Comets are best described as "dirty snowballs". They are about five
miles in diameter and travel around the Sun in highly elongated orbits.
As they approach the Sun, its heat causes some of the comet's icy material
to evaporate and then the solar wind pushes these loosened particles
away, creating the long, beautiful tails.
believed initially that the dark regions, where little to no stars
were seen, are "holes" or voids in
the distribution of stars in the Galaxy. It is now realized that these "holes" are
due to dark nebulae that obscure the light from the more distance
stars behind them.
Lanes are regions of dark nebula
superimposed on a starry background that appear to be long, finger–like
A nebula (pl. nebulae) is a diffuse "cloud" of interstellar gas and
dust. There are a variety of types, including dark, obscuring nebulae
and hot, bright emission nebulae. These nebulae are several light years
across and contain hundreds of times the mass of the Sun; however,
the density is rather low.
terms indicated the apparent distance of the starry background. Usually
there are gas and dust clouds in front of the starry background, and
the obscured regions appear as dark regions or lanes.
use a coordinate system for the positions of celestial objects that
is very similar to the Earth's system of latitude and longitude. Declination is
directly correlated to latitude. The range is from +90o (North
Celestial Pole) to 0o (Celestial
Equator) to –90o (Celestial South
Due to the Moon's gravitational
pull, the Earth slowly wobbles. This causes the Right Ascension and
Declination values to shift. Consequently, an "Epoch" is given with
the coordinates to indicate the date for when these values were exactly
Astronomers measure distances
to nearby stars by the same triangulation technique that surveyors use.
The Parallax of a star is the angular change (in arcseconds) produced
by the orbit of the Earth. The distance in parsecs is equal to the reciprocal
of the parallax angle. Example: If the parallax is 0.05 arcsec, the distance
1 / 0.05 = 20 parsecs.
Astronomers use a coordinate system for the positions of celestial objects that
is very similar to the Earth's system of latitude and longitude. Right Ascension is
similar to longitude except that the parameter range is from 0 to 24 hours instead
of 0 to 360 degrees.
Bayer in the early 1600s (usually) ranked stars in a given constellation
by brightness. The brightest was designated as ,
followed by the constellation name. Subsequent designations would be ,,,
etc. Examples include Ori
John Flamsteed produced
an Atlas containing 54 constellations in the 1700s. The stars were given
consecutive numbers in order of their right ascension.
Most stars are too
faint to have proper names, so they are identified by a specific number
in various star catalogs, such as the Bonner Durchmusterung (BD) or
the Cape Photographic Durchmusterung (CPD). These particular catalogs
number the stars in order of right ascension per declination zone.
Example: the first star (near RA 0 h00m 00s )
in the declination zone from 22o to 23o is BD
Charles Messier was a
comet hunter in the late 1700s who became tired of finding the same nebulous–appearing
objects in the sky. He made a list of 103 such objects. A few examples
include the Andromeda Galaxy (M31) and the Orion Nebula (M42).
Telescopes are described by their aperture (i.e., diameter), for this
characteristic is an indicator of how much light it can gather. The
larger the aperture, the larger the collecting area and therefore
the "deeper" or "fainter" it can see.
This optical system of two lenses has a wide aperture and is free from optical
There are two types of optical
telescopes. Refractors use a large lens at the front of the telescope
tube that focuses the light to an eyepiece at the back of the tube.
There are two types of optical
telescopes. Reflectors use a curved mirror at the back of the tube that
focuses the light to a mirror at the front of the tube, which in turn
reflects the light to an eyepiece.
Verniers are small scales of tick marks attached next to the coordinate
wheels on a telescope. They are used to improve the precision of "reading" the
coordinates to which the telescope is pointed.
These are the mechanical gears
that turn the telescope so that it tracks objects on the sky. Because
the Earth is rotating, the sky appears to spin around us. If a telescope
did not track, then the objects could only be viewed for a short time
before they moved out of the telescope's field of view.