“Tirer des plans sur la comète” is one of the most comical French expressions. The literal meaning is “to draw plans on the comet”. English meanings including:
- to build castles in the air
- to make overly ambitious plans
- to chase rainbows
- pie in the sky
- pipe dreams
A French definition from expressio.fr defines the idiom as to make plans based on theories that are not possible and bound to fail.
Il ne faut jamais tirer plans sur la comète. Il faut et réaliste et baser ses projets sur la realité. One must never build castles in the air. One must be realistic and make plans based on reality.
Here are some synonymous expressions in French:
- faire des châteaux en Espagne – Literally to make castles in Spain
- faire des projets chimériques – Literallly to make fanciful plans