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➤ Mercury in Transit, May 9, 2016!

Transit duration for central europe: 7:12 to 12:33 Central European Summer Time

Transit of Mercury: 15th Nov. 1999, Astronomy Picture of the Day

On May 07, 2003, the planet Mercury will transit the disk of the Sun for the first time since the year 1999. But the observability of a transit of Mercury not only depends on the exact location of the observer, but also like all astronomical events on the weather at that location. These are the reasons why the last observable transit for a location in Europe happened on November 10, 1973.
Mercury Source: APOD
(The event on November 13, 1986 was a typical victim of the foggy European weather-conditions in November). Transits of a planet are rather rare events and only possible for Mercury and Venus. If all locations on Earth are taken into account, there are 13 or 14 observable transits of Mercury per century, while transits of the planet Venus are even rarer: only about 1 Venus-transit per century can be observed (the last one occurred in 1882, but the next one is on June 08, 2004, so be prepared for it!).
The next possibility for the observation of a transit of Mercury in Europe is on May 9th, 2016.

Every 116 days, Mercury passes between Earth and Sun, the so-called inferior conjunction. But some additional circumstances must be fulfilled to allow a real transit of Mercury, because its orbit around the sun has an inclination of 7° relative to that of Earth. For that reason, the biggest part of Mercury's inferior conjunctions happen a few degrees north or south of the solar disk and no transit can be seen from earth. Only when a conjunction occurs nearly the same time when Mercury passes its ascending or descending node (the two points, where its orbit touches the ecliptic), the tiny black disk of Mercury can be seen in front of the bright solar disk.
It is quite similar to an eclipse of the Sun: only when the new moon is very near to one of its nodes, it is in the right place to occult the Sun.

Observation-Tips: For the whole observation of the transit it is absolutely necessary to use a secure filter (e.g. foil-filter) in front of the telescope or other optics. Never look into the sun without a suitable filter! The rules for a secure observation are the same as known from a solar eclipse.

Because of the small diameter of Mercury's disk, the transit can only be observed with a telescope (it is not possible to see the event with the naked eye). But you don't need a professional instrument to see Mercury in front of the sun; a cheap telescope with a small lens or mirror, that can reach a magnification of about 40 x is enough. But don't forget a filter in front of it!

The events occurring during a transit of Mercury are called contacts, like the contacts of an annular solar eclipse. The first contact occurs when the planet's disk is externally tangent with the Sun. This happens always on the eastern (the left) limb of the sun, because the planets Mercury and Venus are always travelling retrograde, when they are in inferior conjunction. Some seconds later, the planet can be seen as a small notch in front of the sun. The entire disk of the planet can be seen some 5 minutes later at the second contact. The black, only 12 arc-seconds small disk of Mercury then crosses the sun within the next 5 hours to reach the third contact, where the planet touches the other limb of the Sun. The transit ends with the fourth contact, when Mercury is complete outside of the Sun's disk.

Contact times:

                       Universal-Time   CEST       Position-Angle

   First   contact        05:12:56     07:12:56         15°
   Second  contact        05:17:24     05:17:24         14°
   Third   contact        10:27:19     12:27:19        292°
   Fourth  contact        10:31:46     12:31:46        291°

   CEST = Central European Summer Time

The complete event will be visible from Europe, Africa and Asia. Japan, Australia, and New Zealand can only observe the start of the transit because the Sun will set there too early. People in Western Africa, eastern North America and Eastern South America will only see the end of the transit.

A brief history of transit-observations:

The first described observation of a Mercury-transit was in 1631. Kepler (1571-1630) published in 1629 an ephemeris, that is based on his famous "tabulea rudolfinae", and predicted a transit of Mercury for November 07, 1631, observable from Europe. Because of bad weather-conditions, only 3 persons were able to observe this transit. One of them was Pierre Gassendi (1592-1655), who used a projection of the sun through his telescope to see the event on a white screen. He used this transit to measure the apparent diameter of Mercury, and determined 20 arc-seconds. This value was much smaller than the Astronomers of these early days of the astronomical telescope have expected.

The tricky mercury wanted to pass by unnoticed; he entered earlier than expected, but could not escape without being discovered; I found him and I have seen him; something never experienced before by anybody; on November 7th, 1631, in the morning.

Johannes Hevelius (1611-1687), an Amateur Astronomer, observed the Mercury-transit of Mai 03, 1661, the same, that the famous Christian Huygens (1629-1695) saw during his stay in London.

Edmund Halley (1656-1742 image on the left), first realized that transits could be used to measure the distance of the Sun, thereby establishing the absolute scale of the solar system from Kepler's third law. The idea for this revolutionary method came from a journey to St. Helena, where Halley was the first person who measured a complete transit of Mercury on November 07, 1677. According to this observation, the 1761 and 1769 expeditions to observe the transits of Venus gave astronomers their first relatively accurate value for the distance of the Sun, the "Astronomical Unit".

Rudolf Wolf, Geschichte der Astronomie; Verlag Oldenbourg, München 1877
Eli Maor, Venus in Transit; Princeton University Press 2000

Maps of Mercury

In 1610 the italian astronomer Galileo Galilei observed Mercury for the first time with a telescope. Giovanni Zupus discovered in 1639 that Mercury shows phases like the Earth's moon. That proved that Mercury circled the Sun inside of the Earths orbit. That first charts of Mercury were not drawn until many years later.

Johann Hieronymus Schroeter produced the first maps of mercury. Schroeter built in Lilienthal (near Bremen, Germany) what was then Europe's largest observatory and founded the "Astronomische Gesellschaft" together with leading European astronomers like Franz Xaver von Zach and Wilhelm Olbers.
Unfortunately Schroeter scetches were not high quality. The detailed charts of Eugenios Antoniadi remained in use for 50 years.

Merkurkarte v. Schroeter Merkurkarte v. Schiaparelli Merkurkarte v. Lowell Merkurkarte v. Antoniadi Merkurkarte nach Mariner 10
J. H. Schroeter (1745-1816)
Source: AVL Lilienthal
G. V. Schiaparelli (1835-1910)
Source: NASA's History Office
P. Lowell (1855-1916)
Source: NASA's History Office
E. Antoniadi (1870-1944)
Source: NASA's History Office
After the Mariner 10 Mission
Source: M31

Mercury Mission: Mariner 10
Planned Mercury Missions: MESSENGER / BepiColombo
Mercury pictures Solarsystem DLR / NASA Photojournal
Mehr about Mercury: The nine planets / Planetscapes

9. Jänner 2003/KE, translated by MR