Sunday, May 1, 2016

Planetary Transits - Rare Astronomical Phenomenon

Planet Mercury is normally seen along the Sun. It rises along with the Sun and is clearly visible at time of Sunset. As it is one of the innermost planets in the solar system, always appear only a small distance away from the Sun in the sky. Mercury is so small and so close to the Sun (always within 28 degrees) that it is difficult to see from Earth during the day, since it is usually lost in the Sun's glare.
But Monday 9 May 2016 and 10 November 2016 a rare astronomical event will be visible from Earth. That is transit of Mercury across the face of the Sun. Mercury will be clearly visible as a black dot on the face of Sun. This transit takes place when Mercury passes directly passes through in between earth and the Sun. Planet Mercury will appear as a dot crossing over the face of Sun, but unlike a solar eclipse, Mercury will only block a small amount of light from the Sun.
Nearly 13 transits of Mercury take place in a century (just once every eight years on average), making it fairly rare events. The last time this happened was in 2006, and the next two occasions will be in 2019 and 2032.

During a transit, Mercury which appears to be only 1/158 of the Sun`s diameter, will be seen as a tiny black dot moving slowly in an East-to-West direction across the Sun. For an Indian observer this would mean that Mercury will move from the "top" towards the "bottom" of the Sun as it moves towards the western horizon.
The 2016 transit commences on May 9th at 11:12 GMT (16:42 IST) and ends later that same day at 18:42 GMT (00:12 IST), with mid-transit taking place at 14:57 GMT (20:29). The total duration is therefore about 7½ hours.
Transit can be viewed from anywhere on the Earth where the Sun is above the horizon at the time of the event.
In India, we will catch only the initial part of the transit before sunset. Most of Central and Southern part of India will see up to the initial 1/4th of the transit and Northern and Western part of India will see the initial 3/8th of the transit. The Eastern part of India will see the initial 1/8th of the transit only. This translates to 2 hrs, 3 hrs and 1 hr respectively. Discounting the last hour (haze, clouds, dust, landscape etc.), that gives us 1, 2 and 0 hrs respectively.
Mercury completes each orbit around the Sun every 88 days, and passes between the Earth and Sun every 116 days. Since Mercury's orbit is inclined seven degrees to Earth's, it intersects the ecliptic at two points or nodes, which cross the Sun each year on those dates. Mercury does pass between Earth and the sun every four months or so, we only get to see it transit when everything lines up just right. If Mercury passes through inferior conjunction at that time, a transit will occur. A transit can only take place when the Earth, Mercury and the Sun are exactly in line in three dimensions.

Circumstances of Mercury Transits
Transits of Mercury can only occur whenever the planet is close to its ascending node or descending node (the points in its orbit where the planet crosses the ecliptic - the apparent path of the Sun against the background stars) heading Northwards or Southwards, respectively. For such an event to take place, the planet must be close to inferior conjunction (i.e. positioned directly between the Earth and the Sun) and also be positioned within about 2° of one of these nodes. Because of the positioning of the nodes in relation to the Earth's orbit, in the present era transits can only take place in the second week of May (descending node) or in the second week of November (ascending node). The ascending and descending nodes of the planets are positioned exactly 180° apart - hence the six-month difference between these two dates.
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