This article was the theme talk at the Beginners meeting October 2006



The two outermost planets of the Solar System are Uranus and Neptune the icy giants. Now that Pluto has been removed from the official list of planets in out Solar System, Neptune is the outer planet. Uranus and Neptune are very similar planets. They are approximately the same size and composition and their appearance is so similar that they could be twins.



Until the invention of the telescope in the early 1600's it was not possible to determine that there were any planets beyond the orbits of the naked eye planets. Mercury, Venus, Mars, Jupiter and Saturn have been known to be different to the stars since humans first looked up into the night sky and thought ‘what are all these tiny lights?' The word planet was derived from a Greek word that means ‘wanderer'. Until the 1600s most scientists still believed that Earth was at the centre of the Solar System and was not a planet. Thanks partly to Galileo and his use of a telescope the view of the Solar System changed to that proposed in 1530 by Nicolaus Copernicus. Copernicus proposed that the Sun was at the centre of the Solar System and Earth was one of the planets. So eventually it was agreed that there were six planets including Earth and this remained until 1781.

Some mathematicians had found that the orbit of Saturn appeared to be influenced by some large object further out in the Solar System so it must have been another planet and it must be a large one. In 1781 William Herschel found this planet using his own home made telescope. Herschel wanted to name his new planet after the king but eventually it was officially named Uranus.

Uranus is 51,118 km in diameter this is about 4 times the diameter of Earth. It orbit is 2,875million km from the Sun and takes 84 Earth years to complete one orbit.

Uranus imaged by Voyager 2 in 1986

Uranus has a very odd tilt on its axis, it is actually tilted over almost 90° on to it side compared to all the other planets. This gives a very strange combination of days and seasons on the planet. Each pole (north and south) face directly towards the Sun once a year, (every 84 Earth years). If we start the year with the Sun shining directly down on the North Pole then the point direct under the Sun will move south as the year progresses. After ¼ of a year (21 Earth years) the Sun will be directly over the equator. After ½ year (42 Earth years) the Sun will be directly overhead at the South Pole. After ¾ of a year (63 Earth years) the Sun will be directly over the equator again. At the end of the year (84 Earth years) the Sun will be directly overhead at the North Pole again.

When we add in the rotation of Uranus, on its axis (17 Earth hours), we have an even more peculiar situation. Imagine we are at a similar point on Uranus as we are here on Earth. If we follow the yearly cycle described in the paragraph above when the Sun is directly over the North Pole, we would be in continuous daylight for about 10 Earth years. As the Sun moves south we would start to get a short night that grew longer until the Sun was directly overhead at midday (after 21 Earth years). There would be 8½ hours each of day and night. The night would then lengthen over the next 21 Earth years until the Sun moves below the southern horizon and it would become night for the next 21 Earth years. The Sun would then begin to rise over southern horizon and the days would begin to lengthen. After another 21 Earth years the day and night would be equal again (8½ hours). The days would then lengthen for about 10 Earth years there would be permanent daylight.

Uranus has 15 known moons with 4 of them over 1000 km in diameter and one over 500 km. These are:

DISTANCE (1000 km)
PERIOD (Earth Days)


Miranda looks as if it has been smashed to pieces and then reassembled. It is thought that at some time shortly after the solar system and planets formed Uranus was involved in a collision with another planet that knocked Uranus on to its side. It is possible that Miranda and possibly some of the other moons were involved in this collision in some way and were smashed to pieces.

Miranda the 5th largest moon of Uranus


After the discovery of Uranus in 1781 some mathematicians calculated that Uranus could not fully account for discrepancies in the orbits of Saturn and Uranus. Both planets appeared to be influenced by another large object further out. In 1846 Neptune was found by two astronomers in Berlin . The new planet was found less than one degree from the position it was calculated to be. Neptune is slightly smaller than Uranus 49,528 km in diameter and orbits the Sun at 4.5 million km and one orbit takes nearly 165 Earth years.

Like Uranus, Neptune is composed mainly of Hydrogen with rock and metal core and an atmosphere approximately 10,000 km deep. Neptune is the densest of all the giant planets with a density of 1,640 kg/m². Surprisingly the atmosphere of Neptune is much more turbulent than that of Uranus despite receiving less than a quarter of the heat and light from the Sun. It is thought that there is a heat source in the core that drives the weather systems. Despite the extremely cold atmosphere at -214°C, Neptune has some of the most violent storms in the solar system, with winds in the storms reaching speeds of over 300km per hour.

Neptune showing some storm features

Neptune has only one large moon but has 7 other known smaller moons. Triton at 2,720 km is larger than all of the moons of Uranus and is also the coldest large moon in the solar system. Triton has a crust of mainly water ice between 150 and 200 km thick. Below this solid crust there is thought to be an ocean of liquid water mixed with Ammonia and Methane. The temperature at the surface is -235°C and is very bright, reflecting up to 90% of the sunlight reaching it.

Triton imaged by Voyager 2 in 1989

Triton also has a very thin atmosphere comprised of Nitrogen. Amazingly this atmosphere is replenished by geysers that eject Nitrogen up to 8 km above the surface. When the plume from the geyser reaches 8 km is suddenly changes direction and travels parallel to the surface for over 100 km across the moon, leaving a dark trail across the surface.



Both planets are observable this month but both are quite low in the south. Uranus is located in constellation of Aquarius and Neptune in Capricornus. The planets are close to opposition that means they are directly outside the orbit of Earth and at their closest and best observing position. Earth is currently overtaking the two planets on the inside so we see them in due south about midnight.

The planets are difficult to find but it is possible even using a good quality pair of 10 x 50 binoculars. In binoculars both planets will only be seen as looking very star like and it will be difficult to distinguish them from the background stars. A 100mm or larger telescope at about 25 to 50x magnification will give a hint of their non-star like appearance. Uranus is larger and brighter than Neptune and will obviously be easier to find. Once it is thought that one of the planets has been found increasing the magnification of the telescope will make the planet appear fuzzy compared to any stars in the field of view. A 100mm telescope may show just a hint of blue and this will be more pronounced in larger telescopes.

The position of Uranus in Aquarius

Uranus will be slightly easier to find because it is close the star ? (Lambda) and will appear slightly blue.

The position of Neptune in Capricornus

Neptune is more difficult to find but it can be found close to the star ? (iota).