The Transits Of Mercury

Given that Mercury is smaller than Venus, more distant from us, and also inclined at a greater angle to the ecliptic plane, you might guess that transits of Mercury occur less frequently even than the rare Venusian transits. But you would be wrong. Mercury crosses the face of the Sun 13 times a century on average.

This does not imply, though, that Mercurial transits are spaced by even gaps of 7.7 years. Like Venus, Mercury follows a cycle with steps of certain length, quantized as multiples of an Earth year, but those steps are uneven. For Venus the steps are a regular sequence of 8, 121.5, 8, 105.5 years, but for Mercury there are interleaved cycles of 7, 13, and 33 years. The outcome is that Mercury's transits may be separated by only 3 years, but there may be up to a 13-year gap.

As for Venus, the dates of Mercurial transits are spaced by six months: they all fall within a few days of May 8 and November 10. Those dates define a position of the Earth in its orbit, and if on either date Mercury happens to be near its appropriate node (descending in May, ascending in November) then a transit will occur.

Another regularity is also produced. In a November transit, Mercury is near its perihelion, making it more distant from Earth, and so its disk appears small: only about one part in 190 of the solar diameter. Conversely, a May transit happens while Mercury is near aphelion, making it appear larger, about one part in 160 of the solar disk. (Recall that Venus appears to be about one part in 30 the solar diameter when in transit, so that Mercury always represents a rather smaller spot passing over the Sun.) This behavior makes May transits slightly easier to follow, but they occur only about half as often as November transits. This is because at aphelion the planet is moving slowest, and consequently it is less likely to pass across the Sun during the critical window. November transits independently follow a cycle with 7-, 13-, and 33-year intervals, while May transits are governed only by 13- and 33-year gaps.

Recent and upcoming transits of Mercury are as follows.

1970: May 9 1973: November 10 1986: November 13 1993: November 6 1999: November 15 2003: May 7 2006: November 8 2016: May 9 2019: November 11

There is then a 13-year wait until 2032 for the next opportunity.

Transits of Mercury typically last several hours, the longest in recent times being the 7 hour 47 minute behemoth of 1878. That which occurred in November 1999 was much briefer, lasting for but 50 minutes. This was barely a transit at all because, depending upon the viewing latitude, Mercury only just managed to break onto the Sun. This is called a graze, a rather rare event. Observers far enough north saw Mercury enter the face of the Sun in its entirety, but not venture far from the edge before terminating its fleeting visit (see Figure 13-1), whereas those further south saw the planet simply skim along the solar limb.

A transit is something well worth seeing at least once in your life, and there is a better window of opportunity in May 2003, when all longitudes from Europe east across Asia to Japan are favored as Mercury traverses the face of the Sun in a much deeper fashion.

A small telescope projecting an image onto a screen is what is needed, or a proper filter fitted to a telescope allowing direct view-

FIGURE 13-1. The transit of Mercury over the edge of the Sun on November 15, 1999, as recorded with an ultraviolet telescope on board a satellite called TRACE (Transition Region And Coronal Explorer). The five dark spots show the movement of Mercury over a time-span of almost 30 minutes.

ing. Mention was made much earlier of the ubiquitous Ha filter used in solar observing. Such a filter is especially useful in this case because it dims the brightness of the solar disk while making the chromosphere and corona visible, because it permits the transmission of only a single red light wavelength emitted by hydrogen. As a result Mercury (orVenus, if you watch in 2004 and 2012) may be seen silhouetted against the chromosphere before and after it meets the solar limb, whereas a simple gray (neutral-density) filter leaves the chromosphere virtually invisible.

Let us leave Mercury with a historical note. The first recorded transit was seen from Paris in November 1631. Pierre Gassendi was able to watch Mercury cross the Sun's face after receiving Kepler's prediction, confirming that the calculations were correct. A few other European astronomers who had heard of Kepler's work did likewise. The transit of Venus in the following month was unseen due to geographical considerations: there was not the time for observers to travel to the Pacific Ocean from where it could have been seen. We described above how Horrocks watched the Venusian transit in 1639, based upon his own calculations and ignoring the slip made by Kepler. For some reason the transit of Mercury in November 1644 passed unnoticed.

Another Englishman, Jeremiah Shakerley, later computed that a transit of Mercury would occur in 1651, but found it would be night in Britain when it occurred. Accordingly he traveled all the way to Surat in India to observe it. Solar eclipse chasing became a major pursuit in the Victorian era, but perhaps we should accord Shakerley some recognition as the first individual to make an intercontinental voyage in the quest for a glimpse of an eclipse of the fourth kind.

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