Neptune- The farthest planet from sun

     The fourth-largest planet by diameter and the third-largest by mass, Neptune got its name from the Greek god- Poseidon, the god of the Sea.  Neptune is 17 times the mass of Earth and is somewhat more massive than its near-twin Uranus, which is 15 times the mass of Earth but not as dense.

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Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching −218 °C.









   
     Neptune was the first planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led to deduce that its orbit was subject to gravitational perturbation by an unknown planet behind it. Neptune was subsequently observed on 23 September 1846.

     More than four centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights, Neptune's motion would be obvious to him and Galileo would have discovered it. But, alas, cloudy skies prevented observations on those few critical days.

   

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The axial tilt of Neptune is 28.32°, which is similar to the tilts of Earth (23°) and Mars (25°). As a result, this planet experiences similar seasonal changes. The long orbital period of Neptune means that the seasons last for forty Earth years.




The image here shows the size of Neptune in comparison to Earth.





      Neptune has 13 known moons. The largest by far, comprising more mass in orbit around Neptune and the only one massive enough to be spheroidal, is Triton, Unlike all other large planetary moons in the Solar System, Triton has a retrograde orbit, which indicates that it was captured rather than forming in place; it was probably once a dwarf planet in the Kuiper belt. It is close enough to Neptune to be locked into a synchronous rotation, and it is slowly spiraling inward because of tidal acceleration.

            



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   Dark spot on Neptune similar in appearance to Jupiter's Great Red Spot, Like Jupiter's spot, they are anticyclonic storms. However, the interiors of Great Dark Spots are relatively cloud-free, and unlike Jupiter's spot, which has lasted for hundreds of years, their lifetimes appear to be much shorter.










     Neptune is never visible to the naked eye, having a brightness between magnitudes +7.7 and +8.0. Neptune's orbit has a profound impact on the region directly beyond it, known as the Kuiper belt, extending from Neptune's orbit at 30 AU out to about 55 AU from the Sun. Much in the same way that Jupiter's gravity dominates the asteroid belt, shaping its structure, so Neptune's gravity dominates the Kuiper belt.



  Few other characteristics of Neptune:

Escape velocity                    : 23.5 km/s
Equatorial rotation velocity  : 2.68 km/s 
Equatorial radius                  : 24,764 km
Axial tilt                                  : 23.38°
Gravitational acceleration
on surface                             : 11.15 m/s²