How a transit works
Mercury orbits the Sun inside Earth’s orbit, so it regularly passes between the Sun and Earth, an alignment called inferior conjunction. That happens roughly every 116 days. Most of the time, though, Mercury passes just above or below the Sun as seen from Earth, because its orbit is tilted by about 7 degrees relative to Earth’s.
A transit can happen only when inferior conjunction occurs near one of the two points, called nodes, where Mercury’s orbit crosses the plane of Earth’s orbit. Earth lines up with those nodes at about the same time each year: in early to mid May and in early to mid November. That is why every transit of Mercury falls in one of those two months.
May and November transits
November transits are more common than May transits, by roughly two to one. In November, Mercury is near the point of its orbit closest to the Sun, and the window in which a transit can happen is wider. In May, Mercury is near its farthest point from the Sun and closer to Earth, so it looks slightly larger against the Sun, but the window is narrower.
The gaps between transits are irregular, ranging from a few years to more than a decade.
Recent and upcoming transits
| Date | Notes |
|---|---|
| 9 May 2016 | A May transit lasting about seven and a half hours |
| 11 November 2019 | A November transit lasting about five and a half hours |
| 13 November 2032 | The next transit of Mercury |
| 7 November 2039 | The transit after that |
As with all transits, how much you can see depends on whether the Sun is above your horizon during the event. Convert published times, usually given in Coordinated Universal Time, with the time zone converter, and check the Sun’s hours at your location on the sunrise and sunset page.
What a transit looks like
Through a safely filtered telescope, a transit unfolds in four contacts:
- First contact: Mercury touches the Sun’s edge from outside.
- Second contact: Mercury is fully on the solar disk.
- Third contact: Mercury touches the opposite edge from inside.
- Fourth contact: Mercury leaves the Sun.
The planet looks like a small, sharp, perfectly round black dot that drifts slowly across the Sun over several hours. At its size, about 1/150 to 1/200 of the Sun’s diameter, it is easy to confuse with a sunspot at first glance, but sunspots are irregular, have gray outer regions and do not move noticeably over a few hours.
Near second and third contact, observers sometimes see the black drop effect: Mercury seems to stay connected to the Sun’s edge by a thin dark bridge for a moment. It is caused mainly by the blurring of the telescope and atmosphere and by the darkening of the Sun toward its edge.
Why transits mattered
Timing transits from different places on Earth helped astronomers refine their knowledge of the planets’ orbits. Transits of Mercury have also been used to study the Sun’s exact diameter and to test methods later used to find planets around other stars, which are often detected as tiny dips in a star’s light when a planet transits in front of it.
How to watch safely
Never look at the Sun directly, and never point binoculars, a telescope or a camera at it without a certified solar filter securely fitted over the front. Because Mercury is so small, you will need magnification of roughly 50 times or more. Safe options include:
- A telescope with a proper front-mounted solar filter.
- Projecting the Sun’s image through a small telescope onto a white card, with nobody looking through the eyepiece.
- Live streams from observatories during the event.
The same rules that protect your eyes during a solar eclipse apply; see solar eclipse eye safety. For the larger and rarer transit of Earth’s other inner neighbor, see transit of Venus.