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Starman Special
by Jay Ryan
"Inferior Conjunctions and Transits of Venus"

(c) 1996 Jay Ryan

On June 10, 1996, Venus reached Inferior Conjunction with the Sun. On this day, the fast-moving Venus passed between the Earth and the Sun as it "lapped" the Earth in its orbit. The bright Venus disappeared into the sunset , where it had been visible as the "Evening StarĒ for a number of months. In the coming months, Venus will be visible in the morning skies before sunrise, in its alter ego as the ďMorning Star.Ē If youíre up before the Sun this summer and fall, be sure to look for Venus in the predawn sky.

Nevertheless, Venus did not pass directly in front of the Sun during this past Inferior Conjunction. Like all the other planets in the solar system, the orbit of Venus is slightly inclined to the plane of the Earthís orbit (also known as the Plane of the Ecliptic. Venusí orbit is inclined only 3 1/4 degrees and the orbit crosses the Ecliptic at the nodes. During the Inferior Conjunction of 1996, Venus was somewhat past the Descending Node (indicated by the arrow) the node where Venusí orbit crosses to the south of the Plane of the Ecliptic.

Venus was thus south of the Ecliptic on June 10, 1996, and did not cross over the Sunís disc. Venus was invisible, lost in the bright glare of the Sun. However, Venus does occasionally cross over the Sunís disc, an event called a transit of Venus. The next transit of Venus will occur the next time an Inferior Conjunction takes place in June.

Both Venus and the Earth are moving, and so it takes 584 days, or about 1 year, 7 months, for Venus to catch up to the Earth again. Thus, 584 days is the Synodic Period, or the Cycle of Venus, the period between Inferior Conjunctions. In this time, the Earth completes 1 7/12 orbits, but Venus completes 2 7/12 orbits. Thus, the next Inferior Conjunction occurs on January 16, 1998.

Each subsequent Inferior Conjunction in the coming years occurs one year and seven months after the one before, and the planets are displaced accordingly in their orbits. Inferior conjunctions will take place on: August 20, 1999; March 30, 2001; and October 31, 2002. The fifth Inferior Conjunction in the series will again occur in June, this time on June 8, 2004. It so happens that 5 cycles of Venus is very close to 8 Earth years, and so Inferior Conjunctions occur very close to the place where they occurred eight years before. However, this amount is not exact and so there is about a two-day shortfall in the Inferior Conjunctions after an eight-year period. Thus, the Inferior Conjunctions slip backwards bit by bit every eight years.

By the Inferior conjunction of June 8, 2004, Venus will have slipped a bit backward. Thus, Venus will be somewhat closer to the Descending Node than it was in 1996 and will be very close to the Plane of the Ecliptic during the Inferior conjunction of June 8, 2004.

Consequently, Venus passes over the disc of the Sun during Inferior Conjunction, across the southern limb, in a transit. This event is very similar in principle to an eclipse of the Sun, except instead of the Moon, the body of Venus is in front of the Sunís disc. Even a modest telescope with a proper Sun filter will be sufficient to permit viewing of the transit. It will take about 6 hours for Venus to complete its transit of the Sun. The last transit of Venus was on December 9, 1882. This will be the first transit of Venus in 121 1/2 years!

Eight years after that, Venus will have completed another cycle of five Inferior Conjunctions, and will have fallen back another two days from the date of the 2004 event. Thus, this Inferior Conjunction will occur on June 6, 2012. On this date, Venus is on the north side of the Descending Node, but still very near to the Plane of the Ecliptic.

Venus is close enough to the Ecliptic so that another transit will occur. At this time, Venus will cross the northern limb of the Sun, as it approaches the Descending Node. After the next eight year cycle, Venus will be too far from the node at Inferior Conjunction, and will pass to the north of the Sun, lost in the Sunís glare. After that, there wonít be another transit for 113 years, until December 13, 2117, when an Inferior Conjunction occurs on the other side of the Sun at the Ascending Node. Transits are very rare events and we are a privileged generation to have not one but two occur within our time.

See www.transitofvenus.org for more details about this celestial spectacle.  

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