Skip to content
Skip to main content

About this free course


Download this course

Share this free course

An introduction to exoplanets
An introduction to exoplanets

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

2.1  Transits of Mercury and Venus

Figure 3 shows a close-up of the Sun during the 2004 transit of Venus. Venus is in front of the Sun from the Earth’s point of view, and has blocked some of the Sun’s light, appearing in silhouette.

An image of the transit of Venus.
Figure 3  The 2004 transit of Venus, taken with a telescope in Belgium. Venus is passing in front of the Sun, and forms a dark circular silhouette.

The video at the link below is a speeded-up time-lapse sequence of the 2006 transit of Mercury. In reality it took five hours for Mercury to complete its transit.

Transit of Mercury: watch as planet passes in front of the Sun for first time in a decade [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]

In some images of the Sun, you will also see dark patches that are actually on the surface of the Sun. These are caused by sunspots. Figure 4 shows an example.

An image displaying the transit of Mercury on 9 May 2016.
Figure 4  The transit of Mercury on 9 May 2016, with sunspots also visible. Mercury is the left-most dark circle.

It is also possible for other, much closer, objects to get in the way and block some of the light from the Sun, as Figure 5 shows.

An image of Venus, with an aeroplane in transit in front of it.
Figure 5  Transit of an aeroplane during the transit of Venus on 5 June 2012, taken from San Francisco, California, USA by MacNeil Fernandes

Figures 3 and 4 are photographs taken during transits of Venus and Mercury. The obvious difference between them is the size of the silhouette, with that of Mercury being much smaller. That makes perfect sense, because Mercury is less than half the size of Venus.

Astronomers know how big Mercury and Venus are. But if they didn’t, Figure 6 shows that they could work out the sizes of the planets by measuring their transits, given knowledge of three more things:

  • how far away Mercury/Venus is from Earth
  • how far away the Sun is from Earth
  • how big the Sun is.
Described image
Figure 6  Side-on transit diagram for Venus showing how the amount of sunlight that gets blocked depends on both the relative sizes of Venus and the Sun, and on the distances of Venus and the Sun from Earth. Note that the diagram is not to scale – the Sun would be very much bigger than the planets and the distances between them greater if drawn to scale.