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  2. Parsec - Wikipedia

    en.wikipedia.org/wiki/Parsec

    By the 2015 definition, 1 au of arc length subtends an angle of 1″ at the center of the circle of radius 1 pc. That is, 1 pc = 1 au/tan( 1″ ) ≈ 206,264.8 au by definition. [ 9 ] Converting from degree/minute/second units to radians ,

  3. Light-year - Wikipedia

    en.wikipedia.org/wiki/Light-year

    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.

  4. Arc measurement of Delambre and Méchain - Wikipedia

    en.wikipedia.org/wiki/Arc_measurement_of_Delambre...

    The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m⋅s −1, where the second is defined in terms of the caesium frequency Δν Cs.

  5. List of mathematical constants - Wikipedia

    en.wikipedia.org/wiki/List_of_mathematical_constants

    For example, the constant π may be defined as the ratio of the length of a circle's circumference to its diameter. The following list includes a decimal expansion and set containing each number, ordered by year of discovery. The column headings may be clicked to sort the table alphabetically, by decimal value, or by set.

  6. Pendulum - Wikipedia

    en.wikipedia.org/wiki/Pendulum

    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 ...

  7. Surface brightness - Wikipedia

    en.wikipedia.org/wiki/Surface_brightness

    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). [10]

  8. Lux - Wikipedia

    en.wikipedia.org/wiki/Lux

    The lux is one lumen per square metre (lm/m 2), and the corresponding radiometric unit, which measures irradiance, is the watt per square metre (W/m 2). There is no single conversion factor between lux and W/m 2 ; there is a different conversion factor for every wavelength, and it is not possible to make a conversion unless one knows the ...

  9. Wind wave - Wikipedia

    en.wikipedia.org/wiki/Wind_wave

    where g is the acceleration due to gravity, 9.8 meters (32 feet) per second squared. Because g and π (3.14) are constants, the equation can be reduced to: = when C is measured in meters per second and L in meters. In both formulas the wave speed is proportional to the square root of the wavelength.