Learn Spanish (Latin America): El movimiento de ese cometa en su órbita es muy rápido. - The movement of that comet in its orbit is very fast.

Cometa is one of those tricky words whose meaning changes depending on its gender. El cometa (masculine) means the comet in space, while la cometa (feminine) means the kite you fly in the sky. Since we are talking about orbits, it is masculine here, so we use ese.

Why do we use ese instead of eso for that?

Ese is the masculine singular adjective used right before a noun to point out a specific thing, like ese cometa. Eso is a neuter pronoun used for abstract ideas, general situations, or unknown objects, and it never goes directly in front of a noun.

Why do we use es instead of está for is very fast? Isn't speed something that can change?

Even though speeds can vary, here we are describing the inherent, general characteristic of this specific comet's movement. When defining the normal quality or fundamental nature of a noun (in this case, el movimiento), we use the verb ser (es).

Why is it rápido ending in -o when the closest noun is órbita ending in -a?