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
Near-Earth object for spacecraft. , five near-Earth comets and three near-Earth asteroids have been visited by spacecraft, and probes are en route to two more NEAs. Plans to mine NEAs commercially have been drafted by private companies. The major technical astronomical definition for Near-Earth objects (NEOs) are small Solar System bodies with orbits around the Sun that by definition lie partly between 0.983 (perihelion) and 1.3 (aphelion) astronomical units (AU; Sun–Earth distance) away from the Sun. Thus, NEOs are not necessarily currently near the Earth, but they can potentially approach the Earth relatively closely. However, the term is also used more flexibly