NEPTUNE - The Eighth Planet
After Uranus had accidentally been discovered, astronomers were long perplexed that, even allowing for the perturbations of Jupiter and Saturn, Uranus's orbital motion was less predictable than that of the other planets.
The discrepancy was finally resolved in 1845 and 1846 by two astronomers, John Adams (1819-1892) in England and Urbain Leverrier (1811-1877) in France. By a brilliant application of the law of gravitation they arrived independently at the conclusion that there must be a disturbing body beyond the orbit of Uranus.
Leverrier's results were communicated to Johann Galle (1812-1910) of the Berlin Observatory, who received the information on September 23,1846. Within half an hour of searching Galle located the new planet among a group of eight stars whose positions had been charted on a recently prepared map. Recent historical research suggests that Galileo probably saw Neptune in December 1612 and January 1613, fully 233 years before Galle found it, but he did not recognize that it was a planet. We know that Neptune passed extremely close to Jupiter during that time.
Looking at Neptune through a telescope, we see a slightly flattened, bluish-green, almost featureless disk. Observers at times have reported irregular, indistinct' markings and a bright equatorial zone, although observations of the planet are very difficult to make and subject to. some degree of doubt.
Neptune's diameter is about 3.5 times that of the earth. Its mass is 17 times greater and its mean density is one-third that of the earth. Neptune is about 30 AU from the sun or 30 times the earth's distance. Thus the angular diameter of the sun is one-thirtieth of what it is from the earth. For us the sun has an angular diameter of 0.5°, or 30 minutes of arc, so that from Neptune its angular diameter is 1 minute of arc. We have tried to illustrate this very pronounced difference in Figure 6.11. At a distance of 30 AU Neptune's orbital, or sidereal, period is almost 165 years. Thus it has yet to complete one orbit of the sun since its discovery in 1846.
The larger of Neptune's two satellites, Triton, was discovered less than a month after the planet itself. It orbits Neptune in about 6 days in a direction opposite to the planet's eastward rotation. The orbital plane in which Triton moves is inclined to the equatorial plane of Neptune. Triton appears to be somewhat larger than the moon, but its mass is only about 80 percent that of the moon, producing a lower mean density.
The smaller satellite, Nereid, takes nearly a year to swing around Neptune in a highly elongated ellipse, varying between about 1.5 million and almost 10 million kilometers from the planet. Nereid's orbital plane is also inclined to Neptune's equatorial plane. Neptune's satellites are distinctly different from those of Uranus. Their inclined orbital planes and Nereid's elongated orbit continue to prompt speculations on their origin.
