Our solar system is composed of eight major planets, the innermost Mercury to the outermost Neptune.

Mercury is the smallest planet. It has no atmosphere or moon. One year on Mercury is 88 days long. The planet has a dense iron core; it is the second most dense planet in our solar system. It has no atmosphere, perhaps because of its close proximity to the sun. Several times a year Mercury can be observed passing in front of the sun, in an event we call a transit, which is observable from our home planet. Mercury is one of five planets visible to the naked eye but because it is so near to the sun it is best observed in twilight or before dawn. The surface temperature of the planet can range from 840 degrees Fahrenheit (450 degrees Celsius) to night temperatures as low as minus 275 Fahrenheit.

Next out from the sun is Venus, sometimes referred to as “Earth’s Sister” for being similar in size and having a similar orbit around the sun. Once thought of as a balmy paradise, we find that it is anything but pleasant. Dense cloud cover on Venus in fact creates a runaway greenhouse effect. Rain of sulfuric acid gives the impression that the planet is what we think hell must be like. Also, Venus has no moon. The planet Venus is the third brightest object in the sky after the sun and moon.

Then we have the earth, the “Big Blue Marble” on which all life as we know it exists. We, of course, have one moon and are 92.8 million miles from the sun.

Mars the “Angry Red Planet” has a thin atmosphere and polar ice caps. Mars is named for the Roman god of war. It has two moons appropriately named Phobos (Fear) and Deimos (Terror). To the Greeks Mars was known as “Aries,” their god of war. Ancient Chinese astronomers referred to Mars as “The Red One.” Mars to our eyes is certainly red from sand on the surface covering darker material below. Early astronomers saw the shifting red sand that uncovered the dark material and thought this to be seasonal growth of vegetation. At its closest, Mars is 33.9 million miles away ... but this changes as the earth and Mars travel in their orbits.

Next out we have the planet Jupiter, a “gas giant.” Jupiter is a very different world; to the Romans Jupiter was the king of the gods and ruler of the sky. The magnetic field of Jupiter is some 14 times greater than that of the earth. The orbit of Jupiter around the sun is about 11.86 years.

Jupiter has many moons—79, in fact, the most well known the four Galilean moons discovered by Galileo in 1620—Io, Calisto, Ganymede and Europa. Galileo first observed these moons as they traveled in their orbit around the planet. Galileo thought it was another solar system altogether. Jupiter is the fastest rotating planet in our solar system—through a telescope it is seen to bulge a bit at the equator. And the clouds on Jupiter form bands at the equator due to its rapid rotation. Jupiter has another remarkable feature ... the great red spot that can at times be seen through a telescope. It is actually a storm that has raged in Jupiter’s atmosphere for some 500 years.

Next we have the ringed wonder Saturn. Through a small telescope, the majestic rings become readily visible. Saturn is the second largest planet in our system after Jupiter. Saturn is named for the Roman god of agriculture and has 53 named moons but has 62 with confirmed orbits. Saturn’s largest moon is Titan, which is bigger than the planet Mercury! Saturn was observed by Galileo in 1620. Because of the fabulous ring system, Galileo called Saturn “The Planet with Ears.” Galileo also thought the rings were two large moons on either side of the planet. Galileo’s telescope was quite primitive by today’s standards.

Uranus and Neptune, our most distant worlds, are positioned some 1.6 billion miles from earth at the closest for Uranus and some 4.553 billion kilometers or 2.829103 x 10 (9th) power for Neptune! Neither worlds are visible to the naked eye. Uranus has 27 moons we know of and Neptune has 13 known moons with one more waiting confirmation. Both Uranus and Neptune are similar in composition; both have “bulk” chemical compositions that differ from the other gas giant planets such as Saturn and Jupiter. For this reason, scientists refer to Uranus and Neptune as “Ice Giants,” as they do differ from the other gas giants. Uranus’s atmosphere is similar to Saturn and Jupiter, being mostly comprised of helium and hydrogen ... but it contains more ice formed from methine, water and ammonia.

Finally we arrive at the minor planet, Pluto. Discovered in 1930 by Clyde Tombaugh, Pluto was originally thought to be the ninth planet in our solar system. But after the discovery of similar sized objects in the Kuiper belt in 2005, Pluto was predesignated as a minor or dwarf planet. Pluto has three moons, the largest being Charon. One side of Charon always faces Pluto, much like one side of our moon always faces the earth.

And now you know the basics of the neighborhood in which we all live.

Victor C. Rogus (F.R.A.S.) Fellow of the Royal Astronomical Society, London, is a former Arcadian living in Sedona, Arizona.


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