Neptune
is one of the two planets that cannot be seen without a telescope. Neptune is about 30 times as far from the sun as is Earth. Pluto is the only planet farther from the sun than Neptune. But every 248 years Pluto moves inside Neptune's orbit for about a 20-year period, during which it is closer to the sun than Neptune. Pluto crossed Neptune's orbit on Jan. 23, 1979, and will remain within it until March 15, 1999.
Neptune's diameter is about 30,500 miles (49,100 kilometers), or almost 4 times that of Earth. The planet is about 17 times as massive (heavy) as Earth, but is not so dense as Earth (see MASS; DENSITY). It has eight satellites (moons). Astronomers have also detected several rings around Neptune.
Neptune travels around the sun in an elliptical (oval-shaped) orbit. Its mean distance from the sun is about 2,798,800,000 miles (4,504,300,000 kilometers). Neptune goes around the sun once about every 165 earth-years, compared to once a year for Earth. As Neptune orbits the sun, it spins on its axis, an imaginary line through its center. Neptune's axis is not perpendicular (at an angle of 90°) to the planet's path around the sun. The axis tilts about 30° from the perpendicular position (see PLANET [illustration: The axes of the planets]). Neptune spins around once in about 16 hours and 7 minutes. |
