Momentary means belonging to the moment, lasting for a short time.
Momentous means important.
1. There was only a momentary loss of the great vision in that immortal poet.
2. A man should not purchase life-long pain for momentary pleasure.
3. A momentary feeling of fear.
1. On that momentous occasion all the patriots in the land joined hands and opposed the tryannical government.
2. A momentous event.