How a transit of Venus works
Venus orbits the Sun inside Earth’s orbit and passes between the Sun and Earth about every 584 days. Usually it passes above or below the Sun as seen from Earth, because its orbit is tilted by a little over 3 degrees relative to Earth’s. A transit happens only when this alignment falls very close to one of the nodes, the two points where Venus’s orbit crosses the plane of Earth’s orbit.
Earth lines up with those nodes in early June and early December, so every transit of Venus in the current era happens in one of those two months.
The pattern of pairs
Transits of Venus follow a pattern that repeats every 243 years:
| Gap | Years |
|---|---|
| Within a pair | 8 years |
| Between pairs | 121.5 years |
| Within a pair | 8 years |
| Between pairs | 105.5 years |
The 8-year spacing works because 8 Earth years are almost exactly equal to 13 Venus years, so the planets return to nearly the same arrangement. The match is not perfect, so after one or two transits the alignment drifts away from the node and a long wait follows.
Past and future transits
| Date | Notes |
|---|---|
| 1631 | Predicted by Johannes Kepler; not observed, as it was not visible from Europe |
| 1639 | First observed transit, by Jeremiah Horrocks and William Crabtree in England |
| 1761 and 1769 | The great international expeditions |
| 1874 and 1882 | Observed photographically by many national expeditions |
| 8 June 2004 | First transit in living memory |
| 5–6 June 2012 | The most recent transit |
| 10–11 December 2117 | The next transit |
| 8 December 2125 | The transit after that |
Dates of transits that span midnight, such as 2012 and 2117, differ by time zone. You can convert event times with the time zone converter and the world clock.
Measuring the size of the solar system
In the 17th century, astronomers knew the relative distances of the planets from the Sun, but not their actual size in kilometers. Edmond Halley proposed that if observers far apart on Earth timed a transit of Venus precisely, the small differences in their timings could reveal the distance from Earth to the Sun, now called the astronomical unit.
That idea launched some of the first big international science projects. For the transits of 1761 and 1769, observers traveled to sites around the world. James Cook sailed to Tahiti to time the 1769 transit. Results were limited by the black drop effect, in which Venus seems to cling to the Sun’s edge as it enters and leaves, blurring the exact moment of contact. Even so, the combined measurements narrowed the Earth-Sun distance far better than before.
During the 1761 transit, observers including Mikhail Lomonosov noted a thin, bright ring of light around Venus as it moved onto the Sun. This was light passing through the planet’s thick atmosphere, which is still seen during transits today.
What a transit looks like
Venus appears as a sharp, round, black disk, about 1/30 of the Sun’s width. It takes about six hours or more to cross the Sun completely, entering at one edge and leaving at the other. The four key moments, called contacts, are the same as for a transit of Mercury: first touching the Sun’s edge, fully on the disk, touching the opposite edge, and leaving.
How to watch safely
The Sun is just as dangerous to look at during a transit as on any other day. Use only:
- Certified solar eclipse glasses, which can show Venus as a small dot.
- Telescopes or binoculars fitted with a certified solar filter over the front.
- Projection of the Sun’s image onto a card.
Never look through unfiltered optics at the Sun. Full guidance is in solar eclipse eye safety. For other sky events that will happen in your lifetime, see the eclipse calendar.