Earth's rotation: Difference between revisions

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	== Rotation period ==		== Rotation period ==

			The Earth rotates at a rate of 15 degrees per hour and takes 24 hours to complete a full 360 degree rotation,the imaginary lines of longitude organised around the Earth's rotational characteristics contain this information where 15 degrees at the equator represents both 1 hour time difference and 1037.5 miles so that in 24 hours,the planet turns a full equatorial circumference of 24901.5 miles.Historically,the average 24 hour day is derived from the rotation of the Earth to natural noon,although unequal,the natural noon cycle provides the basis for both the average 24 hour day and rotation as a constant -
	] (or ''obliquity'') and its relation to the ] and ].]]
			"Here take notice, that the Sun or the Earth passeth the 12. Signes, or makes an entire revolution in the Ecliptick in 365 days, 5 hours 49 min. or there about, and that those days, reckon'd from noon to noon, are of different lenghts; as is known to all that are vers'd in Astronomy." Huygens http://www.xs4all.nl/~adcs/Huygens/06/kort-E.html
	] planet like the Earth, the ''']''' is shorter than the ]. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again but the Sun is not (1→2 = one stellar day). It is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day).]]
			The Earth's average rotation period to natural noon provides the basis for the progression of 24 hour days,from the 24 hours of Monday into the 24 hours of Tuesday,from Tuesday into Wednesday etc and was adapted as the standard for determining longitude using 4 minutes for each degree of rotation,15 degrees per hour and a full rotation in 24 hours.http://en.wikipedia.org/Marine_chronometer

	Earth's rotation period relative to the Sun (true noon to true noon) is its '''true solar day''' or '''apparent solar day'''. It is the ] of the ] and thus depends on the ] of Earth's orbit and the tilt (]) of Earth's axis. Both vary over thousands of years<ref></ref> 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.{{#tag:ref\|When Earth's eccentricity exceeds 0.047 and perihelion is at an appropriate equinox or solstice, only one period with one peak balances another period that has two peaks.<ref name=Meeus/>\|group=n}} The true solar day tends to be longer near ] when the Sun apparently moves along the ] through a greater angle than usual, taking about {{nowrap\|10 seconds}} longer to do so. Conversely, it is about {{nowrap\|10 seconds}} shorter near ]. It is about {{nowrap\|20 seconds}} longer near a ] when the projection of the Sun's apparent movement along the ecliptic onto the ] causes the Sun to move through a greater angle than usual. Conversely, near an ] the projection onto the equator is shorter by about {{nowrap\|20 seconds}}. Currently, the perihelion and solstice effects combine to lengthen the true solar day near {{nowrap\|December 22}} by {{nowrap\|30 mean}} solar seconds, but the solstice effect is partially cancelled by the aphelion effect near {{nowrap\|June 19}} when it is only {{nowrap\|13 seconds}} longer. The effects of the equinoxes shorten it near {{nowrap\|March 26}} and {{nowrap\|September 16}} by {{nowrap\|18 seconds}} and {{nowrap\|21 seconds}}, respectively.<ref name=Meeus>Jean Meeus, ''Mathematical astronomy morsels'' (Richmond, Virginia: Willmann-Bell, 1997) 345–6.</ref><ref></ref><ref></ref>

	The average of the true solar day over an entire year is the '''mean solar day''', which contains {{nowrap\|86,400 mean}} solar seconds. Currently, each of these seconds is slightly longer than an ] second because Earth's mean solar day is now slightly longer than it was during the 19th century due to ]. The mean solar second between 1750 and 1892 was chosen in 1895 by ] as the independent unit of time in his ]. These tables were used to calculate the world's ] between 1900 and 1983, so this second became known as the ]. The SI second was made equal to the ephemeris second in 1967.<ref></ref>

	Earth's rotation period relative to the ]s, called its '']'' by the ] (IERS), is {{nowrap\|86,164.098 903 691}} seconds of mean solar time (UT1) {{nowrap\|(23{{smallsup\|h}} 56{{smallsup\|m}} 4.098 903 691{{smallsup\|s}}}}, {{nowrap\|0.997 269 663 237 16}} ] ]s).<ref name=IERS></ref>{{#tag:ref\|Aoki, the ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time".<ref>Aoki, ''et al.'', "", ''Astronomy and Astrophysics'' '''105''' (1982) 359–361.</ref>\|group=n}} Earth's rotation period relative to the ] or moving mean vernal ], misnamed its '']'',<ref group=n>Sidereal day is arguably a misnomer because the dictionary definition of ] is "relating to the stars", thus fostering confusion with the stellar day.</ref> is {{nowrap\|86,164.090 530 832 88}} seconds of mean solar time (UT1) {{nowrap\|(23{{smallsup\|h}} 56{{smallsup\|m}} 4.090 530 832 88{{smallsup\|s}}}}, {{nowrap\|0.997 269 566 329 08}} SI days).<ref name=IERS/> Thus the sidereal day is shorter than the stellar day by about {{nowrap\|8.4 ms}}.<ref>''Explanatory Supplement to the Astronomical Almanac'', ed. P. Kenneth Seidelmann, Mill Valley, Cal., University Science Books, 1992, p.48, ISBN 0-935702-68-7.</ref>

	Both the stellar day and the sidereal day are shorter than the mean solar day by about {{nowrap\|3 minutes}} {{nowrap\|56 seconds}}. The mean solar day in SI seconds is available from the IERS for the periods {{nowrap\|1623–2005}}<ref> Graph at end.</ref> and {{nowrap\|1962–2005}}.<ref></ref>

	]
	Recently (1999–2009) the average annual length of the mean solar day in excess of 86,400 SI seconds has varied between {{nowrap\|0.3 ms}} and {{nowrap\|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.

	The angular speed of Earth's rotation in inertial space is {{nowrap\|7.2921159 {{E\|−5}}}} ]s per SI second (mean solar second).<ref name=IERS/> Multiplying by (180°/π radians)×(86,400 seconds/mean solar day) yields 360.9856°/mean solar day, indicating that Earth rotates more than 360° relative to the fixed stars in one solar day. Earth's movement along its nearly circular orbit while it is rotating once around its axis requires that Earth rotate slightly more than once relative to the fixed stars before the mean Sun can pass overhead again, even though it rotates only once (360°) relative to the mean Sun.<ref group=n>In astronomy, unlike geometry, 360° means returning to the same point in some cyclical time scale, either one mean solar day or one sidereal day for rotation on Earth's axis, or one sidereal year or one mean tropical year or even one mean ] containing exactly {{nowrap\|365.25 days}} for revolution around the Sun.</ref> Multiplying the value in rad/s by Earth's equatorial radius of {{nowrap\|6,378,137 m}} (] ellipsoid) (factors of 2π radians needed by both cancel) yields an equatorial speed of {{nowrap\|465.1 m/s}}, {{nowrap\|1,674.4 km/h}} or {{nowrap\|1,040.4 mi/h}}.<ref>Arthur N. Cox, ed., '''' p.244.</ref> Some sources state that Earth's equatorial speed is slightly less, or {{nowrap\|1,669.8 km/h}}.<ref>Michael E. Bakich, '''', p.50.</ref> This is obtained by dividing Earth's equatorial circumference by {{nowrap\|24 hours}}. However, the use of only one circumference unwittingly implies only one rotation in inertial space, so the corresponding time unit must be a sidereal hour. This is confirmed by multiplying by the number of sidereal days in one mean solar day, {{nowrap\|1.002 737 909 350 795}},<ref name=IERS/> which yields the equatorial speed in mean solar hours given above of {{nowrap\|1,674.4 km/h}}.

	The permanent monitoring of the Earth's rotation requires the use of ] coordinated with the ], ], and other ] techniques. This provides the absolute ] for the determination of ], ], and ].<ref></ref>

	Over millions of years, the rotation is significantly slowed by gravitational interactions with the ]: see ]. However some large scale events, such as the ], have caused the rotation to speed up by around 3 microseconds.<ref>, Nature, December 30, 2004.</ref>

	== Precession ==		== Precession ==

Revision as of 20:36, 13 April 2010

An animation showing the rotation of the Earth.

Earth's rotation is the rotation of the solid Earth around its own axis. The Earth rotates towards the east. As viewed from the North Star Polaris, the Earth turns counter-clockwise.

Rotation period

The Earth rotates at a rate of 15 degrees per hour and takes 24 hours to complete a full 360 degree rotation,the imaginary lines of longitude organised around the Earth's rotational characteristics contain this information where 15 degrees at the equator represents both 1 hour time difference and 1037.5 miles so that in 24 hours,the planet turns a full equatorial circumference of 24901.5 miles.Historically,the average 24 hour day is derived from the rotation of the Earth to natural noon,although unequal,the natural noon cycle provides the basis for both the average 24 hour day and rotation as a constant - "Here take notice, that the Sun or the Earth passeth the 12. Signes, or makes an entire revolution in the Ecliptick in 365 days, 5 hours 49 min. or there about, and that those days, reckon'd from noon to noon, are of different lenghts; as is known to all that are vers'd in Astronomy." Huygens http://www.xs4all.nl/~adcs/Huygens/06/kort-E.html The Earth's average rotation period to natural noon provides the basis for the progression of 24 hour days,from the 24 hours of Monday into the 24 hours of Tuesday,from Tuesday into Wednesday etc and was adapted as the standard for determining longitude using 4 minutes for each degree of rotation,15 degrees per hour and a full rotation in 24 hours.http://en.wikipedia.org/Marine_chronometer

Precession

The axis of the Earth's rotation tends, like the axis of a gyroscope, to maintain its orientation with respect to inertial space. External forces acting on Earth from the Sun, Moon, and planets cause deviations from the fixed orientation. The large, periodic shift of the Earth's axis is called precession, while the smaller corrections are nutation and polar motion.

Physical effects

The velocity of the rotation of Earth has had various effects over time, including the Earth's shape (an oblate spheroid), climate, ocean depth and currents, and tectonic forces.

Origin

An artist's impression of protoplanetary disk.

It is theorized that Earth formed as part of the birth of the Solar System: what eventually became the solar system initially existed as a large, rotating cloud of dust, rocks, and gas. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements ejected by supernovas. As this interstellar dust is inhomogeneous, any asymmetry during gravitational accretion results in the angular momentum of the eventual planet. The current rotation period of the Earth is the result of this initial rotation and other factors, including tidal friction and the hypothetical impact of Theia.

Evidence of Earth's rotation

In the Earth's rotating frame of reference, a freely moving body follows an apparent path that deviates from the one it would follow in a fixed frame of reference. Because of this Coriolis effect, falling bodies veer eastward from the vertical plumb line below their point of release, and projectiles veer right in the northern hemisphere (and left in the southern) from the direction in which they are shot. The Coriolis effect has many other manifestations, especially in meteorology, where it is responsible for the differing rotation direction of cyclones in the northern and southern hemispheres. Hooke, following a 1679 suggestion from Newton, tried unsuccessfully to verify the predicted half millimeter eastward deviation of a body dropped from a height of 8.2 meters, but definitive results were only obtained later, in the late 18th and early 19th century, by Giovanni Battista Guglielmini in Bologna, Friedrich Benzenberg in Hamburg and Ferdinand Reich in Freiberg, using taller towers and carefully released weights.

Foucault pendulum

The most celebrated test of Earth's rotation is the Foucault pendulum first built by physicist Léon Foucault in 1851, which consisted of an iron sphere suspended 67 m from the top of the Panthéon in Paris. Because of the Earth's rotation under the swinging pendulum the pendulum's plane of oscillation appears to rotate at a rate depending on latitude. At the latitude of Paris the predicted and observed shift was about 11 degrees clockwise per hour. Foucault pendulums now swing in museums around the world.