Equator is derived from medieval Latin word "aequator", in the phrase "circulus aequator diei et noctis," meaning ‘circle equalizing day and night’, from the Latin word "aequare" meaning ‘make equal’. The latitude of the Earth's equator is, by definition, 0° (zero degrees) of arc. The Equator is one of the five notable circles of latitude on Earth; the other four are both polar circles (the Arctic Circle and the Antarctic Circle) and both tropical circles (the Tropic of Cancer and the Tropic of Capricorn). The Equator is the only line of latitude which is also a great circle — that is,
Longitude of the periapsis with part of it being measured in the plane of reference and the rest being measured in the plane of the orbit. Likewise, any angle derived from the longitude of periapsis (e.g., mean longitude and true longitude) will also be compound. Sometimes, the term "longitude of periapsis" is used to refer to "ω", the angle between the ascending node and the periapsis. That usage of the term is especially common in discussions of binary stars and exoplanets. However, the angle ω is less ambiguously known as the argument of periapsis. "ϖ" is the sum of the longitude of ascending node