to the Sun: The first is geothermal power, which is derived from geothermal energy, 20% of which remains from the original planet formation and 80% of which is derived from ongoing radioactive decay. The second source is the Moon's effects on tides and the associated methods of power generation. The third is nuclear power derived from uranium and other fissile elements. Ultimately, however, the overwhelming proportion of human-generated power derived from fossil fuels and thence from photosynthetic plants makes this line a very good approximation to the truth. The three other power sources are available because of the Sun's influence on our early solar system, so Idle's statement is correct in a literal (if somewhat pedantic) sense, even if not for practical purposes. Idle's figures for the size of the Milky Way galaxy are roughly correct. He understates the speed at which the Sun orbits the "galactic central point" by an order of magnitude (12 million miles a day compared to "1 million miles a day"), but he gives an estimate accurate to one significant figure for the total time per orbit. This is "two hundred million years" according to the song, compared with accepted figures of 220 to 250 million years. The song says that we are "thirty thousand light years from galactic central point", again correct to within one significant figure of 25,000 light years from the centre of the Milky Way. The song also states that the galaxy is "a hundred thousand light years side to side". This would make the galactic radius 50,000 light years, which is accurate. Australian astrophysicist Bryan Gaensler has stated that Idle's estimation of the thickness of the Milky Way, at 16,000 light years, is more accurate than the official 'textbook' figure of 6,000 light years. However, the song's position on this was later confused by Idle's performance of the song in his Not the Messiah show (2007) where the figure he sings is only 6,000 light years. The reason for the confusion has since been explained in a message from Idle on the official Monty Python website. "There was some smug website pulling apart all my original figures for the song (written circa 1981) so for the 2003 Tour (or maybe 2000) I 'updated' them. Now you tell me I was right all along! Not sure where I got my figures originally but tell the bastards to make up their minds."