Introduction to Neptune
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 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.
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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