F - |
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Focal Length | The distance between a lens (or mirror) and the point at which the image of an object at infinity is |
| brought to focus. The focal length divided by the aperture of the mirror or lens is termed the focal |
| ratio. |
J - |
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Jovian Planets | Any of the four gas giant planets that are at a greater distance from the sun than the terrestrial |
| planets. |
K - |
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Kuiper Belt | A region beyond the orbit of Neptune extending to about 1000 AU which is a source of many short |
| period comets. |
L - |
|
A | |
| With 31,557,600 seconds in a year, the |
| trillion mi). |
M - |
|
Magnitude | Magnitude is a measure of the brightness of a celestial body. The brightest stars are assigned |
| magnitude 1 and those increasingly fainter from 2 down to magnitude 5. The faintest star that can be |
| seen without a telescope is about magnitude 6. Each magnitude step corresponds to a ratio of 2.5 in |
| brightness. Thus a star of magnitude 1 is 2.5 times brighter than a star of magnitude 2, and 100 times |
| brighter than a magnitude 5 star. The brightest star, Sirius, has an apparent magnitude of |
| full moon is |
| point of the apparent magnitude scale is arbitrary. |
Meridian | A reference line in the sky that starts at the North celestial pole and ends at the South celestial pole |
| and passes through the zenith. If you are facing South, the meridian starts from your Southern |
| horizon and passes directly overhead to the North celestial pole. |
Messier | A French astronomer in the late 1700’s who was primarily looking for comets. Comets are hazy |
| diffuse objects and so Messier cataloged objects that were not comets to help his search. This |
| catalog became the Messier Catalog, M1 through M110. |
N - |
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Nebula | Interstellar cloud of gas and dust. Also refers to any celestial object that has a cloudy appearance. |
North Celestial Pole | The point in the Northern hemisphere around which all the stars appear to rotate. This is caused by |
| the fact that the Earth is rotating on an axis that passes through the North and South celestial poles. |
| The star Polaris lays less than a degree from this point and is therefore referred to as the "Pole Star". |
Nova | Although Latin for "new" it denotes a star that suddenly becomes explosively bright at the end of its |
| life cycle. |
O - |
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Open Cluster | One of the groupings of stars that is concentrated along the plane of the Milky Way. Most have an |
| asymmetrical appearance and are loosely assembled. They contain from a dozen to many hundreds |
| of stars. |
P - |
|
Parallax | Parallax is the difference in the apparent position of an object against a background when viewed by |
| an observer from two different locations. These positions and the actual position of the object form a |
| triangle from which the apex angle (the parallax) and the distance of the object can be determined if |
| the length of the baseline between the observing positions is known and the angular direction of the |
| object from each position at the ends of the baseline has been measured. The traditional method in |
| astronomy of determining the distance to a celestial object is to measure its parallax. |
Parfocal | Refers to a group of eyepieces that all require the same distance from the focal plane of the |
| telescope to be in focus. This means when you focus one parfocal eyepiece all the other parfocal |
| eyepieces, in a particular line of eyepieces, will be in focus. |
Parsec | The distance at which a star would show parallax of one second of arc. It is equal to 3.26 |
| 206,265 astronomical units, or 30,800,000,000,000 km. (Apart from the Sun, no star lies within one |
| parsec of us.) |
Point Source | An object which cannot be resolved into an image because it to too far away or too small is |
| considered a point source. A planet is far away but it can be resolved as a disk. Most stars cannot |
| be resolved as disks, they are too far away. |
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