then-current means: current (up to date) at that time in the past.

Then-current is written with a hyphen because the two words "then and current" both (act as adjectives) describing/modifying the noun "theories".

then-current is called a “compound modifier” or “phrasal adjective” or "compound adjective" (compound means two words joined together as one word with a hyphen between the two words, e.g.,

bathroom = compound noun

work-related tasks = the compound adjective "work-related" describes/modifies the noun "tasks".

You asked a good question.

You could also say, for example, "In 1930, the then-president decided to ..." This makes clear that you are referring to the president who was in office in 1930.

Hi Tania,

Yes in answer to your questions "then-current" means at the time, at the particular period or moment in the past that is being discussed in this text.

Hi Tanya, great question!

The text is saying that Galileo made a discovery that disproved (upended) the theories that were generally accepted at the time of his discovery.

"then-current" is how you talk about something that used to be up to date (current) at a time in the past. In this case, we are talking about the theories that were current as of 1610.

This is tricky because we are talking about something that used to be current.