Astronomical Glossary


aphelion - The point of an orbit closest to a planet or other celestial body.

aperture - The diameter of a telescope's primary mirror or lens.

apogee - The point of an orbit furthest from a planet or other celestial body.

ansae - The Latin word for handles, used to refer to the edges of Saturn's rings.


bolide - The same as a fireball, a very bright meteor. A bolide may explode and produce a sound.


catadioptric telescope - A telescope that uses lenses and mirrors to collect and focus light. Examples of this design include Maksutov-Cassegrains telescopes (MCT) and Schmidt-Cassegrains telescopes (SCT).

celestial equator - An imaginary line that extents from out the Earth's equator and is projected onto the sky. The celestial equator divides the sky into the northern and southern hemisphere.

Central Meridian (CM) - The central meridian is an imaginary line through the center of the disk from the south pole to the north pole, dividing the disk into equal halves, independent of phase. By noting the time a feature transits the CM, it is possible to determine the period of rotation of surface or atmospheric features.

circumpolar - A celestial object such as a star, constellation, or deep-sky object that is so close to the celestial pole that it never rises or sets from an observers location.

conjunction - This occurs when two bodies appear to pass near each other in the sky as seen from Earth and their right ascension (R.A.) or declination become the same. An inferior conjunction is when a body such as Mercury or Venus passes between the Earth and the Sun, and superior conjunction when a planet passes behind the Sun.


declination - The angular distance north (+) or south (-) of the celestial equator.

Dew Point vs. Relative Humidity, or When to Expect Dew - Often before we go out to observe at night we wonder if we will have to deal with dew. One way to get an indication is to watch the weather forecast or Weather Channel and see what the dew point is, and what the forecast low temperature is. The closer these two numbers are, the more likely there may be dew. While it may seem like the relative humidity is a good indicator, a better one is the dew point. This is because the relative humidity is only a relative measure of how much water vapor the air can hold. To put it another way, the amount of water vapor the air is holding expressed as a percentage of the amount the air could hold at a particular temperature, and warm air can hold more water vapor then cold air. So when an air parcel with a given amount of water vapor cools, its relative humidity goes up. When an air parcel is warmed, its relative humidity drops.

This is why the dew point is a much better indicator of how much moisture an air parcel (or airmass) is carrying. If at sunset the air temperature is 50 degrees Fahrenheit, the dew point is 15 degrees, and clear skies are forecasted all night, the likelihood of dew forming is low, even if the relative humidity is going up. On the other hand if the air temperature at sunset is 50 degrees, the dew point is 42 degrees, and clear skies are forecasted all night, it is probably a good idea to have the dew zappers ready.


ecliptic - The apparent path of the Sun, Moon, and planets through the night sky. In the Northern Hemisphere the ecliptic is low in the sky during the summer and high in the sky during the winter.

eclipse - Occurs when one celestial body passes through the shadow of another celestial body. For example, an eclipse of the moon occurs when the moon passes through the Earth's shadow. An eclipse of the sun occurs when the Earth passes through the moon's shadow.

equinox - The time of year when the Sun crosses the celestial equator. This occurs around March 21st and September 23rd producing the vernal equinox and autumnal equinox respectively in the Northern Hemisphere.


focal ratio - The ratio between the focal length of a telescope or camera lens and the aperture of the objective. For example, a telescope with an aperture of 200mm and focal length of 2000mm has a focal ratio of f/10. Telescopes with focal ratios of between f/4 and f/6 are usually considered to be rich field telescopes or RFT's.

following and preceding - When observing the planets or other celestial object with a telescope with an even number of reflections, such as a Newtonian reflector, or refractor used without a star diagonal, south is at the top. Features on the disk appear to rotate from the right hand side to the left-hand side of the disk (when using a telescope of different configuration, note that north may be at the top, and the planet may appear to rotate from left to right). The right-hand side of the disk (as defined by the Central Meridian or CM) is the following side of the disk; the left-hand side is the preceding side. Using an accurate watch, pencil, and notepad it is possible to make CM timings. When a feature, such as the GRS, approaches the CM, note the time to the nearest minute the time its preceding edge transits the CM. Do the same for its center and following edge. In our example of the GRS, the notation would read GRSp, GRSc, GRSf, which translates to GRS preceding, GRS center, GRS following. As shorthand notation for other dark or light features, you can use the letter D to indicate a dark feature, W to indicate a white feature. These timings will give you an estimate of when the feature transited the CM. Since features in Jupiter's atmosphere tend to drift over time, these timings will give an estimate of its drift rate relative to other features.


Gegenschien - Also called the counterglow, it is the brightening of the zodiacal light in the direction opposite the Sun along the ecliptic. It is produced by sunlight reflected from meteoric dust lying in the plane of the solar system.


limiting magnitude - Is equal to the faintest star visible to the unaided eye, through a telescope, or binoculars.


magnification - The focal length of the telescope divided by the focal length of the eyepiece. For example, a 20mm eyepiece used with a telescope with a focal length of 2000mm would produce a magnification of 100x. The shorter the telescopes focal length, the lower the magnification the same eyepiece would produce. For example, a 20mm eyepiece used with a focal length of 650mm would produce a magnification of 32.5x.

mare - Is the Latin word for sea, and is used to refer to large dark areas on the moon and Mars. Before the invention of the telescope it was assumed that these large dark areas on the moon were seas.

meteor - The luminous streak or trail that appears in the sky when a meteoroid glows as it passes through Earth's atmosphere.

meteorite - The metallic or stony material of a meteoroid that hits the ground.


nova - A sudden increase in the brightening of a star.


opposition - The position of a planet, as seen from Earth, when it is opposite of the Sun in the sky. This usually corresponds to the planets closet approach to the Earth but it can be a few days before or after the date of opposition.

Depending upon when the planet is at opposition however determines how high in the sky it will appear, because the ecliptic is low in the sky during the summer for Northern Hemisphere observers, but is high in the sky during the winter months. As an example let's take a look at two Mars oppositions, 2003 and 2005. Although Mars was closer to Earth and larger in diameter in 2003 then 2005, it was actually be better placed for observers in the Northern Hemisphere in 2005 when it was much higher in the sky:

Date of Mars Opposition Date Nearest to Earth Diameter, Declination City Altitude
August 28, 2003 August 27, 2003 25.11", -15 49' Chicago, IL 30 15' 33"
New York, NY 30 56' 35"
Los Angeles, CA 37 25' 53"
Key West, FL 42 6' 41"
Sydney, Australia 64 57' 32"
November 7, 2005 October 30, 2005 20.17", +15 54'' Chicago, IL 63 30' 0"
New York, NY 64 49' 12"
Los Angeles, CA 71 8' 28"
Key West, FL 80 53' 3"
Sydney, Australia 40 23' 12"


parallax - The apparent shift in the position of a celestial object when viewed from different positions.

penumbra - The region of a partial shadow that takes place during an eclipse.


quasar - A quasi-stellar radio source.


reflecting telescope - A telescope that uses mirrors to collect and focus light, such as Newtonian reflectors.

refracting telescope - A telescope that uses lenses to collect and focus light, such as apochromatic refractors.

right ascension (R.A.) - Is the celestial equivalent of longitude, as measured eastward from the vernal or autumnal equinox and spring equinox.


seeing - Is an estimate of how steady the atmosphere is. There are two scales, both related. One was developed by Eugene Antoniadi, a well-known planetary observer. The second one was developed by the Association of Lunar and Planetary Observers (ALPO).

On Antoniadi's scale a seeing of I corresponds to perfect seeing with steady images even at high magnification; II is intervals of perfect seeing with occasional periods of less stable seeing; III denotes fair seeing with frequent unsteady images so that medium magnifications are used; IV is poor seeing that offers only occasional glimpses of detail; while V is very poor with blurred images even at low power.

On the ALPO scale a seeing of 9-10 corresponds to Antoniadi's seeing of I or perfect seeing; 7-8 on the ALPO scale would be very good seeing corresponding to Antoniadi's II; 5-6 on ALPO scale would be good seeing corresponding to III on Antoniadi's scale; 3-4 on ALPO's scale would be fair seeing corresponding to IV on Antoniadi's scale; while 1-2 on ALPO scale would be poor seeing corresponding to V on Antoniadi's scale. You can use whichever scale you wish as long as you keep track of which one it is.

An easy way to remember is to think of seeing in terms of the highest magnification you can use when observing the planets. If you can employ a magnification of 50x or 60x per inch of aperture or greater and the image is still steady then your seeing would correspond to I on Antoniadi's scale and 9-10 on ALPO's scale, or excellent seeing.

If you can employ similar magnification or a little lower, say between 30x to 40x per inch of aperture, but there are occasional periods of less stable seeing then your seeing would correspond to II on Antoniadi's scale and 7-8 on ALPO's scale, or very good seeing.

If you were able to only employ medium magnifications, say between 20x to 30x per inch of aperture, your seeing would correspond to III on Antoniadi's scale and 5-6 on ALPO's scale, or good seeing.

If you are observing and using between 20x to 30x per inch of aperture or less but only occasional glimpses of detail then your seeing would correspond to IV on Antoniadi's scale and 3-4 on ALPO's scale, or fair seeing.

If your are using low power and only seeing blurred images then you were seeing would correspond to V on Antoniadi's scale and 1-2 on ALPO's scale, or poor seeing.

Some general considerations about local seeing effect. Natural vegetation, such as a grassy field, makes a good observing location. Manmade objects, such as asphalt parking lots, streets, and buildings, absorb solar radiation during the day, and release it slowly at night. This can adversely affect seeing.

When possible, try to observe when the planet is 45 degrees or higher above the horizon as you will be looking through less atmosphere and the seeing tends to be better. Also, the seeing is often steady right after sunset, as well as after midnight as much of the heat that has been absorbed by the surface of the earth has been released by then. This can be particularly true around sunrise.

As a general rule, when the jet stream is located over your observing area, the seeing is not very good. The same holds true for high altitude air turbulence. There are a couple of websites that can give you idea of the position of the jet stream, as well as turbulence. Try:

72 Hour Jet Stream Forecast

Air Turbulence Potential

Seeing Forecast

Before a cold front comes through the seeing can be good, but gets worse once it passes. As the high pressure system moves over your location and begins to move away the transparency starts to drop but the seeing can get better. The seeing is often poor after a cold front has passed through your observing area.

As a general guide, in the Northern Hemisphere when the wind flow is from the north or northwest, it brings colder and drier air with it, which is unstable as the cold air moves over the warmer surface (depending upon where you live). This is particularly true during the day, and it may take time for this instability to settle down at night. During the sky often appears to be blue down to the horizon. This suggests that the transparency that night will be good, but the seeing may not be. On such nights it may be better to observe deep-sky objects (assuming the light from the moon will not interfere) since the seeing for the planets may not be very good. These kinds of conditions are more common during the winter months.

When the wind flow is from the southwest or south, it brings warmer and more moist air with it (once again depending upon where you live). The sky often appears to have more of a milky appearance to it, particularly near the horizon. This suggests that the transparency that night will not be as good, but the seeing may be. On such nights it may be good for observing the moon and planets, but not necessarily for observing faint detail in deep-sky objects. These kinds of conditions are more common during the summer months.

Some of the best seeing for the planets can occur on warm and hazy summer nights when the transparency is poor, and only the brightest stars are visible. These same kinds of weather conditions can occur during the early fall as well, or in the spring.

Other times when the seeing can be very good is early morning before sunrise when most of the heat absorbed by the surface has been released. Ground fog is another good indicator of a stable atmosphere, but just be careful with dew forming on the telescope optics.

Note that these are general guidelines, and there are times when the seeing can be good even when the transparency is good, as there are times when the transparency is poor and the seeing is poor as well.

Another thing to keep in mind is that simply looking to see with the unaided eye how much the stars are twinkling at night does not always give you an idea of how stable the seeing is for observing the planets through the telescope. I have seen some nights where to the unaided eye the stars were twinkling quite a bit, but the seeing was actually pretty good for the planets. On other nights the stars didn't seem to be twinkling much at all to the unaided eye but the seeing was poor for the planets.

solstice - the time of year when the Sun is at the most northern or southern point of the ecliptic. In the Northern Hemisphere summer solstice occurs when the sun is in the zenith at the tropic of Cancer about June 22nd, while the winter solstice occurs when the sun is over the tropic of Capricorn about December 22nd.

sunspot - these dark spots that often appear in groups on the sun's surface are cool regions in the sun's photosphere and are associated with strong magnetic field lines.


transparency - Is equal to the faintest star visible to the unaided eye. Many observers use the stars in Ursa Minor, The Little Dipper, to determine the transparency. This web site shows the water vapor that affects transparency over the United States.


umbra - The region of full shadow that takes place during an eclipse.


variable star - A star whose magnitude changes over time.


white dwarf - A faint, very dense star that has the radius about the size of the Earth and is the remnant core of a red giant star.


zenith - Is the point directly overhead on the celestial sphere.

zodiacal light - A band of light visible along the ecliptic, the zodiacal light is produced by sunlight reflected from meteoric dust lying in the plane of the solar system.