Apologist!
In Michael Okuda’s subtitle commentary on the Star Trek II: The Wrath of Khan: Director’s Edition DVD, he says that Walter Koenig likes to joke about that. Koenig’s favorite theory is that, while Khan was wandering around off camera, he wanted to use the bathroom, but Chekov spent way too much time keeping him waiting – so when Chekov finally emerged, Khan made sure never to forget his face and vowed to take revenge on him some day. 
What with alien earwigs “wrapping themselves around the cerebral cortex,” and the explosion of Ceti Alpha VI sending out “shockwaves” through space causing Ceti Alpha V to “shift its orbit,” ST2:TWoK has far worse things to worry about than whether Khan ever saw Chekov before.
Heretic!
Speaking of the shifting orbits thing, I never understood how Ceti Alpha VI exploding would make them mistake Ceti Alpha V as being VI. VII would now be VI, not V.
That’s bugged me for years.
I was just thinking the same thing. When they enter a system, do they really spend time counting the planets, or do they just match up Planet V with what their computer says Planet V should look like. Seriously, though, the idea that anyone could mistake one planet for another is pretty silly.
If Ceti Alpha V “shifted its orbit” in such a way that it would later appear at a position where Ceti Alpha VI was expected to appear, and if the surface features of Ceti Alpha VI had never been surveyed before, then the crew of the reliant could easily have mistaken Ceti Alpha V for Ceti Alpha VI.
Planets are really small when compared to the size of the star system they’re in. Trying to locate and count each planet before establishing whether or not the planet beneath them was in fact the 6th farthest from Ceti Alpha may have just been considered a waste of time.
You’d think, however, that the name “Ceti Alpha” would have rung a bell with Chekov some time earlier during their mission, considering how he recognized the name “Botany Bay.”
Botany Bay=easy to remember
hundreds of systems with random greek letters as names=hard to distinguish
Okay, I did some researching. The first season of TOS took place in 2267 and the events of ST:TWOK took place in 2285 according to the Star Trek Encyclopedia. According to Khan in the movie (I FFWed to that part to hear it first hand) Ceti Alpha VI blew up six months after being marooned there. Since he was marooned there about halfway through the first season, that means it was around 2268 when the orbits started shifting.
That gives them seventeen years.
After that, I looked up info on the planets in our solar system. The planets that are closest to each other ar Venus and Earth at a mean distance of 26,000,000 miles or 2.36 light minutes.
That means that assuming Ceti Alphas V and VI were as close as Earth and Venus, the explosion would have had to have enough force to push a planet in the opposite direction at a speed of 1,529,411.76 miles per year or 1.55 light seconds.
Someone with a better idea of physics can try to explain the rest of this, like how big the explosion would have to be to move a planet at that speed. And how the people living on the planet managed to live. :dubious:
If a probe in the system had determined that Ceti Alpha V was habitable, they must have known plenty about it, like its mass, radius, and oblateness. So they go into the Ceti Alpha system to map a previously-unmapped planet, and see that it has identical parameters to Ceti Alpha V. Clearly, the only reasonable conclusion is that it’s Ceti Alpha VI, and it’s just a big coincidence, and it’s not even worth looking around for Ceti Alpha V to make sure they didn’t make a mistake. 
Achernar:
If Ceti Alpha V get hit hard enough by the explosion of Ceti Alpha VI to shift its orbit – and in order to produce enough force to do so, especially across the vacuum of space through which shock waves cannot propagate, Ceti Alpha VI would have had to have gone nova – then it would have also gotten hit hard enough for its physical parameters to change.
And besides, even if its mass and radius weren’t affected by the explosion, but everyone knows that in the Star Trek universe, all extrasolar planets have exactly the same mass and radius as the Earth. That’s why all alien planets have a surface gravity of exactly 1 g.
it doesn’t have to be opposite. observe the situation:
5
6
where 5 is Ceti Alpha 5 and 6 is Ceti Alpha 6. 5 is closer, but at this angle of rotation 6 is below it. six then explodes, but in giant zit explosion way
5
*
6
where * is the exploding debris shooting at 5. five then gets nailed and goes off orbit., regravitifiying up several million miles back, in the place of six. Meanwhile, another chunk of 6 heads forward to about where 5 was and gets stuck in an orbit there, and is erroroniously labeled as 5 with a do not go there warning when the USS NoFactChecker flew by for a rescan.
Or whenever you see something like that, Q did it!
Tars:
Where is Ceti Alpha (the star they’re orbiting) in those ASCII art drawings, relative to the two planets?
That’s fair, except that we see people living on the surface. An explosion with enough power to change the mass of a planet in any significant way would have screwed it over in a Praxis-like way. Although come to think of it, we saw someone on Praxis too. Hmm…
Yeah, but Khan had the fact of his vastly superior intellect which enabled them to survive.
I’m, at a rough guess, some forty billion times more intelligent than you, and even I don’t know why you just said a planet nova-d.
In Saturn’s rings are “shepherd” moons that regularly swap orbits.
But, Khan Noonian Sihng says it exploded, so now all of us have to figure out whether or not he lied just to manipulate circumstances.
new picture
Sun
5
*
\
6
Sun
5
\
6
uh, deep six that 6 in picture 2 and make it a . for debris
I feel like I’ve got a Ceti eel wrapped around my cerebral cortex.
Do you think a couple of Tylenol(s) would help?