Posted by Steve Holmes2 at 17:15 on 2011 Mar 13
I suspect alignments of this sort will be more common than one might think, but will definitely come in "seasons".Given the slow motion across the sky of the gas giant planets, it is clear that once the J-S-U-N-(P) order has been established it will persist for many years. Conversely, as Mercury and Venus move very quickly (one circuit of the sky per year, approximately) in this time there will be many occasions for the order Me-V to happen while they are "to the right of" the sun but "to the left of" Jupiter. The only problem child is thus Mars, as its motion is quite fast and so its oppositions, for example, shift by over 45deg per occasion.The problem might thus reduce to what is the maximum angle allowed between Mercury and Neptune/Pluto? More than 180deg and all planets will not be simultaneously visible, but they will still be in order, nonetheless.For example, on page 348 of "More Mathematical Astronomy Morsels" (aka Morsels II), Jean Meeus tabulates the occasions from 1979 to 2020 when all five naked-eye planets are simultaneously visible in the night sky. This shows that in December 2004 and January 2005 they are arranged in order of distance. Further checks with my astronomy programme shows that Uranus, Neptune and Pluto are also in order, and to the west of Saturn. All planets are thus in the correct order. However! There is a very large gap between Saturn and Uranus and so the outer three are not visible at the same time as the inner five, and even in the best case (11th December) by the time you get to Pluto the arc is almost 360deg. Does this count, therefore?[Amazingly, for a couple of hours before midnight on 11th Dec, the moon is between the Sun and Mercury and so every one of the "heavenly bodies" is then in the correct order! This will of course not be visible at all in the UK, however.]While J-S-U-N-P will persist for many years, Jupiter will get more than 180deg in front of Saturn about 10yrs after being close to it. It will then be to the east again after about another 10yrs, but the pair will now both be to the west of the U-N-P grouping. It will be another 20yrs (the Jupiter/Saturn conjunction period) until they both emerge to the east of the U-N-P grouping again (but of course much further east). I would thus expect that there would have been "correct order" groupings 20 or 40yrs before the 2004/5 set – before, because Saturn was already well to the east of Uranus in 2004/5. Anyone care to check?