Friday, 2 September 2011
The Tides and the Moon
The tides on the Earth are mostly generated by the gradient in intensity of the Moon's gravitational pull from one side of the Earth to the other, the tidal forces. This forms two tidal bulges on the Earth, which are most clearly seen in elevated sea level as ocean tides. Since the Earth spins about 27 times faster than the Moon moves around it, the bulges are dragged along with the Earth's surface faster than the Moon moves, rotating around the Earth once a day as it spins on its axis. The ocean tides are magnified by other effects: frictional coupling of water to Earth's rotation through the ocean floors, the inertia of water's movement, ocean basins that get shallower near land, and oscillations between different ocean basins. The gravitational attraction of the Sun on the Earth's oceans is almost half that of the Moon, and their gravitational interplay is responsible for spring and neap tides.
Over one lunar month more than half of the Moon's surface can be seen from the surface of the Earth.
The libration of the Moon over a single lunar month.
Gravitational coupling between the Moon and the bulge nearest the Moon acts as a torque on the Earth's rotation, draining angular momentum and rotational kinetic energy from the Earth's spin. In turn, angular momentum is added to the Moon's orbit, accelerating it, which lifts the Moon into a higher orbit with a longer period. As a result, the distance between the Earth and Moon is increasing, and the Earth's spin slowing down. Measurements from lunar ranging experiments with laser reflectors left during the Apollo missions have found that the Moon's distance to the Earth increases by 38 mm per year (though this is only 0.10 ppb/year of the radius of the Moon's orbit). Atomic clocks also show that the Earth's day lengthens by about 15 microseconds every year, slowly increasing the rate at which UTC is adjusted by leap seconds. Left to run its course, this tidal drag would continue until the spin of the Earth and the orbital period of the Moon matched. However, the Sun will become a red giant long before that, engulfing the Earth.
The lunar surface also experiences tides of amplitude ~10 cm over 27 days, with two components: a fixed one due to the Earth, as they are in synchronous rotation, and a varying component from the Sun. The Earth-induced component arises from libration, a result of the Moon's orbital eccentricity; if the Moon's orbit were perfectly circular, there would only be solar tides. Libration also changes the angle from which the Moon is seen, allowing about 59% of its surface to be seen from the Earth (but only half at any instant). The cumulative effects of stress built up by these tidal forces produces moonquakes. Moonquakes are much less common and weaker than earthquakes, although they can last for up to an hour – a significantly longer time than terrestrial earthquakes – because of the absence of water to damp out the seismic vibrations. The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972.
Poseidon (Greek: Ποσειδῶν) was the god of the sea, and, as "Earth-Shaker," of earthquakes in Greek mythology. The name of the sea-god Nethuns in Etruscan was adopted in Latin for Neptune in Roman mythology: both were sea gods analogous to Poseidon. Linear B tablets show that Poseidon was venerated at Pylos and Thebes in pre-Olympian Bronze Age Greece, but he was integrated into the Olympian gods as the brother of Zeus and Hades. Poseidon has many children. There is a Homeric hymn to Poseidon, who was the protector of many Hellenic cities, although he lost the contest for Athens to Athena.
[http://en.wikipedia.org/wiki/Neptune_%28mythology%29]
Neptune (Latin: Neptūnus) was the god of water and the sea in Roman mythology and religion. He is analogous with, but not identical to, the Greek god Poseidon. In the Greek-influenced tradition, Neptune was the brother of Jupiter and Pluto, each of them presiding over one of the three realms of the universe, Heaven, Earth and the Netherworld. Depictions of Neptune in Roman mosaics, especially those of North Africa, are influenced by Hellenistic conventions.
Unlike the Greek Oceanus, titan of the world-ocean, Neptune was associated as well with fresh water. Georges Dumézil suggested that for Latins, who were not a seafaring people, the primary identification of Neptune was with freshwater springs. Like Poseidon, Neptune was worshipped by the Romans also as a god of horses, under the name Neptunus Equester, a patron of horse-racing.
[http://en.wikipedia.org/wiki/Poseidon]
Ganesha
Ganesha (Sanskrit: गणेश; IAST: Gaṇeśa; About this sound listen (help·info)), also spelled Ganesa or Ganesh, also known as Ganapati (Sanskrit: गणपति; IAST: gaṇapati), Vinayaka (Sanskrit: विनायक; IAST: Vināyaka), and Pillaiyar (Tamil: பிள்ளையார்), is one of the deities best-known and most widely worshipped in the Hindu pantheon. His image is found throughout India and Nepal. Hindu sects worship him regardless of affiliations. Devotion to Ganesha is widely diffused and extends to Jains, Buddhists, and beyond India.
Although he is known by many other attributes, Ganesha's elephant head makes him particularly easy to identify. Ganesha is widely revered as the Remover of Obstacles and more generally as Lord of Beginnings and Lord of Obstacles (Vighnesha (Sanskrit: विघ्नेश; IAST: Vighneśa), Vighneshvara (Sanskrit: विघ्नेश्वर; IAST: Vighneśvara), patron of arts and sciences, and the deva of intellect and wisdom. He is honoured at the beginning of rituals and ceremonies and invoked as Patron of Letters during writing sessions. Several texts relate mythological anecdotes associated with his birth and exploits and explain his distinct iconography.
Ganesha emerged a distinct deity in clearly recognizable form in the 4th and 5th centuries CE, during the Gupta Period, although he inherited traits from Vedic and pre-Vedic precursors. His popularity rose quickly, and he was formally included among the five primary deities of Smartism (a Hindu denomination) in the 9th century. A sect of devotees called the Ganapatya, (Sanskrit: गाणपत्य; IAST: gāṇapatya), who identified Ganesha as the supreme deity, arose during this period. The principal scriptures dedicated to Ganesha are the Ganesha Purana, the Mudgala Purana, and the Ganapati Atharvashirsa.
[http://en.wikipedia.org/wiki/Ganesha]
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