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Uranus

All of the previous-mentioned planets have been known since ancient times. Uranus was discovered only in 1781 by William Herschel who wished to name it after his royal patron King George III. It can just be seen with the naked eye, provided you know exactly where to look, on a perfectly clear night.

Uranus' orbit is twice as far from the Sun as Jupiter's, 10.2 a.u. It has a synodic period of 370 days and exhibit retrograde motion for 152 days. You probably have noticed that as the planets get farther away from the Sun, their synodic periods get close to one Earth year and their period of retrograde motion approaches half a year. Uranus is moving so slowly that its own motion has little effect on its movement through our sky. The movement of the Earth mostly determines its motion across the sky from night to night. For six of our months, Uranus appears to move in one direction as the Earth passes by it in the same direction. During the next six months, the Earth is on the backstretch of its orbit and is moving in the opposite direction so that Uranus now moves (very slowly) in retrograde motion. Another way of looking at Uranus is to remember that synodic period of an exterior planet lasts as long as it takes for the Earth to catch up. Since Uranus is moving extremely slowly, it takes the Earth only a few days more than a year to catch up.

Uranus is half the diameter of Saturn has one unusual feature. No planet except possible Mercury stands absolutely straight up; all the planetary axes have a tilt. The Earth's equator tilts 23 degrees from the plane of the Solar System. Uranus’ equator tilts 98 degrees. It is almost perpendicular to the other planets. Uranus' North Pole (or perhaps it would be better to call it the East Pole) occasionally faces directly towards the Sun.

You will need a large telescope to see any of Uranus' 15 satellites or its surface features.


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