Uranus was discovered in 1781 by William Herschel

Uranus was discovered in 1781 by William Herschel. This was the first new planet discovered in over 2000 years.

Uranus: • Semi-major axis: 19. 2 A. U. • Eccentricity: 0. 046 (18. 3 A. U. to 20. 1 A. U. )

Orbital period: 84. 1 years. Only two and threefourths revolutions since its discovery.

Uranus is barely visible to the naked eye, if you know exactly where to look. It moves so slowly it was not recognized as a planet before Hershel.

Uranus did not follow the path predicted by astronomers. No elliptical orbit fit the observed locations.

The only explanation was that another body was exerting gravitational influence on Uranus orbit and perturbing its orbit.

The mathematics used to predict the location of this unknown body is called perturbation theory.

In the 1840’s, two mathematicians independently determined the new planet’s mass and location.

John Adams (English) reached the solution in September of 1845, but could convince no British astronomers to look for the new planet.

In June 1846, Urbain Leverrier, a Frenchman, came up with the same answer as Adams. No one was able to find anything during the summer of 1846.

In September of 1846, Johann Galle (German) began a search from Berlin. He found the new planet within 1 or 2 degrees of the predicted location, on his very first try.

It was named Neptune, and Adams and Leverrier were given joint credit for the discovery. (Mysteriously, Galle was excluded. )

Neptune was the first planet found using Newton’s laws of motion and Kepler’s laws of planetary motion.

Actually, Adams’ and Leverrier’s calculated orbits diverged from the actual orbit very quickly. If the search for the planet Neptune had been undertaken a few years earlier or a few years later using this information, Neptune would not have been anywhere near the search area.

Neptune: • Semi-major axis: 30. 1 A. U. • Eccentricity: 0. 010 (29. 8 A. U. to 30. 4 A. U. )

Orbital period: 164. 8 years. Since its discovery Neptune has barely made two revolutions!

Neptune cannot be seen without a telescope, but it may have been seen by Galileo, though he would have had no idea it was a planet.

Uranus: Diameter: 51, 100 km (4. 0 times Earth’s) 28 Mass: 8. 7 x 10 g (14. 6 times Earth’s) 3 Avg. Density: 1. 2 g/cm

Uranus Earth Saturn

Neptune: Diameter: 49, 500 km (3. 9 times Earth’s) 29 Mass: 1. 0 x 10 g (17. 2 times Earth’s) 3 Avg Density: 1. 7 g/cm

The densities suggest that the rocky cores make up a greater percentage of the planet’s volume than on Saturn and Jupiter.

The cores of all four gas giants are probably comparable in size, mass, and composition.

Uranus’ day is 17. 2 hours. Atmosphere rotates differentially, 14. 2 hrs at the poles and 16. 5 hrs at the equator (faster at the poles). The poles rotate faster than the equator, the opposite was true for Saturn and Jupiter.

Uranus’ axial tilt is 98°. It is tipped on its side. The north pole of Uranus, at some time during its orbit points directly at the Sun. Half a Uranian year later, its south pole points at the Sun.

An observer near the north pole when it is pointed toward the Sun would see the Sun move in gradually enlarging circles, completing one loop (in a counterclockwise direction) every 17 hours.

The Sun would dip lower in the sky each day until, eventually, the Sun would set, and the night would get longer and longer with each progressing day.

Twenty-one years after all this started (the summer solstice), it is the autumnal equinox, and day and night are each 8. 5 hours long.

The days would continue to get shorter until, one day, the Sun would fail to rise at all. This period of darkness is equal to the period of constant daylight.

After this “winter” period, the Sun eventually rises again, then the vernal equinox, then back to northern summer as the north pole points at the Sun again.

Near the equator, summer and winter would be equally cold seasons, as the Sun never rises far above the horizon. Spring and fall would be the warmest times, as the Sun passes almost overhead each day.

Some speculate that Uranus is tilted in this way as the result of a catastrophic event, but there is no direct evidence to support this; and no theory to explain it.