21 and 7 years make up the Earth-Mars synodic period. The ecliptic plane is where the orbits of Earth, Mars, and the cycler trajectory are located.
Synodic times are significant because they frequently correspond to direct observables, but physical rotation periods do not. This significance was crucial for determining the planets' orbital periods throughout the history of astronomy.
Synodic period: The amount of time it takes for a body in the solar system, such as a planet, the Moon, or a man-made satellite to return to the same or nearly the same location with respect to the Sun as seen from the Earth.
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