In a transit, Mercury appears as a small black dot moving across the face of the Sun, but it isn't close to the Earth. [Image: InfoBonaire]

When a transit of Venus occurred in 2004, no one alive had ever seen one. Anyone who missed it – and the well-publicized transit of 2012 – will probably never see one. The next one is in December 2117. However Mercury also transits the Sun, and these transits happen more often. But what is a transit and what do we learn from it?

What is a transit and how often do they occur?
When we see a heavenly body passing in front of another one, it's called a transit. You can think of it as a sort of mini-eclipse, since the transiting body seems very small and doesn't cover much of the object behind it. For example, when either of Mercury or Venus is correctly aligned between us and the Sun, we see it as a small black disk crossing the face of the Sun.

In order to get the necessary alignment with Mercury (or Venus), the planet must be between us and the Sun – this is called inferior conjunction. But that doesn't mean we're aligned, because Mercury's orbit is at a tilt to Earth's orbit. There are just two places where the orbits intersect, and they are called nodes. They're not labeled on this diagram showing the orbits of Mercury and Earth, but you can probably see where they are.

If Mercury is at a node at inferior conjunction, a transit occurs. This doesn't happen often, on average thirteen times a century.

What astronomers have learned from transits of Mercury

Planetary tables
Possibly the first thing that astronomers learned from transits was how inaccurate their old planetary tables were. The first prediction of a transit of Mercury was made by Johannes Kepler (1571-1630), based on the tables which he had compiled. His calculations showed the transit occurring on November 7, 1631. Although Kepler didn't live to see it, his new tables got a boost from the accuracy of this prediction. French scientist Pierre Gassendi (1592-1655) observed the transit and published an account of it.

Size of planets
From Gassendi's account astronomers learned something else from the transit: Mercury was much smaller than everyone had assumed. He had viewed the transit with the same set-up he used for studying sunspots. And when he first spotted the planet, at first he assumed that it was a sunspot. However from its movement, Gassendi realized that it was Mercury, but it was no more than one-sixth the size of what he was expecting. His observation encouraged a major rethink about the sizes of the planets.

Edmond Halley is inspired
Edmond Halley (1656-1742) was the first person to observe a complete transit of Mercury, which he did on the island of St Helena in November 1677. In 1716 he published a paper in which he described how transits of Mercury and Venus could be used to measure the distance to the Sun. Observers in different parts of the globe could make measurements which would, along with some geometry, give the Earth-Sun distance. It would be similar to the triangulation that surveyors use. But he concluded that Venus transits would work better than Mercury transits.

Determining longitude? Probably not.
Captain James Cook and astronomer Charles Green observed the November 1769 transit of Mercury from New Zealand. The place is now called Mercury Bay and a plaque proclaims, “Near this spot on 10 November 1769 James Cook and Charles Green observed the transit of Mercury to determine the longitude of the bay.”

This is often listed as one of the uses of a transit of Mercury, but New Zealand freelance writer and amateur astronomer Bill Keir points out that

the journals and logs contain no further entries about the observation and no longitude derivation is given that is specifically attributed to the transit observation. Indeed, the records are more than a little confusing on the whole matter, and are notable for their lack of detail about such an important event.

With the equipment available in those days it's unlikely that the timings would be sufficiently accurate to calculate longitude. Cook, in any case, was an accomplished surveyor and cartographer and the measurements he made with a sextant would have been adequate for the job.

Earth's rotational velocity
Interestingly, transits of Mercury still have their uses. For example, there is historical data from transits, and it's able to contribute to an understanding of variations in day length over several centuries.

Measuring the Sun
A group of astronomers timed the transits of 2003 and 2006 using NASA's Solar and Heliospheric Observatory (SOHO). They obtained a measurement of the Sun's diameter more accurate than any other so far. This helps in understanding solar structures and solar dynamics which affect the Earth.

More transits
The only Solar System planets we can see transiting the Sun are Mercury and Venus. Interestingly, there was a transit of Mercury in 2014 that was not visible from Earth, but was imaged by the Curiosity rover on Mars. This was the only occasion on which such a transit has been observed from another heavenly body.

And of course, Earth also transits the Sun as seen from Mars and beyond. There was such a transit on Mars in 1984, but no one there to see it. The next one isn't due until 2084. Perhaps there will be people on Mars by then.

Reference:
Bill Keir (2010) Captain Cook's longitude determinations and the transit of Mercury — common assumptions questioned, Journal of the Royal Society of New Zealand, 40:2, 27-38, DOI: 10.1080/03036758.2010.482971