This occurs when two planets are the same number of degrees of arc North or South of the Ecliptic.

a line running east and west along the same latitude. The 49th Parallel is the western boundary between Canada and the United States.

east-west circle on the Earth’s surface

an imaginary line around the Earth parallel to the equator

a line that is due East and West in the PLSS Datum

That circle passing through all points of a given latitude and inscribed on the earth's surface by a small circle parallel to the equator.

a circle on the surface of the earth that is parallel to the equator. All points on a given parallel have the same latitude.

A circle, or approximation of a circle, on the surface of the Earth, parallel to the Equator, and connecting points of equal latitude; a circle of the celestial sphere parallel to the ecliptic, and connecting points of equal celestial latitude.

An imaginary line circling the surface of the earth that is parallel to the equator; a line of latitude.