To the Greeks he was Poseidon; to the Romans, Neptune, and his power was second only to Jupiter, king of the gods. Jupiter and Neptune were brothers, sons of Saturn, and while Jupiter held dominion over the earth and the sky, Neptune ruled the oceans, and resided in the cold, dark depths of the deep.

Like its mythological namesake, the planet Neptune also resides in the cold dark depths - of the solar system. At the astounding distance of 2.8 billion miles from the Sun, Neptune is the last and farthest of the gas giants found in the outer solar system. It is so far away that its surface temperature never gets above -364 ° F (-220 ° C), and the reflected sunlight bouncing off its surface - which travels at a speed of 670 million mph - takes over 4 hours to reach Earth. This means that the image of Neptune we see is over 4 hours old. It is so far away that even the extraordinary resolving power of the Hubble Space Telescope is limited in the detail it can capture.

Fortunately, NASA's Voyager 2 spacecraft visited Neptune in August, 1989, taking 12 years to make the journey from Earth. It is the only spacecraft ever to visit this distant world, the farthest object ever surveyed up close by human technology. At its closest, Voyager 2 came within 3,000 miles of Neptune. The photo below was taken as Voyager 2 approached Neptune from a distance of 9.2 million miles (14.8 million kms).

Neptune is composed largely of hydrogen and helium, as all the gas giants are, but like Uranus, it also has enough methane to give it a distinctly blue appearance.

Neptune has 13 moons, one of which, Triton, with a diameter of 1,680 miles (2,700 kms), is almost as large as Earth's Moon. Pictured below in a photo by Voyager 2, Triton is the coldest measured object in the solar system, with a surface temperature of -391 ° F (-235 ° C). It is also the only large moon in the solar system that orbits its host planet in the opposite direction to the planet's rotation, referred to as a retrograde orbit.

Voyager 2 discovered that Neptune was circled by a set of rings just as the other three gas giants are. By masking the bright face of the planet, it was able to capture the faint rings, shown below. Click on photo to enlarge.

Neptune was the first planet discovered by calculation, rather than observation. In 1846, scientists were perplexed because Uranus didn't follow the orbit it was mathematically predicted to. It was proposed that its deviations might be caused by the gravitational pull of another planetary body, outside its orbit, and the location of that body was carefully calculated. When telescopes were pointed in that direction - there was Neptune.

Neptune is the smallest of the gas giants, just slightly smaller than Uranus. Still, with a diameter of 31,410 miles (50,538 kms), 60 Earths could fit inside it.

One day on Neptune lasts 16.05 hours, and one year lasts 164.8 Earth years. Since the International Astronomical Union (I.A.U.)reclassified tiny Pluto as a Dwarf Planet in August, 2006, Neptune now has the official distinction of being the last and farthest planet in the solar system.

Neptune is too far away to see with your naked eye. With binoculars, if you know right where to look, you might just be able to make out a very faint bluish starlike object. Even with your telescope, it still looks pretty much like a star. If you view it regularly, you will see it slowly change position against the background of immovable stars.

The finder chart below will help you locate Neptune. The chart shows that as Earth makes its way around the Sun, Neptune appears to change direction every six months or so, zig-zagging back and forth across the sky. Because of its great distance, however, it does not travel very far or very fast, so once you find it for the first time, it is relatively easy to find it again.

Finder Chart for Neptune, November 2009 - December 2010

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