fr en es pt
Contact the author rss astronoo
 Automatic translation  Automatic translation Category: matter and particles
Updated May 16, 2015
 Rotation of the Earth and measurement of time

Image: The rotational speed of the Earth at the equator is 1 674.364 km / h. The number of rotations of the Earth itself is about 365,2425 per year, 365.2425 days sidereal (rotation relative to the celestial reference system). However, all the movements of the earth are irregular and vary gradually over time. Moon with tidal effects slows the rotation of the Earth itself. Day length on Earth slowly increasing by about 2 milliseconds per century. Thus 300 million years ago, years lasted 400 days, meaning that days were 22 hours long. When the Earth formed, 4.5 billion years ago, years lasted 1434 days meaning that days were  6.1 hours long.

 Accuracy of the second
  Atomic clock fountain of cesium atoms
A gnomon (Greek literally "one that knows or examines") is an astronomical instrument used to determine the height of the sun. In prehistoric times, the gnomon is a simple stick planted in land that can indicate the solar time. At that time, the man lives crop and livestock, it does not feel the need to have a greater time precision. During antiquity, the gnomon and sundial enough to measure, on a clear day, hour day. Today the gnomon is a simple vertical rod placed on a table, the length of the shadow projected at a given latitude, inquires on the height of the sun. Projected on the curves of a sundial, the extremity of the shade determines the time throughout the year.

1997 © − Astronomy, Astrophysics, Evolution and Earth science.

Electromagnetic spectrum
of the spectrum...
Leap second
Leap second...
Cosmological and physical constants