A photograph of the planet Pluto is not included because no space probe has visited it, as yet.
Note: The image size is approximately the same in all photos. The image size does not indicate relative size. Orbital size is given in Astronomical Units (A.U.). One A. U. is the distance from the Earth to the Sun.
Mercury is the planet that is nearest the Sun. Its mean distance from the Sun is 0.39 A. U. Its orbit has an eccentricity of 0.20. (Note: A circle has an eccentricity of 0.0. The orbits of all the planets are ellipses, but they are all very nearly circular.) Its orbit is inclined to the plane of the ecliptic by 7 degrees (The plane of the ecliptic is the plane of the Earth's orbit. All planets orbit in planes that are nearly the same as that of the Earth.) . Mercury has a diameter of about 3100 miles and orbits the Sun in about 88 days.
Our Earth is 1.00 A. U. from the Sun, by definition. This distance is about 93,000,000 miles. The orbit has an eccentricity of 0.017 and is inclined to the plane of the ecliptic by 0.0 degrees (again, this is a definition.) The diameter is about 7900 miles and we orbit the Sun in 365+ days.
Mars (the red planet) orbits the Sun at a mean distance of 1.524 A. U. The orbit's eccentricity is 0.093 at an inclination of 1.9 degrees. Its diameter is 4200 miles and it orbits the Sun in 687 days.
Jupiter is the largest planet. (Four of the planets, Jupiter, Saturn, Uranus, and Neptune, are called giant planets.) Jupiter has a large number of satellites (Moons.) The largest of these moons are nearly as large as the planet Mercury. It orbits at a mean distance of 5.203 A. U. The orbit has an eccentricity of 0.048 at an inclination of 1.3 degrees. Its diameter is 87,000 miles and it takes 11.86 years to orbit the Sun.
Galileo was the first to observe Saturn through a telescope. At first, he thought it was a triple planet. Later he discovered that it has an extensive series of rings. This made it a very unusual sight. Modern obervations has shown that other planets also have rings but they are not nearly as spectacular. Saturn also has a large number of satellites. Saturn orbits the Sun at a mean distance of 9.54 A. U. Its orbit has an eccentricity of 0.056 with an inclination of 2.5 degrees. Its diameter is 72000 miles and it orbits the Sun in 29.46 years.
All of the previously described planets were known to the ancients because they can be seen without the aid of a telescope. Uranus was discovered in 1781 by William Herschel. It orbits the Sun at a distance of 19.19 A. U. The orbit has an eccentricity of 0.047 and an inclination of 0.8 degrees. The diameter is 31,000 miles and it orbits the Sun in 84.0 years.
By 1845, the observed position of Uranus differed enough from the predicted position to suggest that another planet was causing the orbit of Uranus to be perturbed. Two mathematicians, Adams (England) and Leverrier (France) set about to calculate the orbit and direction of this new planet. It was found in 1846 by Galle (German) very near the position predicted by Leverrier. It orbits at a distance of 30.07 A. U. It has an eccentricity of 0.009 and an inclination of 1.8 degrees. Neptune's diameter is 33,000 miles and it orbits the Sun in 164.8 years.
The image shown is my attempt to depict Pluto and its Satellite, Charon, according to the best estimates now available. The relative sizes are approximately correct, but it is not possible to show their separation on the same scale. Since the plane of their orbit is such that they eclipse each other as seen from Earth, you can visualize this drawing as being that of Pluto about to move in front of Charon.
Like the orbit of Uranus, the orbit of Neptune also deviates slightly from the predicted form. This led astronomers to believe that there was yet another planet that causes this deviation. Percival Lowell made extensive computations in an attempt to locate this "trans-neptunian" planet. No planet could be located in either of the two positions that Lowell had predicted. A systematic search was planned and execution of this plan was begun in 1929. Clyde W. Tombaugh was selected to conduct this search. Since all of the other planets orbit near the plane of the ecliptic, it was concluded that the search only needed to be made in a small region either side of this plane. The search was made in the following manner. Every night during the dark of the moon, photographs were made in the direction opposite to that of the Sun. Each night, the photographic plates were developed and briefly scanned for quality. If they were satisfactory, they were stored for later examination. (If not satisfactory, new photographs were taken the next night.) While the moon made the sky too bright for photography, Tombaugh examined the plates using a blink microscope. A blink microscope allows an operator to look at two plates taken on successive nights. The operator gets the two images super-imposed and then looks at small regions of them alternately. Since the background stars do not change position, any object that moves must have changed position between the two exposures. On January 21, 1930, Tombaugh discovered an object that moved between successive exposures by the right amount to be the sought after object. In order to cover the entire circumference of the sky, a complete search would require a year. This search took almost that much time. Tombaugh told me that if he had been able to start the search a month earlier, he would have found Pluto almost immediately. Fortunately, Harvard Observatory has had a program of making photographs of the sky for numerous years. With the aid of these photographs and knowing the approximate position of Pluto, it was found on these photographs dating back over 10 years before the time of its discovery. This gave astronomers enough data to calculate its orbit almost immediately. Unfortunately, the mass of the Pluto system is not sufficient to account for the perturbations of Neptune.
Pluto orbits at a mean distance of 39.44 A. U. Its orbit is so eccentric, 0.249, that its orbit actually crosses the orbit of Neptune. It also has a high inclination, 16. degrees. The diameter of Pluto is about 1416 miles and that of Charon 721 miles. Pluto has a reddish color and Charon is gray. They orbit about their common center of mass in 6.39 days and are separated by 12,000 miles. Their orbital period around the Sun is 248.4 years.
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