is a star, candle, glowing metal, or gas, it is the
spectrum of a star, candle, glowing metal, or gas. An instrument to
see these spectra is called a spectroscope.
Considering the infinite variety of light, and its easy modification and
absorption, we should expect an immense number of spectra. A mere
prism disperses the light so imperfectly that different orders of
vibrations, perceived as colors