Rotation of the Earth




Until recent times the rotation of the Earth has served as the basis for timekeeping. The assumption was made that the rotational speed of the Earth was essentially constant and repeatable, and that the length of the day which resulted from this constant rotational speed was naturally useful as a measure of the passage of time. Astronomical observations, however, have shown that the speed with which the Earth is rotating is not constant with time. The variations in rotational speed may be classified into three types: secular, irregular, and periodic. The secular variation of the rotational speed refers to the apparently linear increase in the length of the day due chiefly to tidal friction. This effect causes slowing of the Earth's rotational speed and lengthening of the day by about 0.0005 to 0.0035 s per century.



The irregular changes in speed have caused the length of the day to vary by as much as 0.01 s over the past 200 years. Irregular changes consist of so-called decade fluctuations with characteristic periods of 5–10 years as well as variations that occur at shorter time scales. The decade fluctuations are apparently related to processes occurring within the Earth. The higher-frequency variations are now known to be largely related to the changes in the total angular momentum of the atmosphere. See also Atmospheric general circulation.



Periodic variations are associated with periodically repeatable physical processes affecting the Earth. Tides raised in the solid Earth by the Moon and the Sun produce periodic variations in the length of the day of the order of 0.0005 s with periods of 1 year, year, 27.55 days, and 13.66 days. Seasonal changes in global weather patterns occurring with approximately annual and semiannual periods also cause variations in the length of the day of this order. See also Earth tides.



Revolution about the Sun



The motion of the Earth about the Sun is seen as an apparent annual motion of the Sun along the ecliptic. A large number of astronomical observations of the positions of the Sun and other solar system objects have been made and are being made continuously. This information is required to determine the nature of the motion of the Earth about the Sun. Observations are analyzed using the mathematical methods of celestial mechanics to provide improved estimates of the motions of the solar system objects in the future and to describe the past motions of the objects. The description of the apparent motion of the Sun in the sky provides the determination of the orbit of the Earth. See also Celestial mechanics.



The true period of the revolution of the Earth around the Sun is determined by the time interval between successive returns of the Sun to the direction of the same star. This interval is the sidereal year of 365 days 6 h 9 min 9.51 s of mean solar time or 365.25636 mean solar days. The period between successive returns to the moving vernal equinox is known as the tropical year of 365 days 5 h 48 min 45.2 s or 365.24219 days. The length of the tropical year is regarded as the length of the year in common usage for calendars. The period of time between successive passages at perihelion (the closest approach of the Earth to the Sun) is called the anomalistic year of 365 days 6 h 13 min 53.26 s or 365.25964 days. The lengths of the years listed above are given for the year 2000. These values vary slowly as a consequence of the long-period perturbations of the Earth's orbit by other planets. See also Calendar; Perturbation (astronomy).



The mean distance from the Earth to the Sun, or the semimajor axis of the Earth's orbit, was the original definition of the astronomical unit (AU) of distance in the solar system. Its absolute value fixes the scale of the solar system and the whole universe in terms of terrestrial standards of length. The distance between the Earth and the Sun can be determined by a variety of methods. The most precise method relies on measurement of the travel time of electromagnetic signals reflected from objects in the solar system or received from artificial interplanetary probes. The currently adopted value of the astronomical unit is 1.495978706 × 1011 m (92,955,807 mi). See also Astronomical unit.



The eccentricity of the Earth's orbit can be accurately determined from the variable speed of the Sun's apparent motion along the ecliptic and the laws of elliptic motion. The adopted value of the eccentricity is 0.0167086171540.



The fact that the Equator of the Earth is inclined in space by about 23.5° to the orbital plane of the Earth (the ecliptic) causes the Northern Hemisphere to be exposed to the more direct rays of the Sun during part of the Earth's revolution around the Sun. The Southern Hemisphere receives the more direct rays 6 months, or a half revolution, later. This effect causes the seasons. See also Seasons.



Other motions



In addition to the rotation of the Earth and its orbital motion about the Sun, the Earth experiences various small motions about its center of mass. Precession and nutation are examples, and these are caused by the gravitational attraction of the Sun and Moon on the nonspherical Earth. See also Nutation (astronomy and mechanics); Precession of equinoxes.



Because the axis of symmetry of the Earth is not aligned precisely with the axis of rotation, the Earth also executes a motion about its center of mass known as polar motion. This motion, caused by geophysical and meteorological effects on and within the Earth, is not predictable with accuracy, and must be observed continuously to provide the most precise information on the orientation of the Earth. Polar motion is characterized mainly by an approximately 435-day and a 365-day periodic circular motion of the axis of rotation on the surface of the Earth. The radius of the circular motion is of the order of 16 ft (5 m), but this may vary. See also Earth; Planet; Planetary physics.



--------------------------------------------------------------------------------