What does 'standby' mean in this context?

Question

Harry and Ron took their copies of Unfogging the Future back down to the common room, found a table, and set to work on their predictions for the coming month. An hour later, they had made very little progress, though their table was littered with bits of parchment bearing sums and symbols, and Harry's brain was as fogged as though it had been filled with the fumes from Professor Trelawney's fire. “I haven't got a clue what this lot's supposed to mean,” he said, staring down at a long list of calculations.

“You know,” said Ron, whose hair was on end because of all the times he had run his fingers through it in frustration, “I think it's back to the old Divination standby.”

“What - make it up?”

“Yeah,” said Ron, ...

I don't quite understand the meaning of the whole sentence “I think it's back to the old Divination standby.”, especially the word 'standby'. The closest definition I got from dictionaries is:

2. a person or thing ready to serve or be put into service on an emergency basis or as a substitute

But I'm not sure if it's correct. How should we understand it in this context?

Answer 1

You have the correct meaning of 'standby'.

Ron and Harry had tried using Unfogging the Future to solve a problem but that had failed. Their next attempt - their standby method - would be to use Divination.

Answer 2

The other answer is correct in noting that you have understood the term "standby" in the proper manner. However, the answer confuses things a bit with the rest of the sentence. It is not "using Divination" that is the standby. Divination is what they will be using the standby instead of.

Harry and Ron are doing their homework assigned in their Divination class. Unfogging the Future is the textbook for that class. Thus, the "correct" way to do the homework would be to use Unfogging the Future to come up with their predictions. That would be actually using Divination. However, when they try that method they have no success. So they decide to move on to the backup plan, which is to not use Divination (via the textbook), but instead to simply make up the predictions themselves.