Opposition (astronomy)

Because most orbits in the Solar System are nearly coplanar to the ecliptic, this occurs when the Sun, Earth, and the body are configured in an approximately straight line, or syzygy; that is, Earth and the body are in the same direction as seen from the Sun.

The instant of opposition is defined as that when the apparent geocentric celestial longitude of the body differs by 180° from the apparent geocentric longitude of the Sun.

[7] A more exact opposition occasionally occurs with mathematical regularity if the Moon is at its usual sun and Earth-aligning point so that it appears full and happens to be aligning with the ecliptic (Earth's orbital plane) during the descending or ascending phase of its 5° inclined (tilted) orbit, which is more concisely termed at a node of its orbit, in which case, a lunar eclipse occurs.

An inferior conjunction occurs when the two planets align on the same side of the Sun.

The "speed" at which a planet goes around the Sun, in terms of revolutions per time, is given by the inverse of its period, and the speed difference between two planets is the difference between these.