Transit of Mercury: what you need to know and how to watch it

The smallest planet passes the face of the sun today. Here's what you need to know and how you can watch it safely.

The transit of Mercury across the face of the Sun on Monday 9 May is the first for the planet since 2006 and the first to be visible from Britain since 2003. Our next occurs in 2019, but we must wait until 2049 for a more favourable one than this.

We might think that the little innermost planet would cross in front of the Sun every 116 days or so, every time it swings around the near side of its orbit of the Sun. However, that orbit is tilted at 7 to that of the Earth, so Mercury usually sweeps unseen above or below the Sun when it is closest, at its so-called inferior conjunction. The next most inclined orbit is that of Venus at 3.4.