The Resonance of Venus

In Chapter 5, we discussed the remarkable resonance between the Moon's synodic and Draconian periods, which produce eclipses in continuing series. Gravitational interactions between Earth and Venus cause the orbits of the two planets to resonate in such a way that for every eight times Earth orbits the Sun, Venus completes fourteen. So every eight years Venus, Earth and the Sun return to almost exactly the same alignment in space. This creates a remarkable regularity in Venus's apparitions in both the morning and the evening skies. During each eight-year period,Venus will overtake Earth five times, producing five sets of apparitions that repeat themselves over and over each eight years. For example, on June 8, 2004 Venus reached inferior conjunction. Almost exactly eight years and five inferior conjunctions later, on June 6,2012, the planets will again line up in just about the exact same position. The conjunction will occur about forty hours earlier than it did eight years before. Each of the five apparitions is distinctly different from each other. In the apparition that ended in June 2004 Venus emerged from behind the Sun in August of the previous year and needed about four months to appear in the evening sky before the combination of increasing elongation and rising ecliptic brought the planet more rapidly into view. This apparition eventually becomes the most favorable of the five in the cycle because the planet reaches greatest elongation about ten days after the vernal equinox. About this time, Venus stands some 40 degrees high at sunset and sets as much as four hours after the sun. The morning apparition that follows is almost as good. The only difference is that Venus is south of the ecliptic at greatest elongation so its altitude is not quite as high. When Venus next returns to the evening sky for the second apparition of the cycle, it enters the evening sky quickly, but stalls out because it is pulling away from the Sun during summertime so the ecliptic lies progressively flatter with respect to the horizon even as Venus pulls away from the Sun, so the planet never climbs more than 20 degrees above the horizon. After inferior conjunction, in January 2006 the planet will have an equally poor apparition in the morning sky. After six more apparitions, Venus will return to the evening sky in August, 2011 and repeat almost exactly the evening apparition of 2003-2004, reaching inferior conjunction again on June 6, 2012. When Venus reaches inferior conjunction in March, as it will in 2008, it is near the northernmost point of its orbit and from our point of view appears to pass about eight degrees north of the Sun at inferior conjunction. This creates an opportunity each eight years to view the planet on conjunction day both in the evening sky at sunset and the morning sky at sunrise.

Another fascinating aspect of Venus-Earth resonance is that the planet's rotation is timed such that every fifth midnight on Venus, the planet presents exactly the same face to Earth. That means that at the end of each resonance cycle at a given inferior conjunction the planet will always present the same face towards Earth. If we could see surface features both through clouds and in the dark, Venus shows us exactly the same face at its inferior conjunctions in June 1980,1988,1996, 2004 and 2012. Venus will do so again at the inferior conjunctions of June 2020, 2028, 2036 and so on. The term "resonance" takes something of a beating here though because a resonance requires some type of gravitational interaction to occur and for their respective masses; Venus and Earth are just too far apart to have any effect on each other's rotational periods. It would appear that the rotational resonance of Venus is one of the more remarkable coincidences in science.

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