If you have an astronomical fact or definition that is not shown here, then please email me at rj_cox@tiscali.co.uk for inclusion.
Astronomical Unit (a.u.) is the mean Earth-Sun distance of 149,597,890 km or 92,955,820 miles
Right Ascension (RA) the angular distance 0° to 360° around from the meridian (great circle) which passes through the vernal equinox and the poles. Measured in the same direction as the solar motion, east to west. Units used are hours, minutes and seconds of time, where:
1 hour = 15°, 1 minute = 15' and 1 second = 15"
Declination (Dec) is the angular distance measured in degrees, minutes and seconds of arc north (0° to +90°) or south (0° to -90°) of the celestial equator.
Precession the Earth's axis moves in space and describes a small circle. The period of which is about 25,750 years. Therefore as the poles and celestial equator also move, the positions of stars and other deep space objects will change position over time.
Focal Ratio
The `speed' of a telescope's optics, found by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera. Fast f/4 to f/5 focal ratios are generally best for wide field observing and deep space photography. Slow f/11 to f/15 focal ratios are usually better suited to lunar, planetary, and binary star observing and high power photography. Medium f/6 to f/10 focal ratios work well with either. An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large. Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio - so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio.
Magnitudes
Spectral Types
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