## The Largest Spinny Bar on, err, is Earth.

On this most terrestrial of days, we here at the Spinny Bar Historical Society wanted to take a brief time out from our whirlwind tour of activity to bring you some fun Earthday Facts!

(shout out to Wikiscient).

Earth’s rotation period relative to the Sun (true noon to true noon) is its

true solar dayorapparent solar day. It is the derivative of the equation of time and thus depends on the eccentricity of Earth’s orbit and the tilt (obliquity) of Earth’s axis. Both vary over thousands of years^{[1]}so the annual variation of the true solar day also varies. Generally, it is longer than the mean solar day twice a year and shorter twice a year.^{[n 1]}The true solar day tends to be longer near perihelion when the Sun apparently moves along the ecliptic through a greater angle than usual, taking about 10 seconds longer to do so. Conversely, it is about 10 seconds shorter near aphelion. It is about20 seconds longer near a solstice when the projection of the Sun’s apparent movement along the ecliptic onto the celestial equator causes the Sun to move through a greater angle than usual. Conversely, near an equinox the projection onto the equator is shorter by about 20 seconds. Currently, the perihelion and solstice effects combine to lengthen the true solar day near December 22 by 30 mean solar seconds, but the solstice effect is partially cancelled by the aphelion effect near June 19 when it is only13 seconds longer. The effects of the equinoxes shorten it near March 26 and September 16 by 18 seconds and 21 seconds, respectively.^{[2]}^{[3]}^{[4]}The average of the true solar day over an entire year is the

mean solar day, which contains 86,400 mean solar seconds. Currently, each of these seconds is slightly longer than an SI second because Earth’s mean solar day is now slightly longer than it was during the 19th century due to tidal acceleration. The mean solar second between 1750 and 1892 was chosen in 1895 by Simon Newcomb as the independent unit of time in his Tables of the Sun. These tables were used to calculate the world’sephemerides between 1900 and 1983, so this second became known as the ephemeris second. The SI second was made equal to the ephemeris second in 1967.^{[5]}Earth’s rotation period relative to the fixed stars, called its

stellar dayby the International Earth Rotation and Reference Systems Service (IERS), is 86,164.098 903 691seconds of mean solar time (UT1) (23^{h}56^{m}4.098 903 691^{s}, 0.997 269 663 237 16 SI days).^{[6]}^{[n 2]}Earth’s rotation period relative to the precessing or moving mean vernalequinox, misnamed itssidereal day,^{[n 3]}is 86,164.090 530 832 88 seconds of mean solar time (UT1) (23^{h}56^{m}4.090 530 832 88^{s}, 0.997 269 566 329 08 SI days).^{[6]}Thus the sidereal day is shorter than the stellar day by about 8.4 ms.^{[8]}

**Source:** Wikipedia