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Frequently Asked Questions about Transits


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What is a transit?

A transit is the crossing of a planet or any other celestial body in front of the Sun. Among the Solar System planets, only Mercury and Venus can transit across the Sun because they orbit closer in than the Earth. The Moon can also transit across the Sun and this is what we know as a solar eclipse.

What can we see during a transit?

During a transit we can see the disc of the planet projected on the bright surface of the Sun. The planet appears as a small dark disc (because of the brightness contrast between the night hemisphere of the planet and the surface of the Sun) that changes in position with time. The apparent size of Venus seen from Earth is about 30 times smaller than the size of the sun. By comparison, Mercury shows an apparent disk about 160 times smaller than the sun.

How can we observe a transit?

NEVER LOOK AT THE SUN DIRECTLY. Looking at the Sun directly without protection or trough eyeglasses (including sunglasses), telescopes or any other instrument not designed for solar observation may result in severe eye injury and blindness.

Because of the small apparent size of Mercury, it will be impossible to observe the planet's disc during the transit unless aided with an optical instrument. The safest way to observe the transit is by projecting the image of the Sun trough a telescope onto a white screen (see figure below). In addition, this procedure permits the simultaneous observation of the phenomenon to a group of people.

The case of Venus is slightly more favourable, because of its larger size it might be observable with the naked eye (ALWAYS UTILIZING A SOLAR FILTER). However, the projection method will yield a better viewing of the event.


How often does a transit of Mercury occur?

The last transit of Mercury were:

  • November 15, 1999. This traffic, however, was not visible from any point of Spain in any of its phases.
  • November 10, 1973, visible from Spain.
  • May 7, 2003, the last transit visible from Spain. Webcasted by ServiAstro
  • November 8, 2006. Invisible from Spain
And a transit of Venus?

The last pair of transits of Venus occurred on June 5 2012 and June 8 2004.

The transit of 2012 was hardly visible from the mainland, the 2004 change was visible in all its stages from all over the peninsula. The previous pair of transits of Venus occurred in 1874 and 1882. Only the second was partially visible from Spain.

When did the last transit take place?

The last transit of Mercury was on November 15, 1999. This transit, however, was not visible from Europe during any of its phases. The last transit visible from Europe occurred on November 10, 1973.

In the case of Venus the last pair of transits took place during 1874 and 1882. Only the second one was observable from Europe.

When will the next transits occur?

The next transit of Mercury will be on November 11, 2019 and from the mainland will be visible only in part, because the sun will set before the transit finished.
The next pair of transits of Venus will occur the years 2117 and 2125.

How long does the May 9 transit last for?

The transit of Mercury of May 9, 2016, has an approximate duration of 7 hours and 30 minutes.

In the most faourable cases, a transit of Mercury can last for up to 8 hours. On average, the transits that occur on November are shorter than those occurring on May, because the former take place when Mercury is closer to its perihelion (minimum orbital distance from the Sun) and, by Kepler's Second Law, it moves faster than near the aphelion (in years with transit, Mercury has its aphelion shortly before the beginning of May).

The maximum duration of a transit of Venus is also slightly over 8 hours.

How does Mercury look like?

Mercury is the planet closest to the Sun. It is prossibly the poorest known planet of the Solar System (besides Pluto). It has only been visited by one space mission, Mariner 10, in 1973 and 1974, which was just able to take images of one of its hemispheres. The picture resulting from Mariner's 10 observations is that of a planet without atmosphere and with a surface populated with numerous impact craters.

Some characteristics of Mercury
(from Astronomía General Teórica y Práctica. D.Galadí-Enríquez & J. Gutiérrez Cabello)
Mass 0.055 Solar masses
Equatorial radius 2439 km
Mean density 5.43 g/cm3
Rotation period 58.646 days
Orbital period 87.969 days
Average distance to the Sun 0.3871 AU (57900000 km)
Orbital eccentricity 0.206
Orbital inclination 7.0 degrees

Who was the first person to observe a transit?

The first observation of a transit of Mercury was registered in 1631 by the French astronomer Pierre Gassendi. Even though the telescope, necessary to observe the transits, was in use since until approximately 1610, no one observed the transits of 1615, 1618 and 1628, because their occurrence was not known. The release of new tables with more accurate positions of the planets (the Rudolphine Tables of Kepler in 1627) made possible the prediction and observation of the transit of 1931. The first transit of Venus was observed in 1939 by the briton J. Horrocks. As it turns out, Kepler had predicted a transit of Venus for 1931. However, despite the efforts of Gassendi, the transit was not observed because it occurred during nighttime in France (this fact was obviously unknown to Gassendi because of the limited accuracy of Kepler's tables). Kepler, however, did not predict a transit for 1639. It was Horrocks himself who, on October 1639, when comparing Kepler's tables with the older and inaccurate tables of Lansberg, realized that the latter were predicting a possible transit of Venus on November of the same year. Horrocks confirmed the prediction with his own calculations but the short time available caused that the event was not widely announced. Thus, only Horrocks, his brother Jonas and his friend Crabtree were aware of the occurrence with enough anticipation. Both Horrocks and Crabtree observed the transit and took some measurements but it seems that Horrock's brother never observed it.

What scientific importance has a transit?

Currently, the transits of Mercury are just a curiosity with almost no scientific interest. The same can be said of the transits of Venus.

Among the few contributions that Mercury's transits can make today is the study of Earth's rotational velocity. When the circumstances of a transit observed in the past are known (i.e., contact times and observing site), one can eventually infer small variations of the length of the day along the centuries.

The situation, however, was radically different up to the XIXth century. Until then, the transits of Venus, as suggested by Edmond Halley in 1716, had been used to attempt an accurate measure of the mean distance between the Earth and the Sun, namely the Astronomical Unit. Many scientific expeditions were organized to observe the transits of 1761, 1769, 1874 and 1882. But, in spite of all the efforts, several observational limitations, most notably the black drop effect, made impossible a sufficiently accurate determination of the Astronomical Unit through the method of the transits of Venus. During the second half of the XIXth century and along the XXth century, new methods were developed to determine the Earth-Sun distance, none of them implying the transit of Venus or Mercury across the Sun. The past strong scientific interest of these events was therefore lost.

Currently, only phenomena derived from the transits such as the mentioned black drop effect are subject of observation and study.

What is the "black drop" effect?

Shortly after the internal contact between the discs of the Sun and the planet (Mercury or Venus) something strange occurs. Instead of separating clearly from the solar limb, the disc of the planet seems to stick for a few seconds to the solar disc, and experiences a deformation in the appearance of a black drop. The same phenomenon repeats again just before the last internal contact.

The effect of the black drop prevents an accurate registration of the times of contact between the disc of the planet and the Sun, and was the chief cause of the failure of the attempts to determine the Earth-Sun distance using Halley's method. The main cause of the occurrence of this phenomenon is the turbulence of Earth's atmosphere, albeit the solar disc being slightly darker at the edge than at its center (known as limb darkening) is also partly responsible. We can simulate the black drop effect by joining our fingers slowly and observing the resulting shadow. Just before they reach contact a meniscus-shaped shadow arises and seems to join the fingers together, similarly to what occurs between the limb of the Sun and the planet.

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If you want a more complete explanation of the black drop effect follow the link TOM1999.jpg (in English, very technical).

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