I don’t understand why you say that.
Consulting an ephemeris of Venus, we find it decreasing in right ascension–that is, moving retrograde from east to west–between December 23, 2005 and February 4, 2006. On February 4 it will resume prograde motion and begin gaining in right ascension. Note that Venus is approaching inferior conjunction at which time it will move from the evening to the morning sky.
Stranger on a Train? Sorry. Don’t know him. Never heard of him.
As I was saying, I thought remembered that Venus and Mercury never retrograded, but that was a long time ago, and I could be wrong.
Here’s a simple way to see that inferior planets must also exhibit retrograde motion.
The motion of a planet (say, Mars, as seen from Earth) across the sky is described by the rotation of the vector EM which points from Earth to Mars. Most of the time this vector rotates in one direction, “prograde,” but near superior conjunction (when Mars is opposite the sky from the Sun) it rotates in the other direction, retrograde.
Now think of Earth, as seen from Mars. Now the governing vector is just ME, the negative of the vector EM. Clearly the directions of rotation of EM and ME are the same, so Earth-as-seen-from-Mars is retrograde at the same time as is Mars-as-seen-from-Earth (that is, at Earth’s inferior conjunction with Mars).
Of course, as stated earlier, inferior conjunctions happen at relatively low angular separations from the Sun, so they are difficult to view.
'Cause I was thinking of an observer using the Sun (or rather, orientation with respect to the Sun) as the reference frame. However, if your reference frame is the stellar background, then the inner planets to indeed appear to engage in retrograde motion. Mea culpa.
Stranger
Well that sounds very logical, but let me check for myself. Hang on a minute…
Okay, I’m back. I just got back from Venus (and boy are my arms tired), which, as Freddy pointed out, is in retrograde right now. I looked back to the Earth, and I’ll be damned if it’s not in retrograde, too. How about that!
There’s nothing like a little empirical evidence to support a plausible theory!
Actually, Mercury and Venus would appear to travel back and forth across the Sun. From our perspective, they’d be circling it, but we’d be looking at the circle edge on.
But you’d see them recede (well, assuming that you could see them well enough to measure distance from size or albedo) and disappear behind the Sun (if it didn’t burn right through your retina). From a standpoint of using the Sun as a fixed reference (i.e. your observer is always pointed toward or the same orientation relative to, the Sun), the inner planets do not display apparent retrograde motion. Because I deal with rockets, satellites, and so forth, I was thinking of it in terms of orbital ballistics. However, an astronomer, and particularly one who believed in geocentricity, would use the relatively invariant stellar background as a reference, in which case the inner planets do demonstrate a retrograde lag and lead loop.
In any case, no celestial body in a simple two body configuration will display epicyclic behavior in proper motion; it’s all a matter of perception by the observer. Some bodies, like Neptune’s moon Triton, display a retrograde orbit, meaning that it orbits in a direction opposite of planetary rotation, but it still follows Kepler’s Laws. A three-body or N-body system could create retrograde proper motion but it’s highly unlikely such a system would be naturally stable if the distortion were significant, and as with any N-body problem the system would be easily perturbed to an unstable state.
Stranger