The nine planets

Express tour of the solar system

All eight planets can be seen with a small telescope; or binoculars. And large observatories continue to provide much useful information. But the possibility of getting up close with interplanetary spacecraft has revolutionized planetary science. Very little of this site would have been possible without the space program. http://en.wikipedia.org/wiki/Planet

The IAU has changed the definition of "planet" so that Pluto no longer qualifies. There are now officially only eight planets in our solar system. Of course this change in terminology does not affect what's actually out there. In the end, it's not very important how we classify the various objects in our solar system. What is important is to learn about their physical nature and their histories. http://nineplanets.org/

The planets

Mercury
Venus
Earth
Moon
Mars
Jupiter
Saturn
Uranus
Neptune

The worlds chosen here are my personal favorites (aside from Earth, of course!) but have no other special significance.

Earth

Earth is the third planet from the Sun and the fifth largest: 

        orbit:        149,600,000 km (1.00 AU) from Sun
        diameter:  12,756.3 km
        mass:        5.972e24 kg

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). 

It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth is just another planet. 

Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful. 

Moon

The Moon is the only natural satellite of Earth: 

        orbit:        384,400 km from Earth
        diameter:  3476 km
        mass:        7.35e22 kg

Called Luna by the Romans, Selene and Artemis by the Greeks, and many other names in other mythologies. 

The Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time. 

The Moon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only extraterrestrial body to have been visited by humans. The first landing was on July 20, 1969 (do you remember where you were?); the last was in December 1972. The Moon is also the only body from which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively mapped by the little spacecraft Clementine and again in 1999 by Lunar Prospector. 

Mars

Mars is the fourth planet from the Sun and the seventh largest: 

      orbit:        227,940,000 km (1.52 AU) from Sun
      diameter:  6,794 km
      mass:        6.4219e23 kg

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. 

Mars has been known since prehistoric times. Of course, it has been extensively studied with ground-based observatories. But even very large telescopes find Mars a difficult target, it's just too small. It is still a favorite of science fiction writers as the most favorable place in the Solar System (other than Earth!) for human habitation. But the famous "canals" "seen" by Lowell and others were, unfortunately, just as imaginary as Barsoomian princesses. 

Saturn

Saturn is the sixth planet from the Sun and the second largest: 

        orbit:        1,429,400,000 km (9.54 AU) from Sun
        diameter:  120,536 km (equatorial)
        mass:        5.68e26 kg

In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and the father of Zeus (Jupiter). Saturn is the root of the English word "Saturday".

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). 

Saturn was first visited by NASA's Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini (a joint NASA / ESA project) arrived on July 1, 2004 and will orbit Saturn for at least four years. 

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