Astronomical Objects

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.

Dark Holes
Barnard 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 shapes.

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.

Stratum, star stratum, stellar stratum, substratum
These 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.

Coordinates and Distances

Declination δ
Astronomers use a coordinate system for the positions of celestial objects that is very similar to the Earth's system of latitude and longitude. Declination δ is directly correlated to latitude. The range is from +90o (North Celestial Pole) to 0o (Celestial Equator) to –90o (Celestial South Pole).

Epoch 1875.0
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 correct.

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 is
1 / 0.05 = 20 parsecs.

Right Ascension α
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 α is similar to longitude except that the parameter range is from 0 to 24 hours instead of 0 to 360 degrees.

Naming Conventions

Bayer Star Designation
Johann Bayer in the early 1600s (usually) ranked stars in a given constellation by brightness. The brightest was designated as α, followed by the constellation name. Subsequent designations would be β, γ, δ, etc. Examples include α Ori and ρ Oph.

Flamsteed Star Number
John Flamsteed produced an Atlas containing 54 constellations in the 1700s. The stars were given consecutive numbers in order of their right ascension.

Star Catalog Numbers
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 +22o0001.

Messier's Number
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.

Petzval System
This optical system of two lenses has a wide aperture and is free from optical aberrations.

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.

Worm Wheels
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.