During the afternoon of Monday, November 11, we can see the silhouette of Mercury as it passes in front of the Sun. The transit will begin at 13h35m (Peninsular time) and will last until sunset. For your observation a small telescope will be needed. Planetary transits were decisive in the history of astronomy to determine the size of the solar system. Today they can be used as an exercise in citizen science. But attention! Never look at the Sun with the naked eye, or with binoculars, or with astronomical instruments without adequate protection.
Thirteen per century
Astronomers we call 'planetary transit' the passage of a planet before the surface of the Sun. When passing a planet in front of the king star a mini-eclipse occurs: we see how its round black silhouette is cut, backlit, on the solar surface and how it goes along it as the planet describes its orbit around our star. From a given planet you can only observe the transits of the innermost planets. Thus, from Earth we can only observe the transits of Mercury and those of Venus. But an observer (or a space vehicle) located on Jupiter can also observe the transits of Earth and Mars.
For a transit to take place, it is necessary that the Sun, the planet in question and the Earth be almost perfectly aligned in this order, something that only happens when the planet crosses, between the Sun and the Earth, exactly along the plane of the Orbit of our planet. This does not happen often. From Earth it is only possible to observe thirteen transits of Mercury per century and thirteen transits of Venus per millennium .
After the transit of Mercury on November 11, it will not be possible to observe another until November 13, 2032 (from Spain, only in the Balearic Islands). The next one, on November 7, 2039, will be from all the Spanish territory (although incompletely from the Canary Islands). To see a transit of Venus will have to wait until December 8, 2125.
Transit ephemeris
On November 11 the outer edge of Mercury will 'touch' the outer edge of the Sun (first contact or outside contact) at 13h35m (Peninsular time). After two minutes, the silhouette of the small planet will already be completely within the solar surface (second contact or inner contact). Half of the traffic, with Mercury at the center of its trajectory on the king star, will be given at 16h19m.
The third and fourth contacts, with the planet touching the inner and outer edge of the solar disk, will be towards 19h03m and will not be visible from the Peninsula because, for example, in Madrid the Sun sets that day at 18h01m. But the traffic will be visible in its entirety from the Canary Islands. If the sky is clear, it will be impressive to be able to see the sunset with the silhouette of Mercury still being cut before the solar disk.
The black spot of Mercury before the Sun will not be visible to the naked eye or with binoculars. It is necessary to use a small telescope of at least 50 magnifications. For the observation of the Sun you always have to take precautions (see the end of this article).
Transits that made history
The measurement of distances in the solar system was one of the greatest problems of astronomy since ancient times. Already in the 17th century, the only data available on the distance from Earth to the Sun had been obtained by Giovanni Cassini , the first director of the Paris Observatory, who had measured the parallax of Mars from his observatory and from Guyana, estimating the Earth-Sun distance in 140 million kilometers.
The measurement of distances in the solar system was one of the greatest problems of astronomy since ancient times. Already in the seventeenth century the only data available on the distance from Earth to the Sun had been obtained by Giovanni Cassini, the first director of the Paris Observatory, who had measured the parallax of Mars from his observatory and from Guyana, estimating the Earth-Sun distance in 140 million kilometers.
The great British astronomer Edmund Halley had realized, in 1716, that planetary transits could be used to accurately measure the distance from Earth to the Sun using the parallax method and Kepler's third law. He then devised an ingenious method to measure the distance to the Sun by observing the transits of Venus that, due to the larger size of the planet and its greater proximity, were much easier to observe than those of Mercury.
The measurement of distances in the solar system was one of the greatest problems of astronomy since ancient times. Already in the 17th century, the only data available on the distance from Earth to the Sun had been obtained by Giovanni Cassini, the first director of the Paris Observatory, who had measured the parallax of Mars from his observatory and from Guyana, estimating the Earth-Sun distance in 140 million kilometers.
The great British astronomer Edmund Halley had realized, in 1716, that planetary transits could be used to accurately measure the distance from Earth to the Sun using the parallax method and Kepler's third law. He then devised an ingenious method to measure the distance to the Sun by observing the transits of Venus that, due to the larger size of the planet and its greater proximity, were much easier to observe than those of Mercury.
Citizen science
Nowadays Halley's method can be used to measure the Earth-Sun distance again using transits. The principle is very simple: two observers located in distant parts of the Earth are needed to take simultaneous images of the traffic. Today we can help ourselves with good watches and GPS to measure the time at the moments of contacts and the exact positions on the planet.
The great British astronomer Edmund Halley had realized, in 1716, that planetary transits could be used to accurately measure the distance from Earth to the Sun using the parallax method and Kepler's third law. He then devised an ingenious method to measure the distance to the Sun by observing the transits of Venus that, due to the larger size of the planet and its greater proximity, were much easier to observe than those of Mercury.
However, in addition to the comparison of the measurements made between different positions, it is necessary to superimpose exactly the images of the Sun obtained in both places . This is easy when there is a spot on the surface of the Sun that serves as a reference to overlap. However, being in a solar minimum, the surface of the Sun now appears immaculate, without any detail that allows to superimpose exactly two different images. But there are also ways to get around this difficulty.
If you are an amateur astronomer and have minimal equipment, you can find technical advice to perform the measurements and even a partner who observes from another corner of the planet in this discussion forum of the Cloudy Nights web portal.
And if on November 11 it is cloudy at your place of observation, you can always use the solar images taken by the GONG solar telescope network: a collection of identical telescopes spread all over the world (one of them on the Teide) and that It has a good image file.
Never look directly at the Sun
Mercury is too small and too far away to see its transit to the naked eye, to observe it a telescope is needed. In any case, observing the traffic means looking at the Sun, something for which all necessary precautions must be taken.
Never look at the Sun with the naked eye and much less with binoculars or an astronomical instrument that does not have adequate protection. If a telescope is used, you must remove the search engine and use a full-opening sunscreen or a screen to project the image. The projection screen is the best and safest system. Aiming at the Sun is very simple, you don't have to look through the tube, just look for the position where the shadow of the tube is minimal.
Rafael Bachiller is an astronomer and director of the National Astronomical Observatory (IGN).
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