Neptune Help

By — McGraw-Hill Professional
Updated on Sep 16, 2011

Introduction to Neptune

Neptune, named after the mythical god of the sea, is more than half again as far from the Sun as is Uranus: 4,504 million kilometers (2,799 million miles). This is 30.06 AU. Neptune receives only about 1/900 as much sunlight per unit area as does the Earth. If you’re into electronics, acoustics, or physics, you might get some idea of the difference by noting that a ratio of 1:900 is equivalent to approximately 30 decibels (dB). If light were sound and the Sun shining on the Earth were like a loud vacuum cleaner, then the solar illumination on Neptune would be like a small fan running at low speed.

Finding And Observing Neptune

As is the case with the other outer planets, we see Neptune best when it is at or near opposition (Fig. 7-8), although its absolute brightness does not vary much as the Earth revolves around the Sun. Neptune cannot be seen with the unaided eye; powerful binoculars or, better yet, a good telescope is necessary to observe it. You need to know exactly where to look; the Weather Underground or Celestron CD-ROM maps can be used to locate it. Even when viewed through a large amateur telescope, Neptune will only look like a blue star.

The Outer Planets Jupiter The Year And The Day

Figure 7-8. The orbits of Earth and Neptune, to scale. As with the other outer planets, the best viewing is at opposition.

The Year And The Day

Neptune is tilted on its axis by 29½ degrees. This is to say, the equatorial plane of Neptune intersects its orbital plane at an angle of 29½ degrees. This compares with 23½ degrees for the Earth. The seasonal variations in the Sun’s path across the sky on Neptune would be somewhat familiar to Earthlings, except for one fact: No human would ever live long enough to see all four Neptunian seasons go by. As a matter of fact, no human likely would be able to survive long enough beneath the sapphire-blue haze and snow-white clouds of Neptune to eat a decent supper, let alone carry out a lifetime’s research.

Neptune takes 165 Earth years to make one complete journey around the Sun. Its orbit is almost a perfect circle, so any seasonal effects on Neptune’s climate must be caused entirely by the tilt of its axis and not by variations in the amount of sunlight it receives.

Composition, Atmosphere, And Weather

In composition, Neptune is thought to be similar to Uranus, but it is more dense. Neptune is about 49,500 kilometers (30,800 miles) in diameter; this is a little less than four times the diameter of the Earth (Fig. 7-9). Neptune generates more internal heat than Uranus; in this respect it more nearly resembles Jupiter and Saturn. Neptune is more blue in color than Uranus, and astronomers are not quite certain what is responsible for this vivid sapphire hue.

The Outer Planets Jupiter Composition, Atmosphere, And Weather

Figure 7-9. The equatorial diameter of Neptune is about the same as that of Uranus, four times the diameter of Earth.

When Voyager passed by Neptune in 1989, it proved to have a more interesting atmosphere, at least visually, than its aquamarine cousin. There were dark spots and bright clouds; one of the clouds raced around Neptune independently of other weather phenomena and was named Scooter for this reason. There was an oval-shaped spot of deep indigo, similar in shape to but about half the diameter of Jupiter’s Great Red Spot. This system had winds that blow faster than those on any other planet in the Solar System, approximately three times the speed of the wind in a maxitornado on Earth. The Great Dark Spot , which was as large in diameter as the Earth, disappeared after a few years. Apparently the self-sustaining forces that keep storms alive on Jupiter are not as effective on Neptune.

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