The obvious answer is 24 hours, but, as Nicholas Mee discovers, that would be far too simple. In fact, the length of a day varies throughout the year. If you plot the position of the Sun in the sky at the same time every day, you get a strange figure of eight which has provided one artist with a source for inspiration.
Sophie Butchart finds mysterious patterns in the solar system
These are busy times for Plus. Apart from celebrating our 50th issue by putting together a bumper collection of in-depth articles, news and reviews, we're also working on various special projects, some of which have already gone live, and some of which will reach you over the next few weeks and months. Here's a sneak preview of what's in store for 2009...
It's International Year of Astronomy and all eyes are on Galileo Galilei, whose astronomical observations 400 years ago revolutionised our understanding of the Universe. But few people know that Galileo wasn't the first to build a telescope and turn it on the stars. That honour falls to a little-known mathematician called Thomas Harriot, who excelled in many other ways too. Anna Faherty takes us on a tour of his work.