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Wednesday, June 6, 2012

Transit of Venus

On 06 June 2012, a transit of Venus will be visible in the Philippines. The entire transit (all four contacts) is visible in Greenland, North and Central America, Pacific Is., Australasia, Asia (including the Philippines), E. Africa and most of Europe as shown in Figure 2.
Figure 2

The transit or passage of a planet across the face of the Sun is a relatively rare occurrence. As seen from Earth, only transits of Mercury and Venus are possible (an eclipse of the Sun occurs when the Moon passes directly in front of the Sun. Since the planets Mercury and Venus orbit inside the path of the Earth around the Sun, they too can come between the Earth and the Sun. However since these planets have a tiny apparent diameter as seen from Earth, the Transit is seen as a small black disk moving across the face of the Sun). On the average, there are 13 transits of Mercury in a century. In contrast, transits of Venus occur in pairs with more than a century separating each pair. Only Eight (8) such events have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874, 1882, 2004 and the upcoming 6 June 2012). The 2004 and 2012 transits form a contemporary pair separated by 8 years. The next two transits of Venus will occur on 2117 and 2125.

The principal events occurring during a transit are characterized by contacts, similar to the contacts of an annular solar eclipse. The transit begins with contact I, the instant when the planet's disk is externally tangent with the Sun. Shortly after contact I, the planet can be seen as a small spot along the solar limb. The entire disk of the planet is first seen at contact II when the planet is internally tangent with the Sun. During the next several hours, the silhouetted planet slowly traverses the brilliant solar disk. At contact III, the planet reaches the opposite limb and once again is internally tangent with the Sun. Finally, the transit ends at contact IV when the planet's limb is externally tangent to the Sun. Contacts I and II define the phase called ingress, while contacts III and IV are known as egress. Position angles for Venus at each contact are measured counterclockwise from the north point on the Sun's disk. Related important information on the forthcoming Transit of Venus are given in the accompanying table and diagram.


There are four named "contacts" during a transit — moments when the circumference of Venus touches the circumference of the Sun at a single point as shown in Figure 3.

a) First contact (external ingress): Venus is entirely outside the disk of the Sun, moving inward
b) Second contact (internal ingress): Venus is entirely inside the disk of the Sun, moving further inward
c) Third contact (internal egress): Venus is entirely inside the disk of the Sun, moving outward
d) Fourth contact (external egress): Venus is entirely outside the disk of the Sun, moving outward.
e) A fifth named point is that of greatest transit, when Venus is at the middle of its path across the solar disk and which marks the halfway point in the timing of the transit.

Geocentric Phases of the 2012 Transit of Venus

Contact I  6:09 AM

Contact II  6:27 AM

Greatest  9:29 AM

Contact III  12:31 PM

Contact IV  12:49 PM

Observing the Transit

The safest way to observe a transit is to project the image of the Sun through a telescope, binoculars, or pinhole onto a screen, but the event can be viewed with the naked eye using filters specifically designed for this purpose, such as an astronomical solar filter with a vacuum-deposited layer of chromium, eclipse viewing glasses, or Grade 14 welder's glass. An earlier method of using exposed black-and-white film as a filter is no longer regarded as safe, as small imperfections or gaps in the film may permit damaging UV rays to pass through. Also, processed color film (unlike black-and-white film) does not contain silver, and is transparent to infra-red. This may result in burns to the retina. Observing the Sun directly without filters can cause a temporary or permanent loss of visual function, as it can damage or destroy retinal cells.

Philippine nights are at their shortest and daytimes are at their longest around the Summer solstice, which falls on June 21 at 7:09 A.M. (Philippine Standard Time). This is the time when the Sun attains its greatest declination of +23.5 degrees and passes directly overhead at noon for all observers at latitude 23.5 degrees North, which is known as the Tropic of Cancer. This event marks the start of the apparent southward movement of the Sun in the ecliptic.


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