Ttyjukl

In Star Trek II: The Wrath of Khan (the one from 1982, not the recent remake), Khan yells "This is Ceti Alpha V!" and they're all like this is a big surprise, we didn't know.

If they had previously charted the planet, wouldn't the computer have told them the planets weren't quite where they should be based on earlier visits? I mean, we can predict the next hundred transits of Venus, shouldn't 23rd century explorers be able to predict where the Ceti planets are less than twenty years after their first visit?

This is explained in the original script and the novelisation;

• It was a pretty unusual star system to begin with. Ceti Alpha V and Ceti Alpha VI rotate around each other in a shared orbit. Some instability would be expected.




• The shock of Ceti Alpha VI exploding moved the main planet out of alignment so the charts would be inaccurate in any case (see below)




• In order to prevent Khan from being released, the planetary charts were deliberately erased.

KHAN: This is Ceti Alpha V ! Ceti Alpha VI exploded six months after we
were left here. The shock shifted the orbit of this planet and
everything was laid waste. Admiral Kirk never bothered to check on
our progress. It was only the fact of my genetically engineered
intellect that enabled us to survive!

On earth, two hundred years ago, I was a prince, with power over
millions -- now, like Prometheus I have been left by Admiral Kirk to
digest my own entrails. - _{Movie Script}

and

"You lie!" Chekov shouted. "I saw the world we left you on! It was beautiful; it was like a garden—flowers, fruit trees, streams … and its moon!" Chekov remembered the moon most clearly, an enormous silver globe hanging over the land, ten times the size of the moon on Earth, for Captain Kirk had left Khan and his followers on one of a pair of worlds, a twin system in which planet and satellite were of a size. But one was living, the other lifeless.

"Yes," Khan said, in a rough whisper. "Alpha Ceti V was that, for a while."

Chekov gasped. "Alpha Ceti V !" The name came back, and all the pieces fell into place: no official records, for fear Khan Singh would free himself again; the discrepancies between the probe records and the data Reliant collected. Now, too late, Chekov understood why he had lived the last few days under an increasing pall of dread. - _{Star Trek II: The Official Novelisation}

We in the 21st Century care about ephemeris data because our current spaceships move among the planets using Hohmann transfer orbits. We need to know where the planets are going to be so we know when to launch. We need to know how fast the planet is moving in its orbit so we know how much propellant to carry to produce the delta-vee needed to get there. We need to know these things because if we don't the ship will miss its target, run out of fuel and then everybody dies.

A 23rd Century starship with antimatter-fueled faster-than-light maneuvering and FTL sensing capability doesn't need to keep track of that kind of detail. They arrive in the system, scan for the planets, find one in roughly the right orbit and with matching surface characteristics and then drive straight toward it. Reliant arrived looking specifically for a barren lifeless planet roughly X million kilometers from the star and they found one. It's sloppy navigation work by our standards, but we don't live in a world where space navigation is as easy as guiding a ship from continent to continent with GPS.

They reasonably thought they were on Ceti Alpha VI because that had been it's orbit. Space is a very large and busy place so obviously Stellar Cartography is kept very busy.
A single planet taking another's place in orbit in a remote solar system could be easily missed.

It's possible also that the star chart could have mistakenly logged the surviving planet as Ceti Alpha VI and removed Ceti Alpha V as being destroyed. No one had been back to confirm this apparent fact since Khan and company had been placed there. They obviously had no idea that it was in fact Ceti Alpha VI that had been destroyed and it's orbit taken over by Ceti Alpha V. Who knew? Thinking that Ceti Alpha V was destroyed is reasonable due to it missing from it's orbit.

The kind of planetary 'musical chairs' that occurred is obviously very rare so their surprise by such an unusual/rare occurrence would be a reasonable reaction.

Chekov though, being there firsthand when Khan was marooned, was quick to put 2 and 2 together and recognized their danger after seeing the ship's name, 'Botany Bay'.

Given the sensors reported:

CHEKOV: "Does it have to be completely lifeless?"

TERRELL: "Don't tell me you've found something."

CHEKOV: "We've picked up a minor energy flux reading on one dynoscanner."

TERRELL: "Damn! Are you sure? Maybe the scanner's out of adjustment."

CHEKOV: "I suppose it could be a particle of preanimate matter caught in the matrix."

And seem to overlook Khan, his merry men and Ceti Alpha Five's only remaining indigenous lifeform. I would say there sensors are more than slightly 'out of adjustment' and they're lucky to know what side of the galaxy there on, much less how many planets are in the system.

We could chalk up the problem to equipment malfunction.

The thing that I just realized is that Kirk MUST HAVE known what had happened to Ceti Alpha V and VI by the time Khan was discovered by Chekov and Terrell.

If you go through the transcripts of 'Wrath of Khan', somehow Kirk knows what Khan 'blames him for', before Chekov or Terrell or anyone else has a chance to explain things. It really is as if Kirk is not too surprised about Khan's vengeful rage.

I think Kirk knew what had happened during Khan's exile, and decided to cover it up and let it go.