|
stellation Taurus, starting today, as it begins its retrograde motion in front of the stars. In astronomy, "retrograde motion" of an outer planet like Jupiter happens when that planet seems to stop and then change its normal eastward direction of motion in front of the background stars. That is what Jupiter does starting today, October 4, 2012.
• Normally, Jupiter moves toward the east in front of the stars as seen from Earth. Tonight, the planet appears poised in front of the stars - moving neither east nor west. Astronomers call this called Jupiter's "stationary" point. Afterwards, Jupiter will begin moving toward the west as seen from our earthly vantage point.
• What's happening here? Has Jupiter really changed its direction of motion in orbit? No. What we're seeing is an illusion, which baffled the ancient astronomers, but which now seems perfectly ordinary.
• Think of Jupiter in orbit around the sun. Jupiter may be a giant planet, but in contrast to Earth it moves like an oxcart in the race around the sun. The Earth's average speed is about 67,000 miles an hour, while Jupiter lumbers along at less than half that speed, or about 29,000 miles an hour.
• Because of its faster speed and shorter distance to go around its orbit, our Earth laps Jupiter about once every 13 months. It's a lot like a fast race car in the inner track passing a slower car in the outer track.
• The race car analogy works well to explain the phenomenon of "retrograde motion." Suppose you are the driver of an inner, faster race car (Earth). As you move faster than a slower race car (Jupiter) on an outer track - say, just before passing it - you see the slower (Continued on page 15)
|
|
