support fluid behavior. Earth tides Earth tides or terrestrial tides affect the entire Earth's mass, which acts similarly to a liquid gyroscope with a very thin crust. The Earth's crust shifts (in/out, east/west, north/south) in response to lunar and solar gravitation, ocean tides, and atmospheric loading. While negligible for most human activities, terrestrial tides' semi-diurnal amplitude can reach about at the Equator— due to the Sun—which is important in GPS calibration and VLBI measurements. Precise astronomical angular measurements require knowledge of the Earth's rotation rate and polar motion, both of which are influenced by Earth tides. The semi-diurnal M2 Earth tides are nearly in phase with the Moon with a lag of about two hours. Galactic tides Galactic tides are the tidal forces exerted by galaxies on stars within them and satellite galaxies orbiting them. The galactic tide's effects on the Solar System's Oort cloud are believed to cause 90 percent of long-period comets. Misnomers Tsunamis, the large waves that occur after earthquakes, are sometimes called tidal waves, but this name is given by their resemblance to the tide, rather than any causal link to the tide. Other phenomena unrelated to tides but using the word tide are rip tide, storm tide, hurricane tide, and black or red tides. Many of these usages are historic and refer to the earlier meaning of tide as "a portion of time, a season" and "a stream, current or flood". See also Notes References Further reading 150 Years of Tides on the Western Coast: The Longest Series of Tidal Observations in the Americas NOAA (2004). Eugene I. Butikov: A dynamical picture of the ocean tides Tides and centrifugal force: Why the centrifugal force does not explain the tide's opposite lobe (with nice animations). O. Toledano et al. (2008): Tides in asynchronous binary systems Gaylord Johnson "How Moon and Sun Generate the Tides" Popular Science, April 1934 External links NOAA Tides and Currents information and data History of tide prediction Department of Oceanography, Texas A&M University UK Admiralty Easytide UK, South Atlantic, British Overseas Territories and Gibraltar tide times from the UK National Tidal and Sea Level Facility Tide Predictions for Australia, South Pacific & Antarctica Tide and Current Predictor, for stations around the world Geodesy Navigation Articles containing video clips