Jupiter and Saturn have been spectacular in the evening sky for the last couple of months - it's about to get even better!

BBC News | SCI/TECH | Spectacular planet show promised: The five planets visible to the naked eye will line up in the sky at the end of April. Astronomers say the rare grouping of Mercury, Venus, Mars, Jupiter and Saturn may not be seen again for a century.

(But what do they mean, "may not"?)