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Duration of retrograde period of superior planets

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Duration of retrograde period of superior planets

All planets revolve around the Sun in the same direction, and their orbital periods are all calculated relative to Earth time, we can measure successive conjunctions and oppositions between the planets according to Earth-view (see fig. i.4, p. i4; this figure shows graphically the ratio of direct to retrograde planetary motion). The superior planets’ periods are, all much greater than the Earth year and, therefore, each of their conjunctions or oppositions to the Sun will occur a bit tater each year. These contacts, usually applied to successive conjunctions, are termed ‘synodic’. The more distant a planet is from the Sun, the slower is its motion, thus a slightly longer period of time passes between each successive conjunction or opposition. Since this is all taking place from Earth-view, we are looking at synodic cycles of the Sun and superior planets.

As we are dealing with retrograde planets, we shall talk about successive opposition points. These opposition points always occur some days later each year, again because of the slow forward movement of each superior planet.

Mars: Of the superior planets, Mars is the only one that breaks the pattern, because its orbital period is slightly less than twice that of Earth. Mars orbits the Sun once every 687 days, while the Earth does so in 3651/4 days. Its opposition cycles relative to the Sun are not annual, as are the cycles of all the other superior planets, but do occur regularly, approximately every two years and two months. Successive conjunctions with the Sun indicate a mean synodic period of 780 days. It is retrograde for approximately 9 per cent of its cycle, which amounts to about sixty to eighty days every twenty- six or so months.

Jupiter: Jupiter orbits the Sun once every twelve years and its annual motion (increment along the ecliptic) is about 300. Its opposition cycle, therefore, occurs about thirty days later each year. Jupiter is retrograde about 30 per cent of the time, or for approximately 110 days per year. Its mean synodic period is 398.9 days.

Saturn: Saturn’s orbital period is about twenty-nine and a half years and its oppositions to the Sun are about twelve to thirteen days later each year. Saturn is retrograde for about 36 per cent of its cycle, or about I3 days per year. Its mean synodic period is 378.1 days.


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