elongation

noun

• 1the action or process of lengthening something: the bird’s tails become forked through elongation of their outer feathers [count noun]:an elongation of the crater in the impact direction
• an elongated part.
• the amount of extension of an object under stress, usually expressed as a percentage of the original length.

• 2 Astronomy the angular separation of a planet from the sun or of a satellite from a planet, as seen by an observer: Mercury reaches greatest elongation on the 1st, when it will be 27 degrees west of the Sun