Something in the Way It Moves: Observing Retrograde Motion: Observer's Notebook for February, 2003

by John Rummel

“... When I trace at my pleasure the windings to and fro of the heavenly bodies I no longer touch the earth with my feet: I stand in the presence of Zeus himself and take my fill of ambrosia, food of the gods.”
– Claudius Ptolemy

Carefully observing the motions of the classical planets was something our ancestors did with great solemnity. Because the geometry of our solar system is so well understood now, it’s rarely done anymore. Nowadays, amateur astronomers are much more interested in looking at the planets rather than at how they move.

All of the superior planets trace out retrograde loops around the time of their oppositions. Since we usually observe the planets anyway around this time, we are invariably looking during the time of their retrograding. However, unless it is pointed out to us, or unless we make it a point to chart their motions against a conspicuous landmark, we usually fail to notice the direction of motion.

I recall watching the Mars opposition of 1999. It was not a great opposition since Mars was never larger than about 16 arcseconds. Remember, however, that we’re not as interested in looking at the planet, but how it moves. That year Mars ended its retrograde loop very close to the first magnitude star Spica. Mars is quite speedy, and watching this U-turn was very easy, and quite satisfying. There’s just something deeply gratifying to me about watching the mechanics of our solar system play out night by night. Maybe I’m imagining a spiritual connection with Kepler or Tycho.

Jupiter spends about 4 months out of every 12 going backwards, or eastward among the stars. Marking Jupiter’s progress this year would seem more difficult than most years. Spending the majority of its time in the constellation Cancer, with no bright stars to mark its path, Jupiter seems to be floating in the void between Leo and Gemini. There is a wonderful landmark in Cancer though–M44 or the Beehive Cluster–and Jupiter will approach and give it a “kiss” as it finishes retrograding this spring.

The ideal way to track Jupiter’s progress as it approaches and then hesitates before the Beehive is with binoculars. The wide field of view will be perfect for viewing Jupiter in the context of the star field surrounding M44. Telescopes are also fine, but use low power so you can get the widest field of view possible.
February opens with Jupiter at opposition (and right in the middle of its retrograde loop), so it’s at its brightest and up all night. At this point it’s about 6 degrees east of the Beehive, but it will close that distance by a little each night.

As February slowly proceeds, Jupiter closes on the cluster gradually, moving about 10 times its own diameter each night. This is sufficiently rapid movement that you should be able to follow its progress easily with binoculars or a small telescope. The closer it gets to the Beehive–and to its stationary point–the slower it will move. By the middle of March, it’s moving only about four times its own diameter each night, and will continue to slow until it finally stops, hesitates, and reverses direction around April 3rd. When it reverses course, it’s only about a degree away from the center of the Beehive, so you should be able to observe both in the same telescopic or binocular field of view.

Will Jupiter’s brightness overwhelm the stars of the cluster? Not at all. Though Jupiter is about 2000 times brighter than the brightest stars in the M44, your eye can easily observe this range of brightness. In June of 1999, I observed Venus in Cancer while vacationing in Door County. At magnitude -4.3, over six times brighter than Jupiter, the stars of the Beehive were still clearly visible as Venus passed almost directly in front of the cluster.

Telescopic viewers will have several chances to observe Jupiter’s progress as the planet passes some sixth magnitude stars on its journey. On February 8-10, Jupiter passes just to the south of two magnitude 6 stars, which should be visible in the same field of view on those nights. Again on March 12-14, it passes through a mini cluster of stars ranging from magnitude 7 to 9. About this same time it’s passing about a degree to the north of Asellus Australis, the 2nd brightest star in Cancer, at magnitude 4.

An exercise such as this is not something you’re likely to impress your neighbors with. If they want thrills, show them Saturn. If you enjoy the more sublime pleasures of amateur astronomy, it’s hard to beat the subtleties of celestial mechanics at work.

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