Asked by: Gareth Brazier, by email
Because of the orientation and tilt of their orbits, the eight major planets of the Solar System can never come into perfect alignment. The last time they appeared even in the same part of the sky was over 1000 years ago, in the year 949, and they won’t manage it again until 6 May 2492.
Fortunately, roughly every half century or so the brightest planets take up positions in the night sky creating the impression of being in more or less a straight line. The last decent display was in April 2002, when Jupiter, Saturn, Mars, Venus and Mercury were strung above the western horizon like a celestial necklace, with a crescent moon as its central jewel. A similar alignment will take place 30 years from now, on 8 September 2040.
And in case you’re worried about the gravitational effects of such an alignment, don’t be: the extra pull on the Earth is negligible. Certain alignments are useful, however. During the 1970s, NASA exploited a special alignment of the planets to send space probes on a ‘grand tour’ of Jupiter, Saturn, Uranus and Neptune with minimal effort. Such an alignment occurs just once every 175 years. Fortunately, it came just after NASA scientists figured out how to put it to use.