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A truly dark sky has a surface brightness of 2 × 10 −4 cd m −2 or 21.8 mag arcsec −2. [ 9 ] [ clarification needed ] The peak surface brightness of the central region of the Orion Nebula is about 17 Mag/arcsec 2 (about 14 milli nits ) and the outer bluish glow has a peak surface brightness of 21.3 Mag/arcsec 2 (about 0.27 millinits).
Meteorological data includes wind speeds which may be expressed as statute miles per hour, knots, or meters per second. Here are the conversion factors for those various expressions of wind speed: 1 m/s = 2.237 statute mile/h = 1.944 knots 1 knot = 1.151 statute mile/h = 0.514 m/s 1 statute mile/h = 0.869 knots = 0.447 m/s. Note:
The siemens (symbol: S) is the unit of electric conductance, electric susceptance, and electric admittance in the International System of Units (SI). Conductance, susceptance, and admittance are the reciprocals of resistance, reactance, and impedance respectively; hence one siemens is equal to the reciprocal of one ohm (Ω −1) and is also referred to as the mho.
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,265 astronomical units (AU), i.e. 30.9 trillion kilometres (19.2 trillion miles).
A value of 9.460 536 207 × 10 15 m found in some modern sources [15] [16] is the product of a mean Gregorian year (365.2425 days or 31 556 952 s) and the defined speed of light (299 792 458 m/s). Another value, 9.460 528 405 × 10 15 m , [ 17 ] is the product of the J1900.0 mean tropical year and the defined speed of light.
Although not defined by the pendulum, the final length chosen for the metre, 10 −7 of the pole-to-equator meridian arc, was very close to the length of the seconds pendulum (0.9937 m), within 0.63%. Although no reason for this particular choice was given at the time, it was probably to facilitate the use of the seconds pendulum as a secondary ...
The metre per second is the unit of both speed (a scalar quantity) and velocity (a vector quantity, which has direction and magnitude) in the International System of Units (SI), equal to the speed of a body covering a distance of one metre in a time of one second.
Rapidly-rotating 20 M ☉ stars take 9.3 million years to reach the red supergiant stage, while 20 M ☉ stars with slow rotation take only 8.1 million years. [173] These are the best estimates of Betelgeuse's current age, as the time since its zero age main sequence stage is estimated to be 8.0–8.5 million years as a 20 M ☉ star with no ...