On November 11 between 12:35 and 18:04 Universal Time, the tiny black silhouette of Mercury will slowly pass the Sun's disc. Such a passage is called a transit of Mercury,1 and we are organizing a project to observe, photograph, and measure the transit.2 Our project will bring people together from all over the world so that they can compare their own observations with those of astronomers at distant places and determine the distance between the Sun and Earth. Additional participants in our project, especially from the American continent, are most welcome.

Since Mercury's orbit around the Sun is inclined with respect to Earth's orbit, Mercury will usually pass above or below the Sun (relative to an observer on Earth) and be invisible during its conjunction with the Sun. Transits of Mercury, therefore, are quite rare (12–13 per century). The 2019 transit will be the last chance until 2032 to see Mercury cross in front of the Sun and to reenact the fascination felt by astronomers in past centuries and by amateurs while observing the transits of Venus in 2004 and 2012.3 

Because of its diminutive size (an angular diameter of less than 10 arcsec in front of the solar disc of more than 1900 arsec), Mercury cannot be observed without magnification. Trying to see the transit by projecting the image of the Sun with a magnifying glass will be difficult. But you can view and photograph Mercury crossing the Sun by using a small telescope equipped with a solar filter. People in Europe and Africa will only be able to view the first part of the transit. For most of North America, the transit will begin before sunrise, but it can be observed until its end. People in South America will be able to view the entire transit.4 

When photographed with a camera so that the lower edge of the images is kept parallel to the horizon, the curvature of Mercury's transit will depend on the geographical position of the observer (see Fig. 1). However, by rotating the images so that north is always at the top, all observers will observe a linear transit of Mercury. Nonetheless, there will still be small differences: (i) distant observers will see Mercury enter the Sun's disc and exit it at times differing by up to 60 s. (ii) The positions of Mercury on the Sun's disc as seen in photos taken at different places at exactly the same time will differ by up to half of the diameter of Mercury's disc (see Fig. 2).5 

Fig. 1.

Superposition of 15 images, each separated by 30 min, of the 2016 transit of Mercury, taken with a horizontally mounted camera from Hannover, Germany.

Fig. 1.

Superposition of 15 images, each separated by 30 min, of the 2016 transit of Mercury, taken with a horizontally mounted camera from Hannover, Germany.

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Fig. 2.

Superimposed 14:00:00 UT exposures of the 2016 transit from Caracas, Venezuela and Hannover, Germany.

Fig. 2.

Superimposed 14:00:00 UT exposures of the 2016 transit from Caracas, Venezuela and Hannover, Germany.

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These parallax effects occur because Mercury is not as far from Earth as is the Sun. By measuring the effect on Mercury's position, we can determine the distances of Mercury and the Sun from Earth. The Earth-Sun distance, called the astronomical unit (au), is the basic benchmark for determining the size of our solar system and, therefore, the basis of our knowledge about the size of the universe. While observing the 1677 transit of Mercury, the famous astronomer Edmund Halley6 realized that transits could be used for measuring the astronomical unit. In the 18th and 19th centuries, several countries organized expeditions to locations all over the world to observe and measure transits of Venus. At the time, this was the best way to determine the value of the astronomical unit.7 

Our project to observe the transit of Mercury offers an excellent opportunity for students and amateurs to increase their awareness of astronomical phenomena and the history and nature of science. The project gives them the chance to repeat historical measurements with modern equipment, to make their own measurement of the au, and to experience the fascination of scientific discovery and the difficulties that had to be conquered in order to achieve a satisfactory accuracy.

1.
More information about transits of Mercury can be found online at <https://en.wikipedia.org/wiki/Transit_of_Mercury>.
2.
Our project is described in detail at <http://www.transit-of-mercury2019.de/>. There we make proposals for photographing Mercury crossing the Sun and offer the possibility of publishing one's own photos and of finding photos taken simultaneously at distant sites. Additionally, we describe methods of measuring Mercury's position with respect to the Sun and an algorithm for deriving the solar parallax from simultaneously determined positions.
3.
See, for example, <http://www.venus2012.de>.
4.
For detailed information about the local circumstances of the transit, see e.g., <http://xjubier.free.fr/en/site_pages/transits/ToM_2019.html> and the Mercury transit calculator, which can be found there.
5.
In 2016, Mercury's distance from the Earth was smaller and its distance from the Sun was larger than they will be in November 2019. Thus, the parallax effect in 2019 will be smaller than it was in
2016
.
6.
“The life of Edmund Halley and his contribution to the method of measuring the distance to the Sun by observing transits of Venus are shortly described at,” <https://en.wikipedia.org/wiki/Edmond_Halley>.
7.
D.
Hudon
, “
A (not so brief) history of the transits of Venus
,”
J. R. Astron. Soc. Can.
98
(
1
),
6
20
(
2004
), available online at <http://people.bu.edu/hudon/transit_jrasc_final.pdf>.