   Absolute magnitude The absolute magnitude Mvis of an astronomical object is the apparent magnitude an object would have if it were placed at a distance of 10 Parsec from the viewpoint. This magnitude is given as order of magnitude. Albedo The albedo is the reflectivity of an object. The albedo represents the ratio of reflected light to incident light. Aphelion The aphelion is the point in orbit where an object is the farthest from its central body. So the aphelion is also the maximum distance of an orbit. The opposite is the perihelion. Apparent magnitude Apparent magnitude mvis is a measure of the brightness of a celestial object as seen from Earth. The lower the number, the brighter the object. This magnitude is given as order of magnitude. Astronomical Unit (AU) The astronomical unit is the average distance from the Earth to the Sun. 1 AU = 149.597.870,61 km = 92.960.116 miles Diameter A celestial bodies diameter is the distance from its surface through the center of the body to the opposing surface, normally measured on the equator. Eccentricity The eccentricity measures the relation between maximum distance a (Aphelion) and minimum distance b (Perihelion) of an elliptical orbit. The eccentricity measures how far from a circular shape an elliptic orbit is. Eccentricity e = √(a² - b²) / a. Ecliptic The ecliptic is the plane on which Earth moves on it orbit around the Sun. Escape velocity THe minimum speed needed to allow an object of small mass (e.g. a space probe) to escape the surface gravity of an object of larger mass is called escape velocity. Force of gravity The force of gravity is the force at which a large body attracts another object towards itself. All objects upon earth experience a force of gravity which is directed "downward" towards the center of the earth and is measured as 1 g = 9,78 m/s². Inclination The inclination of an objects orbit is the angle between the plane of its orbit and the ecliptic for objects orbiting the Sun. The inclination of planets satellite is the angle between the plane of its orbit and the plane of its primary bodies equator. Kelvin Kelvin is based on the absolute temperature scale. The absolute temperature minimum (0 K) is similar to -273,15 ° C in the centigrade scale. 0 K = -273,15 ° C = -460 F 0 ° C = 273,15 K = 32 F Lightyear (ly) The distance light travels in a year in a vacuum at the rate of 300.000 km/s is defined as a lightyear. 1 ly = 9.460.530 × 106 km = 63.239,74 AU Oblateness The measurement oblateness is the difference between a spheroid and a globe. The oblateness is calculated as the difference between polar diameter and equatorial diameter in relation to the equatorial diameter. Oblateness a = (dequ - dpol) / dequ Orbital velocity The mean orbital velocity is the average speed of a planet or satellite in its orbit around the central body. The orbital velocity is at its minimum in the aphelion (maximum distance) and at its maximum in the perihelion (minimum distance). Order of magnitude The order of magnitude is the degree of brightness of a celestial body. The numerical classification 0 is reserved for the brightest stars like Vega. A difference of five orders of magnitude represents a difference in luminance of 100, each order of magnitude equals a multiplicator of 2,512. Parsec (pc) A parsec is defined as the distance from which the axis of Earth's orbit (1 Astronomical Unit) spans an angle of an arcsecond. 1 pc = 3,261633 ly Perihelion The pelihelion is the point in orbit where an object is closest to its central body. So the pelihelion is also the minimum distance of an orbit. The opposite is the aphelion. Rotational period The rotational period is the time a celestial body takes to rotate once on its axis with, respect to the stars. The rotational period is synchronous if it is identical to the orbital period, meaning the celestial body always shows the same side to its central body. Rotational velocity The rotational velocity of a planet of moon is the speed of this bodies rotational motion around its axis. The velocity is measured as the speed a point on the equator inherits on its rotational movement. Sideric period The siderial period is the period of revolution of one object (planet, asteroid or other satellite) around its central body measured relative to the stats. This period is measured on completion of an exactly 360° orbit. Spectral type The spectral type is the classification of a star according to its temperature as measured from the strengths of its spectral lines. This is also the order of luminosity and mass (most luminous and most massive to dimmest and least massive). In order of temperatures from hottest to coolest the spectral types are O, B, A, F, G, K, M. Synodic period The synodic period is the time required for a planet to go from a particular configuration as seen from Earth or respectively Sun, back to that same configuration again. I am most grateful for every correction or sensible improvement of descriptions in this glossary. Please send them as E-Mail to me .
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