| Albedo |
The amount of light reflected from an object. |
| Altitude |
The angle above the observer's horizon. The point directly overhead is known as the zenith. |
| Annular Eclipse |
An annular eclipse is a type of solar eclipse. During an annular eclipse, the Sun looks like an "annulus" or ring. The ring is visible when the Moon does not entirely cover the disc of the Sun during the eclipse. This type of eclipse happens when the Sun is at perihelion and the Moon is at apogee. |
| Aperture |
The diameter of the primary mirror or lens of the telescope. The most basic thing you do in astronomy is scoop up photons, concentrate them, and project them into your eye. The bigger the aperture, the more photons you can grab. You will quickly learn that large aperture telescopes (15+ inches) used by amateur astronomers are affectionately called light buckets. Don't go buy the biggest scope in the store for your first telescope, that one will be your second telescope! I would highly recommend starting with binoculars. |
| Apogee |
The point in an orbit most distant from the body being orbited |
| Astronomical Unit |
The average distance between the Earth and the Sun. The AU is approximately 93 million miles (150 million kilometers). Pluto averages 40 AU from the sun.
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| Azimuth |
The angle clockwise from north along the horizon that lies beneath a star. |
| Bolide |
A brilliant meteor, which may explode during its descent through the Earth''s atmosphere.
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| Cepheid |
A variable star that scientists can use to determine how distant a galaxy, or star cluster is.
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| Conjunction |
When a planet appears to come close to another planet, or star. It only appears to come close because it moves in between the other object, and the Earth.
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| Declination |
The celestial equivalent of latitude on Earth, is Declination. It is measured from the celestial equator, in units of degrees, minutes, and seconds. It increase from 0 degrees to +90 degrees at the north pole, and decreases from 0 degrees to -90 degrees at the South Pole. (See Right Ascension) |
| Diffuse Nebula |
Clouds of thin widespread agglomerations of gas and dust. |
| Dobsonian |
This is a common name used for a Newtonian telescope which has been mounted in a Dobson mount. Designed by John Dobson, this mount suspends the body of the Newtonian in a fork, this gives the telescope movement in the vertical. The fork is then supported by a pedestal that rotates, this allows the telescope to move in the horizontal. Designed to be simple to build, this type of mount is very popular. Mr. Dobson, by the way, does not like the name Dobsonian, he prefers to call them 'street scopes'. |
| Double Star |
A stellar system consisting of two stars orbiting about a common center of mass. Also called a binary star. |
| Eclipse |
The partial or complete obscuring, relative to the observer, of one celestial body by another. |
| Elliptical Galaxy |
Spheroidal distributions of mostly older stars, usually without as much intersteller material found in a spiral galaxy. |
| Equinox |
Either of the two times during a year when the sun crosses the celestial equator and when the length of day and night are approximately equal. The vernal (spring) equinox or the autumnal equinox.
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| Eyepiece |
This is a small tube filled with bits of glass that controls the power of your telescope. There are 3 common diameters used for eyepieces: .965", 1.25" and 2". 1.25" eyepieces are very popular and widely available. Be warned that if you get a chance to use 2" eyepieces, you'll be hooked, just remember that not only are they bigger, they're more expensive. The bits of glass in the eyepiece vary in the amount of glass and their arrangement. Eyepieces have focal lengths like the telescope itself, and they range from about 4 millimeters to about 50 millimeters. Power is determined by dividing the focal length of the telescope by the focal length of the eyepiece. You will eventually acquire a collection of eyepieces, usually 3 or 4 of them spaced out in varying focal lengths. |
| Gibbous |
When the Moon is more than half full, but less than completely full.
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| Globular Cluster |
Gravitationally bound concentrations of approximately ten thousand to one million stars, spread over a volume of several tens to about 200 light years in diameter.
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| Irregular Galaxy |
A galaxy not does not have a regular shape like a spiral galaxy or elliptical galaxy. |
| Light Year |
Contrary to what some science fiction shows might lead you to believe, a light year, is not a measurement of time. It is a measurement of distance, specifically: the distance light travels, in a vacuum, in a year. At about 186,000 miles per second, this distance is approximately 6,000,000,000,000 (6 Trillion) miles (10 Trillion Kilometers). |
| Magnitude |
A numeric representation of the brightness of a celestial object. The higher the number, the dimmer the object. Since this measure is logarithmic, a five magnitude difference represents a 100-fold change in brightness. The Sun is a magnitude -26.8. The full Moon is -12.6. The star Wolf 359 is +13.6.
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| Meridian |
The imaginary great circle that passes through your zenith from north to south, dividing the sky into east and west halves. |
| Nebula |
A cloud of gas and dust.
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| Newtonian |
A type of reflector telescope. Light enters the front of the Newtonian and travels to the rear of the telescope where it is reflected and focused by the primary mirror. The light returns back towards front of the telescope where it is intercepted by the secondary mirror. The secondary mirror is suspended in the center of the telescope near the front. Mounted at a 45 degree angle, the secondary mirror then reflects the light out the side of the telescope, where it then passes through the eyepiece for viewing. The Newtonian telescope in the picture on the left, is mounted on an equatorial mount. |
| Nova |
A star which suddenly flares up to many times its original brightness before fading again.
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| Occultation |
The covering up of one celestial body by another.
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| Open Cluster |
Physically related groups of stars held together by mutual gravitational attraction. They occupy a limited region of space smaller than their distance from us and are all roughly at the same distance. |
| Opposition |
When a planet is exactly opposite the Sun, so that the Earth is between them.
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| Parsec |
A parsec is a measurement of distance and is about 3.3 light years. |
| Perihelion |
That point of the orbit of a planet or comet which is nearest to the sun. |
| Perturbation |
A disturbance in the motion of an object, produced by some force additional to that which causes its regular motion; as, the perturbations of the planets are caused by their attraction on each other. |
| Photometry |
Measurement of the properties of light, especially luminous intensity. |
| Planetary Nebula |
At the end of the life cycle of an average star, like our Sun, the nuclear reactions in its core come to an end, while helium is burned into a shell. This makes the star expand, and causes its outer layers to pulsate. As it becomes more and more unstable, it loses mass in strong stellar winds. The instability finally causes the ejection of a significant part of the star''s mass in an expanding shell. The stellar core remains as an exremely hot, small central star, which emits high energetic radiation.
The expanding gas shell is excited to shine by the high-energy radiation emitted from the central star. |
| Polar Scope |
A polar scope is a small telescope that is used for aligning a larger telescope with the North or South pole star. |
| Power |
In astronomy this is not the important thing that you may think it is. Do not buy a telescope because it's advertised to be 600x (power). I, on average, use 79x for my observing and to date, the highest power I have ever used was 288x. The lowest power I use, is 9x, and that's when I'm using my binoculars. Take a look at Aperture, that's where most of the action is. |
| Precession |
Precession is the small wobbling motion around the Earth's axis that the Earth makes as it spins (just like the wobbling motion of a spinning top). This causes the Earth's axis to point in a slightly different direction over time. Polaris is our pole star now, but this is not permanent because of precession. Precession is caused by the gravitational influences of the Sun and the Moon and has a period of 25,800 years. |
| Quasar |
A very distant immensely bright object.
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| Reflector |
A telescope that uses mirrors as it's means of focusing light. This one should be easy for you to remember, just keep in mind that mirrors reflect. There are many different ways to design a reflecting telescope like the Schmidt-Cassegrain or the Newtonian. |
| Refractor |
A telescope that uses lenses as it's means of focusing light. A refractor offers an unobstructed aperture, that theoretically has the least aberrations of any optical system. A lower quality refractor can sometime exhibit a blue aura, around brighter objects. This chromatic aberration is reduced in apochromatic design refractors. The refractor has a maintenance advantage of being a closed system which keeps dust out of the optical tube assembly. The accompanying picture shows a refractor telescope, that is mounted on an equatorial mount.
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| Right Ascension |
The celestial equivalent of longitude on Earth, is Right Ascension. It is abbreviated to RA, and is measured in hours, minutes and seconds instead of degrees. The 0 hours line of RA passes through the point where the sun crosses the celestial equator at the first moment of spring in the northern hemisphere: the Vernal Equinox. Hours in right ascension are measured eastward from this point, up to 23 hours, 59 minutes. One minute later starts the cycle over at 0 hours. (see Declination)
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| Schmidt-Cassegrain |
A type of reflecting telescope that also uses a lens, called a corrector plate. Light enters the telescope through the corrector plate. It then travels to the rear of the telescope where it is reflected and focused by the primary mirror onto the secondary mirror. The secondary mirror, mounted in the center of the corrector plate, reflects the light back to the rear of the telescope. The light travels back through a hole in the center of the primary mirror, out of the rear of the telescope, where it then passes through the eyepiece. Most of the time a 90 degree diagonal is attached to the rear of this kind of telescope before the eyepiece. This diagonal allows for more convenient viewing. |
| Sidereal Time |
Sidereal time is the time it takes for an astronomical object outside of our solar
system to return to the same spot in the sky in one day, or approximately 24 hours.
In other words, if you carefully marked the position of a star as it transits a fixed point in the sky and time it exactly until it returns to the same position you will find that this time is approximately 4 minutes less than 24 hours. The difference is explained by the fact that because the earth is constantly orbiting the Sun, the time it takes the Sun to return to the same spot in the sky is exactly 24 hours (with very small adjustments made periodically in the solar mean time). |
| Spiral Galaxy |
A huge disk of stars and interstellar matter. The whole mass slowly rotates around a central hub.
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| Summer Solstice |
The longest day of the year. In the northern hemispere the summer solstice usually occurs around June 22nd. See also Equinox and Winter Solstice. |
| Supernova Remnant |
The gaseous remainders of a supernova.
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| Twilight |
There three types of twilight. Civil twilight ends when the center of the sun is 6° below the horizon.
Nautical twilight ends when the sun is 12° below the horizon.
Astronomical twilight ends when the sun is 18° below the horizon. |
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Umbra
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The dark inner part of a sunspot, or shadow.
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| Variable Star |
A star which fluctuates in brightness.
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| Winter Solstice |
The shortest day of the year. In the northern hemispere the winter solstice usually occurs around December 22nd. See also Equinox and Summer Solstice. |
| Zenith |
The point on the celestial sphere that is directly above the observer. |
