Earth's rotation

The Earth's rotation is the rotation of the earth around its own axis. The axis of rotation is called the Earth's axis. The earth rotates to the east, which can be easily checked at sunrise through orientation with a compass. From the Polar Star of view, the earth rotates counter-clockwise.

The rotation vector of the earth has, according to the right-hand screw rule exactly earth- north, and thus almost exactly the Pole Star. All points on the earth's surface, except its two poles, thereby move into the (all local ) east direction. For an observer who is positioned head north on the ground and the stars seen in the zenith, located about an earth-fixed mast tip moves (compared to a very nearby star within a minute visible) also to the east, which, however, in the sky by observing from below, quasi as seen from the earth out, is on the left.

The average duration of one revolution with respect to the cosmic taken as resting background - the mean sidereal day - is 23 h 56 min 4,10 s (which is laid down by the IERS nominal average angular velocity of 7.292115 × 10-5 rad / s corresponds ). The reference points for the precise measurement of the rotation period used, inter alia observed by radio interferometry of extragalactic radio sources nowadays. Until a few decades, however, no resting reference points were available which higher demands would have been enough. The observation of accessible stars were of limited use because of their own movement.

In astronomical practice, therefore, one obtains the rotation usually on the spring equinox, the position can always be calculated with respect to the stars and planets. The time it takes for the earth to occupy the same position relative to the vernal equinox after one revolution, is a sidereal day. Therefore takes a sidereal day only 23 hours 56 min 4.091 s The precession of the Earth, however, is the reason that a sidereal day approximately 8 ms ( ie milliseconds ) is longer than a sidereal day.

Dividing the sidereal day in 24 h * (hours sidereal time ), then the sidereal time is a direct measure of the angle of rotation of the earth. From knowledge of the sidereal time, therefore, the current view of the sky can be determined. In particular, culminates the Spring Point for the relevant observers to 24 h *.

Note the not entirely consistent name: The sidereal day is not related despite its name on the stars, but on the spring equinox. On the star, the sidereal day refers. The English names ( defined by the IERS ) for example, are exactly the opposite: the sidereal day sidereal day is called here, while the sidereal day is called stellar day.

• 3.3.1 precession
• 3.3.2 polar motion
• 3.3.3 polar wandering

Sonnentag

The solar day is the period of one solar maximum to the next level and serves as the basis of our everyday time measurement. It takes an average of 24 hours and is slightly longer than a sidereal day. The difference between the length of the star day and the length of the solar day is due to the annual motion of the earth around the sun. After one complete rotation of the Earth is in its orbit nearly one degree of arc on running (360 degrees in 365 days ). Around this same angle the earth must continue turning until the sun can be seen again in the same direction in the sky, as the day before. This takes about 4 minutes in average.

Since the elliptical orbit of the earth but through the course of the year, with variable speed and because the ecliptic is inclined to the celestial equator, all days of sunshine a year are not the same length. One therefore distinguishes the true solar day as the period between two peaks and the sun is always the same long mean solar day whose length corresponds to the average over a year lengths of true solar days. The mean solar day was divided into 24 hours, by definition. Therefore, our clocks run after a mean sun, unlike sundials, which naturally take the actual sun to the base. The time difference between mean solar time and true solar time is called the equation of time.

Rotation axes

Due to their inertia, the rotational axis of the earth retains its direction ( almost, see below) constant. Your northern end is currently pointing to a point in the sky, which is just a degree away from a star of the constellation Ursa Minor. To this point seems to be for a terrestrial observer of the heavens to rotate once a day. Therefore, the point is called the north celestial pole and said Polarstern. The southern extension shows no striking star.

The axis of rotation is just under 23.5 ° to the Earth's orbital plane inclined ( obliquity of the ecliptic ). Since the Earth's axis maintains its direction in space, while the earth in the course of a year runs once around the sun is on one half of the railway in the northern hemisphere and on the other half inclined to the southern hemisphere of the Sun. On the relevant hemisphere prevails because of the stronger sunlight summer, the other seasons arise accordingly.

Temporal variability

Physical Basics

Due to its angular momentum, the earth makes one rotation. The angular momentum is the product of the rotational velocity of the earth (expressed as an angular velocity ) and its moment of inertia.

Since angular momentum is conserved, it can only be changed by the action of attacking from the outside torque. As the angular momentum vector has both a magnitude and a direction; Constancy of the angular momentum, therefore, means that both the rotational speed and the position of the rotational axis in space remains constant.

The forces acting on the earth torques are very small, so its angular momentum and thus its speed of rotation and the orientation of its axis of rotation remain substantially constant. With precise measurement or observation long periods of time, however, changes over time can be observed.

The rotation speed changes,

• When changes by the action of an external torque, the total angular momentum
• If the magnitude remains constant total angular momentum in various ways on subsystems ( atmosphere / mantle / core of the Earth ) redistributed ( the observations capture only the motion of the system " mantle with crust " )
• When (eg, melting of glaciers ) changes the moment of inertia of the earth due to deformation (eg postglacial land uplift ) or mass redistribution, so that despite a constant total angular momentum of a different rotational speed results ( pirouette effect ).

The position of the axis of rotation in the area change if external torques acting ( precession ). Since the symmetry axis of the earth beyond does not coincide exactly with its axis of rotation, the earth body performs small oscillations about the axis of rotation, so that their intersection point varies by the earth's surface in a range of several meters ( polar motion ).

Variability of the rotation period

Short-term fluctuations

Accurate measurements show that the duration of one revolution, and thus the length of day is not strictly constant. The picture on the right shows the day length since 1962. Shown is the deviation of the measured day length of a nominal, derived from the International System of Units Reference Date with a length of exactly 86,400 SI seconds. After an initial increase in the trend since the early 1970s, is in decline. Such fluctuations, which may comprise several decades to centuries, are probably due to mass displacements in the liquid outer core.

Superimposed on these fluctuations are fluctuations with a period of about a decade. They are probably caused by an angular momentum exchange between the Earth's core and mantle. Even longer-term shifts in the water or ice distribution on the earth surface are likely to play a role.

Particularly marked An annual fluctuation with an amplitude of about 2 ms. You can be attributed to changes in the position and strength of the larger jet streams. Fluctuations on a time scale of days decades caused by the angular momentum exchange between the earth surface and atmosphere (eg, winds blowing against major mountain ranges such as the Andes or the Rocky Mountains ). The latter context is now so well known that meteorological atmosphere models can be used to predict these fluctuations (keyword: Atmospheric Angular Momentum AAM).

Tide -induced deformations of earth and oceans cause bi-weekly, monthly, semi-annual and annual shares of the fluctuations. They are completely predictable and are therefore often subtracted from the observed data in order to bring out the other effects more clearly. They have to be added again on the basis of the respective calculation models prior to administration.

Occasionally, individual events are as visible in the data, for example, mass displacements due to strong earthquakes. The graph shows the effects of a particularly strong El Niño during the winter 1982/83 are clearly visible. The Indian Ocean Tsunami of 2004, the Earth's rotation accelerated so that the day length to 8 microseconds (ie microseconds) shortened. A further acceleration experienced the rotation of the earth on 11 March 2011 after the earthquake in the Pacific Ocean off the coast of Japan: The Earth now rotates slightly faster, "one day is now 1.8 microseconds shorter than before."

Also, transfers of biomass play a certain role. However, the assertion that the earth is in the ( northern) summer slower turn than in winter, because the leaves on the trees increase the moment of inertia ( pirouette effect ) and on the northern hemisphere more trees there than in the southern hemisphere, is not tenable. As the chart shows, the day length in the northern summer is just the shortest, then that is the Earth rotates very quickly. The existing certainly influence of the foliage is so completely obscured by opposing greater effects. One of overlapping effect is, among other things, the redistribution of water masses in the form of snow on the heights of the mountains.

In all of these fluctuations is important to remember that even relatively small influences can add up to significant effects when the exposure time is long enough. In the longer-term fluctuations therefore lower torques or angular momentum redistribution are necessary than with shorter -term.

The current day lengths are usually longer than the reference length of 86 400 days SI seconds. This is because that the SI second ultimately - was derived from that day length, as it existed during the mid-19th century - through several intermediate steps. Due to the below- term increase in the length of day, the days are generally a little longer today than it was then. The excess length of day over the nominal 86,400 s must be regularly offset by a leap second. If the length of the day, such as longer time to 2 ms above the setpoint, the Earth's rotation device to a constant continuous atomic clock every day at 2 ms longer in default. After 500 days, the difference would be accrued to one second: The 500 rotation would be terminated only by one second after midnight ( atomic time ) the 500th day. The watches are then stopped for a second ( " leap second " ) so that the time scale used in everyday life not too far away from the predetermined by the Earth's rotation day - night cycle. This time scale, which is based partly on the plane defined by atomic clocks and therefore strictly equal SI second, but on the other hand, is adjusted by leap seconds to the irregular rotation of the earth by inserting (possibly omitting or) is the Coordinated Universal Time ( UTC). You away with every positive leap second further from the strictly uniform but used only for scientific and technical purposes the International Atomic Time ( TAI ). A leap second is inserted, if the difference between Earth's rotation and UTC threatens over 0.9 s to grow.

In the example about every one and a half years would be required a leap second. This was during the 1980s indeed the case. As the day length graphic can be seen, the day length has again significantly closer to the target value since the mid- 1990s, so that between 1998 and 2005 no leap second was required.

Long-term changes

The tidal friction exerts a braking torque on the earth, so that the length of the day slowly but continuously increasing. In modern measurements, this effect is completely obscured by the fluctuations described above. Since he but added up over longer periods of time, it can be determined with the help of traditional ancient and medieval astronomical observations.

As to the introduction of atomic clocks which was used by the observer time scale always at the sun's path and ultimately compared to the Earth's rotation, it was the same fluctuations and long-term drifting subject as the earth's rotation. On the other hand, are based modern physical models of planetary motion on a strictly uniform over time, how it can be implemented today, regardless of the rotation of the earth with atomic clocks. Specifically, the so-called Dynamic time is used for this purpose. If one converts the movements of the planets back to determine the timing of the observed event in the uniform extending dynamic time and compares this time with the traditional unevenly verlaufenen local time of the observer, we can see a discrepancy fixed, which increases continuously, the further you go into the past returns. For Babylonian reports around the year -700, for example, the traditional local time differs by about five to six hours of that time, which would be expected under the assumption of a constant rotation of the earth. Is to the reports extracted local time therefore always a correction? T to add up to get the corresponding date in Dynamic time and to compare the report with the return statement.

The evaluation of numerous observations from the past 2,700 years shows that the length of the day increased by about 17 microseconds per year during this period, on average. This is in good agreement with the result regardless gained that day length on the one hand increases due to tidal friction by about 23 microseconds per year ( about the conservation of angular momentum derived from the observed influence of tidal friction on the motion of the moon ), while that caused by the postglacial land uplift streamlining of the earth because of the associated pirouettes effect, the length of day by about 6.0 microseconds per year reduced ( since the volume of the earth can not change, the raising near polar regions leads to a shrinkage of the Äquatorwulstes. a bullet with less flattening has a lower moment of inertia ).

For prehistoric times, there is the speed of Earth's rotation from daily growth rings of fossil marine organisms with calcareous skeleton read. When the daily growth is modulated by the monthly change from neap and spring tide or by the annual change of season ( as can also be observed in extant relatives of such organisms ), it can be by counting the rings in principle at least the number of days in the month or identify a year. Corresponding studies indicate, for example, that 400 million years ago, the year was about 400 days, assuming the same annual period or about 21.9 hours per day. For 310 million years ago, however, one days 20 hours could be determined. The results thus permit the assumption that the strength of the tidal friction varied over geological periods of time considerably. Such a variation seems reasonable because most of the energy turnover is probably caused by tidal currents in the shallow shelf seas and their extent and distribution can change dramatically as a result of continental drift.

Mathematical models for the early, just in the pipeline can earth, or about 4 billion years ago, suggest an original day length of only 14 hours. Other scholars assume a rotational period of six to seven hours for this phase of Earth's history.

Variability of the axis of rotation

Precession

Because of their flattening the earth has an equatorial bulge, which is inclined due to the inclination of the earth's axis to the orbital plane. The forces exerted by the sun, the moon and the other planets gravitational forces try to pull the bead in the orbital plane. Due to this torque is subject to the angular momentum of the earth by a slow change in the precession. The Earth's axis maintains its inclination relative to the orbit. The direction in which it is inclined, but pivots through 360 ° in the course of approximately 26,000 years.

A further correction is the nutation, swinging about the rotational axis, with a period of approximately 19 years.

Polar motion

The symmetry axis of the earth is not coincides exactly with the axis of rotation. The rotation is nevertheless stable, as it takes place about the axis with the largest moment of inertia due to the flattening of the earth. Otherwise, the deviation would turn, and lead to a wobble of the earth. Because of the stable situation, the deviation remains limited, and the symmetry axis of the earth performs about once a year präzessionsähnliche movement about the axis of rotation. The point at which the instantaneous axis of rotation penetrates the earth's surface, is characterized thereby an irregular helix having a maximum diameter of about 20 m. This oscillation is composed of two components: a forced by periodic displacements of water and air masses with annual oscillation period and a free oscillation with a period of about 14 months ( Chandler period). The superposition of the two results in the amplitude of the total vibration will vary about six annual rhythm of between about 2 m to about 8 m. In the middle of the pole is drifting slowly toward 80 ° West.

Polar wandering

Displacements of the points of intersection of the axis of rotation of the earth relative to the position of the continents are called polar wandering. Such shifts occur by superposition of several phenomena. Firstly, the lithospheric plates move on the stable in space globe. From the perspective of a place on a continent hiking pole. On the other hand may shift their principal axes of inertia of mass displacements on or in the earth, then tilts the earth in space ( during the rotation axis in space remains stable). Also in this case apparently migrate the poles. According to some scientists, there might have been before about 800 million years ago, such a true polar wander.

Earth rotation parameters

For many applications, astronomy, aerospace, surveying (especially Astro geodesy), etc., the exact knowledge of the instantaneous orientation of the Earth in space is necessary. If the accuracy requirements in a range in which the above-mentioned short-and long -term variations are noticeable, they must be taken into account. For this purpose, the so-called earth rotation parameters are periodically measured and published. they include

• World time correction DUT1 that indicates the difference between the coupled to the variable Earth rotation time scale UT1 and derived from the uniform atomic time Coordinated Universal Time UTC. UT1 is proportional to the earth's rotation and hence a measure of the instantaneous angle of rotation of the earth. The difference DUT1 = UT1 - UTC reflects the irregularity of the earth's rotation resist. Threatens the difference is greater than 0.9 to be s, so a leap second in UTC is added to compensate for the deviation again.
• The polar coordinates x and y. They describe the position of the instantaneous axis of rotation of the earth (more precisely, of the Celestial Ephemeris Pole) with respect to a certain fixed point on the Earth's surface ( the IERS Referenzpols ). The x - axis in the direction of the prime meridian (more precisely, of the IERS Reference Meridian ) and the y- axis in the direction 90 ° west. The unit of measurement milliarcseconds are mostly used ( the distance between the two points on the earth's surface can also be expressed in meters).
• The Himmelspolschwankungen and that describe the observed deviations of the celestial pole of certain mathematical models for precession and nutation. the deviation in length ecliptic, is the deviation of obliquity.

The necessary regularly performed worldwide observations are coordinated by the International Earth Rotation and Reference Systems Service ( IERS ), analyzed and published.

The data obtained are themselves of scientific interest. They contain information about the structure and physical properties of the Earth, changes in shape of the globe, changes in the exact location of Erdschwerpunkts and running within the Earth geophysical processes.

The relevant observations made ​​since the end of the nineteenth century by position measurements of stars or observations of occultations of stars by the moon. It was a five day a determination of the parameters are made. Since the 1970s and 1980s VLBI measurements and GPS observations and laser distance measurements came to appropriate satellite and the moon to it, and it could hourly or even something more frequent readings are taken. Lately can be traced continuously fluctuations by means of ring lasers. The need for the determination of the Earth rotation parameters of rotation and the direction angle can be measured with an accuracy of about half milliarcsecond nowadays. In Central Europe, several research groups working on these issues, including in Hannover ( Jürgen Müller) and in Vienna (Harald Schuh).

The speed at which the earth's surface moves in an easterly direction in the amount of the equator, lies at approximately 1666 km / h, decreasing towards the two poles from by the decreasing extent.

Origin of the rotation

According to the common idea of ​​the solar system was formed from a gas and dust cloud that coalesced due to their own gravity.

If two gas or dust particles move relative to each other, each relative to the other has an angular momentum, provided that they do not move exactly. The existence of an angular momentum is therefore not restricted to a circular motion; a straight line or otherwise arbitrarily moving particle carries with respect to a reference point of an angular momentum, provided its motion seen from this reference point has a sideways component, so it is not pointed directly at the reference point. Consider for example a billiard ball that is not completely centralized hits a second ball. Both balls will rotate after the collision around its vertical axis; the plug end of these rotations angular momentum was removed from the angular momentum, the linearly moving the ball had before the collision with respect to the second ball. Would the balls stick together during the collision, the resulting object would rotate. For the same reason also rotate the clumps formed in a gas and dust cloud, as it is very unlikely that all of their ingredients are exactly centrally clashed. Even after the clumps have grown into larger planetesimals changes each impact of a planetesimals on a proto-planet whose rotation depending on the impact point and angle. The answer to the question " where did the angular momentum " So reads: From the disordered motion of the particles, which is also always wear next to their associated with the movement of linear momentum angular momentum and the angular momenta are not all mutually canceled in the aggregation to the planet. The more compact condenses the resulting body, the faster it rotates (even while maintaining a constant angular momentum ) due to the effect pirouettes.

The direction of rotation of the earth is the same as the direction of rotation in its orbit around the sun, as in almost all other planets also. Only the Venus rotates opposite, and the rotation axis of Uranus lies nearly in its orbital plane.

Proof of Earth's rotation

The rotation of the earth can be detected by physical experiments. It manifests itself by Coriolis and centrifugal forces on the earth's surface. This is reflected, among other things, in the direction of rotation of cloud vortices in low pressure areas.

The earth's rotation the equator causes a centrifugal force which is directed opposite to the force of gravity so that the weight of an object at the equator is smaller than the pole. To demonstrate the Earth's rotation, the following tests may be used:

• Foucault pendulum
• Torsion balance by Loránd Eötvös
• Gyrocompass
• Laser gyroscope
• Trap experiments

• Swivel rod