( I wasn't sure what to do for this until I thought about the line in Snakedance where the Doctor mentions teaching Nyssda and Tegan to read the TARDIS star charts. Also , threw in a callback to Spare Parts :))
The Doctor pointed to the screen; upon which a pattern of white and green lights were currently illuminating a black background.
" It's really quite simple to read" He explained, gesturing as two more small lights came on. " These charts are astronomically correct down to .01 percent of the entire galaxy. "
"I don't really see why we have to do this" Tegan said, her eyes so close to glazing over in boredom. " I mean, when was the last time this rickety old thing go t anything accurate, whether following a star map or whatever. "
"It's a star chart, Tegan" Nyssa corrected, taking in and hanging onto everything the Doctor was saying.
"Well, a star chart. Or an astral map" the Doctor explained, pointing to a particular constellation. "And still, I thought the two of you might find this interesting. This particular constellation is the Cherry Bowl nebula. "
" Oh yes, " Nyssa remarked, upon hearing the name. " I recall that one- before we ended up on Mondas?"
"L-let's perhaps not bring that up." The Doctor said, hands in his coat pockets. " I do wonder if that Cybermat did any more damage to the TARDIS." He shook his head.
Tegan frowned. " All right, now I've got to hear this story. There was a WHAT in here? " Since she'd returned, it seemed like she'd been more and more locked out of the loop the Doctor and Nyssa had.
" Back to the star charts" the Doctor cut in, trying to get back on track. " Right, then there's these clusters of stars. One of them , A11B , is believed to be made of pure ice and snow. And the next one will be colonized by Tegan's descendants in roughly the 33rd century. "