The Sun is our local star and, on average, is around 150 million kilometres (93 million miles) from the Earth (the distance from the Earth to the Sun is called the Astronomical Unit).
Observations with special solar telescopes reveal a number of interesting features on the surface of the Sun. The image to the right shows a few of these: sunspots (1), dark filaments (2) and granulation (3).
As well as seeing Venus against the Sun, it may also be possible to see some sunspots on the surface. A sunspot is a region of the Sun where the magnetic field gets tangled up. This acts to cool the 'surface' of the Sun slightly from around 6000°C to less than 5000°C. As a sunspot is cooler, it gives off less light and so is darker relative to the rest of the surface.
The Sun rotates on its axis and therefore the sunspots can be seen to move across the image. However, the rotation rate of the Sun - how long it takes to spin around once - is between 25 and 30 days, so their motion is very slow. It should be easy to tell the difference between Venus and a sunspot by observing throughout the morning of the transit - Venus will take only 6 hours to pass across the Sun compared with 14 days for a sunspot!
Latest images of the Sun (SOHO, ESA/NASA)