Путь к сокровищу стал на два часа короче

Question

From the Alchemist by Paulo Coelho,

Путь к сокровищу стал на два часа короче.

If I understand this sentence correctly, it means

The path to the treasure stood 2 hours closer.

But why is на used here ?

Answer 1

Стал is the past tense of стать, no longer related in modern Russian to any of стоять's prefixed perfective pairs. It's "the path to the treasure became shorter by two hours".

На is always used when specifying the amount with comparatives; an exception is раз ("times") which uses в, as in в два раза быстрее "twice as fast". This "в for multiplication" is also reflected in adverbialised compounds such as вдвое or втрое. Other than that, even fractions, percentages and logarithmics use на: на две третьих меньше "less by two thirds", на порядок дороже "more expensive by an order of magnitude".