An unseen force called gravitation holds the planets in their pathways, always just so near and just so far from the sun. This is the same force that holds the stars in a Galaxy and keeps the objects on the earth from sliding off into space. Since the attractive power of the sun is tremendous, the planets must travel around their pathways at a tremendous rate in order to develop enough centrifugal force to offset the gravitational pull, and the closer the planet the faster it must fly. [Figure 141: The orbits of the four outer planets, Neptune, Uranus, Saturn and Jupiter, and the orbits of Mars and the Earth, drawn to scale. (Within the orbit of the Earth would appear the orbits of Venus and Mercury, too small to be shown in this drawing.)] Mercury, for instance, travels at a rate of 35 miles a second while Neptune, seventy-five times as far off, has an orbital velocity of only 3.4 miles a second. The following table shows the distances of each planet from the sun and the time it takes it to complete its journey around its orbit, beginning with Mercury, the nearest planet to the sun, and continuing outward. _Distances_ _Period of_ _from the Sun_ _Revolution_ Mercury 35,750,000 miles 88 days Venus 66,750,000 " 225 " Earth 92,500,000 " 365 " Mars 141,000,000 " 687 " Jupiter 457,000,000 " 12 years Saturn 881,000,000 " 29½ " Uranus 1,771,000,000 " 84 " Neptune 2,775,000,000 " 165 " Mercury, Venus, Earth and Mars are sometimes called the inner planets because they form a group nearest the sun but more frequently are called the Terrestrial Planets because they all have solid crusts like the earth. Jupiter, Saturn, Uranus and Neptune, forming a group called the outer or major planets, are more or less in a gaseous condition and are very huge in comparison to the first four. The orbit of Neptune, the farthermost planet, encloses the orbits of all the other planets, and is so very large that it requires 165 years for Neptune to travel around it. This is quite a contrast to the time consumed by little Mercury who flies so close to the sun that he travels around it in 88 days. This, of course, means the length of the year on each of these planets.