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Information about transits

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What is a transit?

A transit is the crossing of a planet or any celestial object in front of the Sun. Mercury and Venus are the only Solar System planets that can give transits, because they are closer to the Sun than the Earth. The Moon can also give transits, and in this case the phenomena is called a Solar eclipse.

During a transit the disk of the planet is seen projected on the bright surface of the Sun.

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Image taken from the 2004 transit in Barcelona

with a CCD camera

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Projection of the 2004 transit

Transit Visibility

Observation times

During a transit we can distinguish some remarkable moments:

  • Contact I, or external ingress: the instant when the planet's disk is externally tangent with the Sun.
  • Contact II, or internal ingress: the instant when the planet's disk is internally tangent with the Sun. The entire disk of the Venus is first seen.
  • During the next several hours, Venus gradually traverses the solar disk at a relative angular rate of approximately 4 arcmin/hr.
  • Greatest transit is the instant of minimum angular separation between Venus and the Sun as seen from Earth's geocenter.
  • Contact III, or internal egress: the instant when the planet reaches the opposite limb and is once again internally tangent with the Sun.
  • Contact IV, or external egress:the instant when the planet's limb is, similarly to contact I, externally tangent to the Sun but at the opposite side of the Sun. The transit ends at this contact.
The next figure illustrates the geocentric observing geometry of each transit across the Sun (celestial north is up).

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Trajectory of Venus across the Sun during the transits of this century.

The 2004 transit crossed the Sun's southern hemisphere while the prediction for 2012 event is that Venus will cross the northern hemisphere.

Global Visibility

The transit is not visible everywhere on Earth. Depending on the altitude of the Sun in a place during the event, the transit will be enterily, partially or not at all visible.

  • The four contacts will be visible from northwestern North America, Hawaii, the western Pacific, northern Asia, Japan, Korea, eastern China, Philippines, eastern Australia, and New Zealand.
  • The Sun sets while the transit is still in progress from most of North America, the Caribbean, and northwest South America.
  • The transit is already in progress at sunrise for observers in central Asia, the Middle East, Europe, and eastern Africa.
  • No portion of the transit will be visible from Portugal or southern Spain, western Africa, and the southeastern 2/3 of South America.

In this Contacts table you can see the four contacts of Venus for more than a hundred international cities.

Table below shows the contact times for some Spanish cities. The Sun will be bellow the horizon almost for all of them. Even in the eastern part of Catalonia and in the last moments of the transit the Sun will be too low to be able to enjoy.

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In this link you can find contact times for every city of the world.

This map represents the visibility of the transit over the world.

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Visibility map of the Transit

Depending on the Time Zone of the observers they will be able to see the transit on 5 afternoon or on 6 morning. The event is invisible in all places where the Sun is not over the horizon during the time of the transit.

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