There’s tremendous excitement at the BBC as Wimbledon finishes & NASA’s New Horizon project looms into view; the BBC will have something cheap to fill it’s summer schedules with.
Even though not a planet (it has not cleared it’s orbit like the other planets) Pluto has at least 5 moons. The major one is called ‘Charon’ (it is so heavy that strictly Pluto/Charon are a binary pair).
Pluto was fortuitously discovered in 1930 at the observatory started by Percival Lowell.
Mercury, Venus, Mars, Jupiter & Saturn had been known of since ancient times, simply because more-ancient peoples watched the night skies & built up an intimate knowledge of the fixed stars & the few moving bodies (which they called ‘planets’, meaning ‘wanderers’). Those 5 mysterious coloured lights in the skies, together with the Sun & Moon, made up the 7 heavenly spheres from which we obtained the names of the days of the week & months of the year:
If you look closely at the months of the year (in a circle is best) you may spot that the ancients somehow knew the order of the planets out from the Sun, and well before Copernicus’ day. How did they do that?
The 5 classical planets (6 including the earth) came down to us from the stone age. The next, Uranus, was discovered as a planet just before the beginning of the modern age (by Sir William Herschel, announced on March 13, 1781). Uranus is so distant from the Sun that it moves very slowly, and is mighty dim. It was the first planet to be discovered using a telescope, although it is (just) bright enough to be seen with the naked eye.
By the middle of the next century, Uranus’ orbit had been calculated with sufficient accuracy that the astronomers realised that it was not quite what it should be. There were perturbations in the flow. The idea took hold that these disruptions in the orbit could be caused by another, more distant planet. French mathematician Urbain Le Verrier made calculations that enabled him to say that “if you look for it on this day it should be here”. His compatriots ignored him, but German astronomer Johann Gottfried Galle looked on the same day that he received his letter (23 September 1846) and the new planet (Neptune) was virtually exactly where Urbain said it would be.
The first 5 planets were discovered by stone-age eyes. Discovery of the 6th planet was power-assisted by use of a telescope. The 7th was discovered by mathematics. Now we are getting into the modern age.
Pluto was discovered by exactly the same method used to find Neptune, but with a huge twist:- Percival Lowell (who in 1894 founded & funded the Observatory that bears his name & which found the planet) was a lousy mathematician. Lowell was a wealthy businessman, and obsessed with finding “Planet X” (the name that he gave to the ‘new Neptune’). The problem is that Neptune is huge & very heavy, whilst Pluto is tiny & so distant that it exists almost in inter-galactic space.
In 1906 Lowell used William Henry Pickering to make the calculations to find Planet X. Pickering was an astronomer that found Saturn’s 9th moon Phoebe in 1899. He also found Saturn’s 10th moon Themis in 1906 (Phoebe exists, but Themis does not). Pickering also believed that the moon contained insects, and that these insects were responsible for changes in the appearance of it’s craters. He also suggested ideas on continental drift (part of modern geology since the 1960s) and the use of aeroplanes as a tool of war (it was 1908, and aeroplanes had yet to be invented). All in all, Pickering was a bit of a strange cove.
Neptune, even though it is so distant, is large enough to cause relatively-large effects on the orbit of Uranus. Pluto, however, is a dwarf planet and unimaginably distant from Neptune. Pickering produced more than one set of calculations, but Lowell’s observatory couldn’t find anything no matter how he tweaked the figures. And here we meet again the huge double-twist:- in our modern age, and using calculations from Voyager’s fly-past, we know that, essentially up to the 10th or 20th decimal point, there isn’t any effect from Pluto upon Neptune and, therefore, that Pickering couldn’t have predicted where it was. The second half of this double-twist? On 19 March 1915, Lowell’s surveys had captured two faint images of Pluto, but they didn’t know what it was, nor name it until 18 February 1930, long after Lowell had died (Pickering died in January 16, 1938). So, Pickering had zero chance of finding it yet was looking in exactly the right place. All in all, a bit of a strange cove.
Lowell died in 1916 & left his observatory a million dollars to continue the search. His widow spent 10 years suing to try to get the endowment back for herself. But failed. The search for Planet X resumed in 1929. The job was given to 23-year-old Clyde Tombaugh, and after a year he found it. Whoopee! The news was made public on 13 March 1930.
Next:- what are they going to call the new planet? The news had gone global & they had thousands of suggestions. Amongst them was one from an 11-year-old English schoolgirl called Venetia Burney.
Venetia was schooled in Oxford and interested in classical mythology. She suggested “Pluto” (also known as Hades), the god of the Underworld (the brother of Zeus, god of the sky, and Poseidon, god of the sea). It made a shortlist of 3 & everyone eligible voted for it. It probably also helped that “PL” were the initials of Percival Lowell, and of ‘Pickering-Lowell’.
It was then natural to call Pluto’s moon ‘Charon’ (the ferryman for Hades, and the reason that coins were placed in the mouth or upon the eyes of the dead, as payment for Charon). Pluto actually has 5 moons: Charon, Styx, Nix, Kerberos, and Hydra.
As a final remark, Pluto’s orbit is highly eccentric, both laterally (it passes inside the orbit of Neptune, and only remains intact due to orbital resonance between the 2 orbits that means that they will never collide). Pluto is also highly inclined to the ecliptic, meaning that it rises above & below that plane. That’s my boy! (Pluto is, of course, devoted to Scorpio).
The New Horizons’ probe was launched in 2006 & has been travelling towards Pluto/Charon at hi-speed ever since. So far it has sent back photos showing Charon as a dark-grey moon & Pluto as a red dwarf planet. The closest approach comes tomorrow (Tuesday 14 July 2015), and masses & masses of data will be radioed home in the following days. I’ll keep you posted.
--------- Alex Kemp