Sixth in order of distance from the sun (counting the asteroids as representing a single body) is the greatest of all the planets, Jupiter. His average distance from the sun is 483,000,000 miles, but the eccentricity of his orbit causes him to approach within 472,500,000 miles at perihelion, and to recede to 493,500,000 miles at aphelion. When in opposition, Jupiter's mean distance from the earth is 390,000,000 miles. This gigantic planet has a mean diameter of 87,380 miles, but is so flattened at the poles and bulged round the equator that the polar diameter is only 84,570 miles, while the equatorial diameter is 90,190 miles, a difference of 5680 miles. This peculiar form is doubtless due to the planet's swift rotation. The axis, like that of Venus, is nearly perpendicular to the plane of the orbit. He makes a complete turn on his axis in a mean period of 9 hours, 55 minutes. The reason for saying “a mean period” will appear in a moment. Jupiter's year is equal to 11.86 of our years, but it comes into opposition to the sun, as seen from the earth, once in every 399 days. The volume of Jupiter is about 1300 times that of the earth, _i.e._ it would take 1300 earths rolled into one to equal Jupiter in size. But its mean density is slightly less than one quarter of the earth's, so that its mass is only 316 times greater than the earth's. The force of gravity on its surface is 2.64 times the earth's. A body weighing 100 pounds on the earth would weigh 264 pounds on Jupiter. It will be observed that Jupiter's mean density is very nearly the same as that of the sun, and we conclude that it cannot be a solid, rigid globe like the earth. This conclusion is made certain by the fact that its period of rotation on its axis is variable, another resemblance to the sun. The equatorial parts go round in a shorter period than parts situated some distance north or south of the equator. It may be supposed that there is a solid nucleus within, but if so, no direct evidence of its existence has been found.