Earth
Earth's rotation period relative to the Sun (its mean solar day) is 86,400 seconds of mean solar time. Each of these seconds is slightly longer than an SI second because Earth's 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's ephemerides 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.
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86164.098 903 691 seconds of mean solar time (UT1) (23 56 4.098 903 691). Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86164.090 530 832 88 seconds of mean solar time (UT1) (23 56 4.090 530 832 88). Thus the sidereal day is shorter than the stellar day by about 8.4 ms. The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005 and 1962–2005. Recently (1999–2005) the average annual length of the mean solar day in excess of 86400 SI seconds has varied between 0.3 ms and 1 ms, which must be added to both the stellar and sidereal days given in mean solar time above to obtain their lengths in SI seconds.
Read more about this topic: Rotation Period
Famous quotes containing the word earth:
“It is the very error of the moon,
She comes more near the earth than she was wont,
And makes men mad.”
—William Shakespeare (1564–1616)
“That air would disappear from the whole earth in time, perhaps; but long after his day. He did not know just when it had become so necessary to him, but he had come back to die in exile for the sake of it. Something soft and wild and free, something that whispered to the ear on the pillow, lightened the heart, softly, softly picked the lock, slid the bolts, and released the prisoned spirit of man into the wind, into the blue and gold, into the morning, into the morning!”
—Willa Cather (1873–1947)
“till seven years were gane and past,
True Thomas on earth was never seen.”
—Unknown. Thomas the Rhymer (l. 79–80)