All the planets revolve around the Sun in the same direction - counter-clockwise, if the Solar System is viewed from the North). This motion is often referred to as direct. Most objects also rotate on their own axis in a counter-clockwise, or direct, sense.
Note : The word rotation is reserved is reserved for spin on its own axis. The orbit about the Sun will use the word revolution.
Due to way that planets are arranged roughly in a plane in the Solar System, they will appear in the Night Sky within roughly the same region of the sky that you can view the Sun - obviously at different times of the day, but along an arc to the South, anyway. They do not remain fixed with respect to the celestial sphere (or fixed stars) and in fact, every so often, they reverse direction with respect to this celestial sphere (or at least the planets further from the Sun than the Earth do).
<---Insert information on Retrograde Motion here>
Quite apart from these retrogressions, their apparent speed of movement across the Celestial Sphere varies.
The planets out as far as Saturn have been known since ancient times.
The three outer planets were discovered using optical aids, by virtue of them being fainter than magnitude 6.
Uranus was discovered by William Herschel from Bath in Britain. He had actually been looking for comets at the time.
Pluto, which today tends to be considered as a captured object of another type rather than actually a planet, was found in 1930. It is obviously inconsistent with current ideas on the formation of the Solar System, which predict the outer planets to be exclusively gas giants.
Details of the Planets
|Distance from Sun (AU)||Mass (in Earth masses)||Equatorial Diameter (km)|
|Pluto||39.44||0.003 approx.||maybe 3,000|
Distances are given in Astronomical Units, where an Astronomical Unit is the average distance from the Sun to the Earth (149,597,870 km).
The stated diameter is the equatorial diameter. In the case of the Earth, the equatorial diameter is about 42 km greater than the polar diameter (42 km being the distance of the Marathon, to the nearest whole number, if that helps you remember the number).
Nevertheless the mass of the Solar System is concentrated in the Sun.
Notice that most 'popular' diagrams of the Solar System are wildly innacurate.
|Rotation Period (days)|
Speed goes down as we go outwards. The Orbital Period can also be called the Sidereal Period. For completeness, I have also added the Synodic Periods to the right - this period is discussed below the table.
|Orbital Speed (km per sec)||Orbital Period (years)||Synodical Period (days)|
|Mercury||49.9||0.241 (88 days)||116|
|Venus||0.615 (225 days)||584|
The Synodic Period is the period as observed by us, which is different from the inherent orbital periods given in the third column. For example, the Synodic Period could be, and usually is presented as, the difference in time between two successive oppositions.
The Synodical Periods for the outer planets are close to one year. Consider Jupiter - if it is at opposition at a particular time, then if it was not moving along its orbit, it would appear at opposition again in one year's time after the Earth has completed one orbit. During this time however, Jupiter has moved along its orbit, but not by much. It takes 11.9 years to complete one orbit, so in one year it will have moved by about 30 degrees. The Earth will have to move another 30 degrees until the planet is at opposition again - but this will only take about one month.
As you go further out than Jupiter, the orbital period goes up, so these planets will move even less (in degrees) along their orbit within one year. Consequently their Synodical Period will be closer to one year the further out the planet is.
Conjunctions and Oppositions
A Conjunction is an alignment such that two bodies in the Solar System have the same right ascension (as seen from the Earth naturally. In practise, the word is usually used to mean the conjunction of a planet with the Sun.
A Opposition is an alignment such that the Earth lies in line between two bodies in the Solar System. In practise, the word is usually used to mean the conjunction of a planet with respect to the Sun. When a planet is at opposition, you would therefore expect it to be at its brightest.
The asteroid belt lies between Mars and Jupiter. Whereas a simple theory of solar system formation would have expected a planet to have formed in this region by the collision and amalgamation of smaller bodies, the influence of Jupiter has assured that any collisions have been more destructive than planet-forming.
Asteroids also occur outside this traditional asteroid belt.
Atens have orbits within the Earth's
Apollos cross Earth's orbit during their own orbit
Amors orbit between 1 and 1.3 AUs (between Earth and Mars). A well known example is Eros.
Because they have never been a part of a planet and have experienced none of the transformation processes that rocks experience in planetary bodies, the asteroids are composed of 'primitive' material, akin to the material of most meteorites.
The first asteroid was discovered by Guiseppe Piazzi in 1801 - this was Ceres, the largest asteroid with a diameter of 1020 km. This was soon followed by Pallas (1802, 539 km diameter) , Juno (1804, 257 km diameter) and Vesta (1807, 536 km diameter).
The total mass of all asteroids is actually very small - about 3% of the Moon's mass.
Asteroids imaged by Galileo on its way to Jupiter include Gaspra and Ida, which has its own moon, Dactyl, of diameter about 1.6 km.
In 1977, Chiron became the first asteroid seen in the outer Solar System - Chiron orbits between Saturn and Uranus. It later brightened and showed a coma similar to a comet's.
In 1992, a similar object was found for the first time beyond Pluto.
Trojans orbit in same path as Jupiter but in a fixed distance in front and behind it (at the Lagrangian Points).
Asteroids are actually named by their discoverers.
Private Eye 1167 (September 2006)
TALKING THROUGH URANUS
DAILY Mail astrologer Jonathan Gainer, Fleet Street's highest-paid hack, is having problems with his predictions thanks to those all-too-predictable print deadlines.
On 16 August he noted the debate raging among scientists about whether Xena is a planet or an asteroid. "Some people have been seriously suggesting it is not a planet - and that Pluto isn't one either, even though it has been treated as such for more than 75 years. Top astronomers are now meeting to discuss this thorny question. It looks as if they are about to confirm that Pluto is a planet, and they are also about to confer full planetary status on Xena. Surprisingly, they also look set to give planetary status to Ceres, the large body discovered in 1801, and Charon, Pluto's Moon. This is amazing. It implies many things..."
On Tuesday 22 August, tension was building. "Once we had celestial symmetry... All that changed after 1781, when Uranus was discovered. After years of experimentation, astrologers attributed it to Aquarius. The 1846 discovery, Neptune, was eventually given to Pisces. By the 1950s we were confident that Pluto belonged to Scorpio. So what will Ceres, Xena and Charon end up ruling?"
By the following day Gainer had all the answers. "In Prague on Friday, members of the International Astronomical Union will vote on whether to create a new class of planet called a Pluton. They are expected to rubber-stamp this proposal. Will Plutons 'rule' zodiac signs? They will have to because Pluto, the ruler of Scorpio, is among them. There are, though, as I explained yesterday, only two vacancies for new ruling planets - yet we have three candidates. Arguably, Charon, Pluto's twin, co-governs Scorpio, leaving Ceres for Virgo or Gemini and Xena for Taurus or Libra. Problem solved? Not quite. More Plutons are sure to be announced soon."
Alas! Gainer had got his dates mixed up, and his predictions entirely wrong. On Thursday 24 August the astronomers decided that Pluto wasn't a planet and there weren't going to be any new classes of planets, or Plutons, called Ceres, Charon or Xena either.
Unfortunately, Saturday's Daily Mail Weekend supplement had already gone to press, containing Gainer's joy at exactly the opposite occurring. "As I predicted on BBC2 a few weeks ago, scientists have accepted, as a planet, the new discovery 2003 UB313," he raved. "That's Xena to the likes of you and me. They have also reclassified Ceres, the asteroid, as a planet - and declared that Pluto's Moon, Charon, is a planet too. By tradition, we astrologers defer to the decisions of astronomers. If they say a heavenly body is important, we treat it with respect, even if previously we have all but ignored it. It's going to take us a while to leam to read these new dials on our dashboard but, eventually, you'll start to see us make even more accurate predictions."
No we won't!
Largest Planet : Jupiter
Highest Surface Temperature : Venus
Time for light to travel between the Earth and Sun : 8 m 20s