To keep time is to tell time using a clock or other time-keeping device. You might use a clock to tell time when you're playing a game, or you might use a clock to tell time when you're working.
The word unchanging refers to something that doesn't change. The sky is always unchanging, and the stars are always in the sky.
To orbit is to go around a planet or a star. The Earth orbits around the sun.
When something is ticking, it's alive and constantly moving. You can hear it ticking if you hold it in your hand and let it go.
Pendulums are devices that are used to measure the motion of objects. They are usually made out of metal, and they have a weight at the bottom.
To vaporize something is to turn it into a vapor. When you vaporize something, you heat it up so that it becomes a gas.
Oscillate means to move back and forth, to swing back and forth. When the price of a stock oscillates, it moves up and down a lot over a short period of time.
To rotate something is to turn it so that it is facing the opposite direction. This is usually done so that it can be used more efficiently.