Perigean spring tide Perigean spring tide A proxigean spring tide (also known as a perigean spring tide) is a tide that occurs three or four times per year when the Moon's perigee (its closest point to Earth during its 29.5-day elliptical orbit) coincides with a spring tide (when the Earth, Sun and Moon are nearly aligned every two weeks). This tide usually adds only a couple of inches to normal spring tides. The Moon's orbit around Earth is elliptical, which causes the Moon to be closer to Earth and farther away at different times. The Moon and the Sun are aligned every two
Earth orbit rendezvous of Project Gemini involving docking with the Agena target vehicle were designed partly to test the feasibility of this program. In the end, NASA employed the Lunar Orbit Rendezvous for the Apollo Program: a Saturn V would simultaneously lift both the Apollo Command and Lunar Modules into low Earth orbit, and then the Saturn V third stage would fire again (Trans-lunar injection) to send both spacecraft to the Moon. This mode had been revived for Project Constellation as the Earth Departure Stage (EDS) and Altair (LSAM), which would be launched into low Earth orbit on the Ares V rocket. The