Rotation of the Earth

Earth rotates slower and slower

The Earth rotates itself around an axis, the velocity at the equator is 1674.364 km/h, this axis is oriented toward the north celestial pole.
For a long time the Earth's rotation was considered the most accurate measure of time passing, but its speed varies over time. The rotational speed of the Earth is not regular, small crises or stuttering time occur quite frequently. Seconds disappear or rather minutes 61 seconds appear. Since the 1960s, 34 seconds were missing because of a slowdown imperceptible but steady rotation of our planet around its axis. Many phenomena tend to slow the speed of rotation. Since the metal core to the atmosphere, each layer rotates unevenly, imposing its rhythm to the other. Earth's mantle, it undergoes gravitational forces of the Moon, the effects of tide and frequently held for billions of years our rotation. This is the entire Earth who suffer the vagaries of the gravitational forces of the solar system, including planets and Sun. Continental drift, ocean currents, weather patterns, earthquakes can change the period of a few microseconds of rotation of the Earth, either by accelerating or slowing in the.
Our Earth is "tossed" the four-dimensional fabric of space-time, as on the surface of a giant trampoline amended by the gravitation of the planets, the Sun and stars. This permanent deformation or curvature of space-time, creates the chaotic waves of gravity. Moreover, the definition of the second, which dates from 1967, includes 9,192,631,770 oscillations of cesium-133 atom, but the second does not divide perfectly the day real, astronomical. It remains a small error that is in addition to all other fluctuations. All of these unpredictable events require us to continuously monitor the rotation period of the Earth and the day length to resynchronize the time between all measuring instruments on the ground and satellite. The importance of this resynchronization is crucial for many systems that are based on the definition of the second.

Many scientific projects and military use this second.
In particular the GPS satellites that must be synchronized since otherwise the distance of their orbit, the miscalculations of positions would not be manageable.
That is why, we regularly catch up with this astronomical time lag.

Image: Rotation of the Earth. All movements of the Earth are irregular and varies continuously over time, many local and cosmic events alter the rate of rotation of the Earth. The speed at the equator is a 674.364 km/h. The number of rotations of the Earth itself is about 365.2425 per year, or sidereal 366.2425 days (rotation relative to the celestial reference system). Since the Earth is not rigorously an undeformable solid mass, the concept of angular speed of rotation must be carefully defined then measured. This is the IERS (International Earth Rotation Service), which places all data online.

How is measured the rotation of the Earth?

The movements of the Earth are constantly monitored and requires the coordinated use of several techniques. To measure the rotation of the Earth, the technicians use the VLBI "Very Long Baseline Interferometry", an international network of radio telescopes and satellite networks such as GPS satellites. For GPS to work correctly, the clocks of the 31 GPS satellites, must be perfectly synchronized with time on Earth. To 18 000 km altitude, if the clocks of the satellites are not synchronized, the values ​​given by the GPS derived from 10 to 12 km per day. A time correction is needed to adapt the GPS to the effects of gravity. Radio telescopes, terrestrial, the VLBI, separated by a few hundred to several thousand kilometers, are a distant object like a quasar and measures the difference in arrival time of radio signals.
The atomic clocks synchronized, to calculate the time difference it takes for the radio wave to reach each of the telescopes. The precision of the observations increases with the distance between two stations and reaches a few tens of microseconds.
This is enough to highlight the daily disturbances. The observation sessions usually involve between 6 and 21 bases and have several tens of distributed sources in the sky, over a period of 24 hours.
A program of daily sessions to determine the fast variations of universal time. VLBI is the only technique capable of measuring all components of the Earth orientation in a manner daily and accurate.
VLBI provides the primary reference for determining the Universal Time of the precession and nutation.

Satellite technology (GPS, SLR, DORIS) allow interpolation daily and short term prediction of universal time between the reference values ​​provided by the VLBI.
They also provide daily values ​​of polar motion. Another technique as the technique of Laser Moon (LLR) is used to determine the obliquity of the ecliptic, and the orientation of the solar system in the extragalactic reference.
The DORIS (Doppler Orbit and Radiopositioning Integrated on Satellite) is used to precisely locate points on the surface of the globe thanks to radar altimeters on board satellites such as SPOT and TOPEX-POSEIDON whose precision is centimeter.
DORIS is particularly suitable for monitoring of natural phenomena (volcanic and tectonic activity, glacier movement, floe drift).
One of the most interesting applications for the measurement of slow movement of tectonic plates using the tags of the network.

NB: The Earth moves like a spinning top around its orbit. The extremity of the axis slowly describes a circle in a horizontal plane, toward the north celestial pole, the precession movement.
A complete cycle of precession movement lasts 25,765 years called great Platonic year.
There is also the attraction of the Moon and the Sun disrupts the precession slightly by adding small oscillations whose period is 18.6 years.
This effect is called nutation.

Image: The Earth moves like a spinning top around its orbit. The extremity of the axis slowly describes a circle in a horizontal plane, toward the north celestial pole, the precession movement.
All movements of the Earth are irregular and vary over time, micro variations, due to the gravitational forces of solar system objects, occur continuously, even local events such as earthquakes, have an impact on its rotation.