The next Venus transit will happen only in 2117

If the subject of astronomy interests you, here is a unique celestial treat you may not want to miss. Astronomers worldwide are gearing up to view a rare event occurring in the solar system. There will a significant alignment of planets (Earth and Venus) with the Sun on June 6 that will not be seen for another 105 years. Clear skies permitting this eagerly-awaited event will be ushered in along with the rising Sun in Chennai at 5.43 a.m.

Occurring after a gap of eight years (Venus transit last occurred on June 8, 2004), the silhouette of Venus will appear like a black moving dot on the sun's northern limb. It will span the surface of the sun in nearly four hours. Finally by 10.22 a.m., the celestial guest will bid adieu to sky watchers all over Tamil Nadu. Globally, the transit will begin at 3.39 a.m. (IST) and end at 10. 22 a.m. (IST) on June 6.

The phenomenon

Planetary transits are a bit like annular eclipses, except that the coverage of the Sun's surface is smaller. “The only planets that cause this are Mercury and Venus as their orbits lie inside the Earth's orbit. When they move between the Earth and Sun, they cover a very small area and there is no noticeable decrease in the amount of sunlight reaching us,” says Dr. P. Iyamperumal, executive director, B. M. Birla Planetarium in Chennai. Transits of Venus are rarer than those of Mercury (13 each century), he says.

Being closer to earth, transits of Venus occur approximately 4 times in 243 years with a strange pattern of frequency. They occur in pairs, separated by eight years, or in a gap of over a hundred years. The reason for these long intervals is because of the difference in the orbital planes of Earth and Venus.

“If Venus and Earth orbited the sun in the same plane as the sun, transits would happen frequently. But since Venus' orbit is inclined to that of earth by about 3.4 degrees, every time Venus passes between the Earth and sun every 1.6 years, the planet usually is a little above or below the sun and invisible in the sun's glare,” says Dr. Iyamperumal. As viewed from the Sun, the orbits cross at two points (called nodes) and it is only at these points that the planets and the sun line up directly.

The transit

Transits begin with contact I when the limb of Venus will be externally tangent with the sun. It will take about a few minutes for the planet's disk to cross the solar limb when it will become internally tangent with the sun at contact II. The period between contacts I and II is known as ingress. At the end of the ingress, the entire disk of Venus will be silhouetted against the sun's disk as it begins its slow passage across the star.

Contact III occurs when Venus touches the opposite limb of the sun. After a few minutes, the planet will exist in the solar disk. Finally, the transit will end with contact IV when Venus's disk will totally leave the Sun and vanish from sight.

As sunrise occurs at 5.43 a.m. in Chennai, only the last two contacts will be visible. Similarly, as the sun rises at different times in different places all over Tamil Nadu, the initial time of observation will be different. But contact III will occur almost uniformly at 10.04 a.m. and contact IV at 10.22 a.m.

But all the four contacts will be visible from north-western 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 when seen from most of North America, the Caribbean and northwest South America.

Transits of Venus are among rare planetary alignments. So far only seven such events have been recorded. 1631, 1639, 1761, 1769, 1874, 1882, 2004. The one that will be occurring this year is the last one of the century, with the next bound to happen only in 2117.

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