Solar System Observers Program – Glossary and Reference

Charles E. Allen, III
4005 St. Germaine Ct.
Louisville, KY 40207
(502) 693-5504
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  • Albedo The proportion of light falling on the surface of a body that is reflected.
  • Ansae Latin for ‘handles’. The appearance of the protrusion of the rings of Saturn on either side of the planet’s disc.
  • Aperture The diameter of the primary light collecting element of a telescope, be it a mirror or lens.
  • Apparition The period of time when it is possible to observe an object in the sky.
  • Arc second A unit of angular measurement. One 60th of an arc minute or 1/3600 of a degree.
  • Eclipse The occurrence of one celestial body’s shadow temporarily falling on another body.
  • Egress To emerge from, as a star or satellite emerging from behind another planetary body.
  • Ingress To disappear behind. The opposite of ‘egress’.
  • Lunar Limb The extreme edge of the visible Moon.
  • Meridian A line on the celestial sphere passing from the North Pole through the Zenith to the South Pole. Can also be the center line of a planetary body drawn through it’s poles.
  • Objective The main light gathering optic in a telescope or binocular.
  • Occultation The passage of one celestial body moving directly in front of another.
  • Opposition The point at which a planet appears in our sky directly opposite the Sun.
  • Phase Defect The extent to which the illuminated area of a spherical body such as a planet or moon differs from a complete circular disk.
  • Prograde Motion of a celestial body moving in its usual direction on the celestial sphere.
  • Red Spot A large hurricane like oval feature in the clouds of Jupiter larger than the Earth. It has been visible since first reported in 1664. Its color density varies over time, sometimes being difficult to see.
  • Retrograde Motion of a celestial body moving in the opposite direction of its usual motion.
  • Terminator The boundary between the lighted and unlighted portions of a celestial body’s surface.
  • Transit The crossing of a celestial object across the observer’s meridian caused by the daily apparent motion of the celestial sphere. Also the passage of a planet across the face of the Sun or of a planet’s satellite across the primary’s disk.


Very Bright
-4 Venus at its brightest
-3 Jupiter at its brightest
-2 Sirius in Canis Major, the brightest star in the sky
-1 Betelgeuse in Orion
0 Vega in Lyra
+1 Spica in Virgo, Deneb in Cygnus, Pollux in Gemini
+2 Polaris, the north star
+3 Megrez, the faintest star in the Big Dipper
+5 Probable naked eye limit in the suburbs
+6 Probable naked eye limit in the country
+8 Neptune
+9 Approximate limit of typical binoculars
+10 Approximate limit of a 60 mm telescope
+11 Approximate limit of a 3-inch telescope
+12 Approximate limit of a 4-inch telescope
+13 Approximate limit of a 6-inch telescope
+14 Approximate limit of an 8-inch telescope
Very Dim (Above Comments assume as dark a sky as possible.)

* The difference in brightness between any successive two numbers is a ratio of two and one half times. Magnitudes are approximate.


A good resource at the telescope is a set of colored filters. Filters can be acquired from various sources. Consult your astronomy magazines. Kodak’s Wratten series can be purchased in over a hundred colors and densities and can be mounted in slide mounts and simply held between the eyepiece and the eye. For longer observations, as when sketching, screw-in filters are available for both 1-1/4 and 2-inch eyepieces. You don’t need to choose between dozens of colors though, only a few will do.

Filters can reduce glare, improve image definition and enhance tonal contrast. Here are some suggestions:

  • A BLUE filter, such as a Wratten #44A, 47B, or 80A, can be used for the detection of high altitude clouds on Mars, white ovals and spots in the belts of Jupiter, and the zones of the clouds of Saturn. It can also be used to cut down glare on a bright Moon.
  • A GREEN filter, such as a Wratten #58, allows you to see more clearly the edges of the Martian polar caps and enhances the belts and Great Red Spot in the clouds of Jupiter.
  • A YELLOW filter, such as a Wratten #8, 12, or 15, can improve markings in the clouds of Venus and enhance Martian dust storms.
  • An ORANGE filter, such as a Wratten #21, is one of the more useful ones you can have. It is used for bringing out detail on Mars, and enhancing some of the zonal detail on Jupiter. An orange filter darkens the blue sky so daytime observations of Jupiter, Venus and the Moon are much improved.
  • A RED filter, such as a Wratten #23A, 25, or 25A, can also be used to enhance contrast on Mars, Jupiter and Saturn. A red filter, however, is fairly dark, so it works best on larger aperture telescopes which give brighter images. Flipping back and forth between red and blue filters can sometimes bring out subtle colorations on the Moon.
  • A POLARIZING filter can cut down glare when observing a nearly full Moon, making it easier to see ray structure. It will also cut down day-time glare.


  • LEVEL ONE – Severely disturbed skies: Even low power* views are uselessly shaky. Go read a good book.
  • LEVEL TWO – Poor seeing: Low power images are pretty steady, but medium powers are not.
  • LEVEL THREE – Good seeing: You can use about half the useful magnification of your scope. High powers* produce fidgety planets.
  • LEVEL FOUR – Excellent seeing: Medium-powers are crisp and stable. High-powers are good, but a little soft.
  • LEVEL FIVE – Superb seeing: Any power produces a good crisp image.

The PRACTICAL LOWEST power magnification for any telescope is approximately 7 times for each inch of aperture. Example: 28X for a 4-inch (100mm) diameter telescope.
The PRACTICAL HIGHEST power magnification for any telescope is approximately 50 times for each inch of aperture. Example: 200X for a 4-inch (100mm) diameter telescope.

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