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After the discovery of Uranus, it was noticed that its orbit was
not as it should be in accordance with Newton's laws. It was
therefore predicted that another more distant planet must be perturbing Uranus' orbit.
Neptune was first
observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently
predicted by
Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn
and Uranus.
An international dispute arose between the English and French (though not, apparently
between Adams
and Le Verrier personally) over priority and the right to name the new planet; they are
now jointly credited
with Neptune's discovery. Subsequent observations have shown that the orbits calculated by
Adams and
Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the
planet taken place a few years earlier or
later it would not have been found anywhere near the predicted location.
jupiter2
BA
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Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and
Venus; at some times Mars is also brighter). It
has been known since prehistoric times. Galileo's discovery, in 1610, of Jupiter's four
large moons Io, Europa, Ganymede and
Callisto (now known as the Galilean moons) was the first discovery of a center of motion
not apparently centered on the
Earth. It was a major point in favor of Copernicus's heliocentric theory of the motions of
the planets; Galileo's outspoken
support of the Copernican theory got him arrested by the Inquisition. He was forced to
recant his beliefs and was imprisone
Saturn has been known since prehistoric times. Galileo was the first to observe
it with a telescope in 1610; he noted its odd
appearance but was confused by it. Early observations of Saturn were complicated by the
fact that the Earth passes through the
plane of Saturn's rings every few years as Saturn moves in its orbit. A low resolution
image of Saturn therefore changes
drastically. It was not until 1659 that Christiaan Huygens correctly inferred the geometry
of the rings. Saturn's rings remained
unique in the known solar system until 1977 when very faint rings were discovered around
Uranus and shortly thereafter around
Jupiter and Neptune).
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Earth is the
only planet whose English name does not derive from Greek/Roman mythology. The name
derives from Old
English and Germanic. There are, of course, hundreds of other names for the planet in
other languages. In Roman Mythology,
the goddess of the Earth was Tellus - the fertile soil (Greek: Gaia, terra mater - Mother
Earth).
Venus has been
known since prehistoric times. It is the brightest object in the sky except for the Sun
and the Moon. Like
Mercury, it was popularly thought to be two separate bodies: Eosphorus as the morning star
and Hesperus as the evening
star, but the Greek astronomers knew better.
Mercurymercur
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Mercury has
been known since at least the time of the Sumerians (3rd millennium BC). It was given two
names by the
Greeks: Apollo for its apparition as a morning star and Hermes as an evening star. Greek
astronomers knew, however, that the
two names referred to the same body. Heraclitus even believed that Mercury and Venus orbit
the Sun, not the Earth.
The outer layers of the Sun exhibit differential rotation: at the
equator the surface rotates once every 25.4 days; near the
poles it's as much as 36 days. This odd behavior is due to the fact that the Sun is not a
solid body like the Earth. Similar effects
are seen in the gas planets. The differential rotation extends considerably down into the
interior of the Sun but core of the Sun
rotates as a solid body.
Mars (Greek:
Ares) is the god of War. The planet probably got this name due to its red color; Mars is
sometimes referred
to as the Red Planet. (An interesting side note: the Roman god Mars was a god of
agriculture before becoming associated with
the Greek Ares; those in favor of colonizing and terraforming Mars may prefer this
symbolism.) The name of the month March
derives from Mars.
Neptune's composition
is probably similar to Uranus': various "ices" and rock with about 15% hydrogen
and a little helium.
Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering
but rather to be more or less uniform in
composition. But there is most likely a small core (about the mass of the Earth) of rocky
material. Its atmosphere is mostly
hydrogen and helium with a small amount of methane.
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