![]() ![]() The opposite of periapsis.Īpogee: The point in its orbit where an Earth satellite is farthest from the Earth. The opposite of perihelion.Īpoapsis: The point in an orbit when the two objects are farthest apart. Earth is at aphelion around July 3 each year. The opposite of periastron.Īphelion: The point in an object’s orbit where it is farthest from the Sun. The Antarctic Circle marks the northernmost point in the southern hemisphere that experiences the midnight sun.Īpastron: The point of greatest separation between two stars that are in orbit around each other. ![]() At the peak of an annular eclipse, the Moon is surrounded by a ring of sunlight.Īntarctic Circle: The line of latitude on Earth’s surface that is 23.5 degrees north of the South Pole. A rotating body also possesses angular momentum in its spin.Īnnular eclipse: A solar eclipse where the Moon passes directly in front of the Sun but is too far from Earth to completely cover the solar disc. Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion. If the orbit is elliptical, the radius will vary and since the mass is constant, the velocity must change. According to the laws of physics, the angular momentum of any orbiting body must remain constant at all points in its orbit that is, momentum cannot be created or destroyed. Many astronomers believe supermassive black holes may lie at the center of these galaxies and power their explosive energy output.Īngular momentum: A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit. The Sun has an absolute magnitude of +4.8.Īccretion: The accumulation of dust and gas onto larger bodies such as stars, planets, and moons.Īccretion disk: A relatively flat sheet of gas and dust surrounding a newborn star, a black hole, or any massive object that is growing in size by attracting material.Īctive galactic nuclei (AGN): A class of galaxies that expell massive amounts of energy from their centers, far more than ordinary galaxies. It is used to compare the true, intrinsic brightnesses of stars. A B C D E F G H I J K L M N O P Q R S T U V W X Y ZĪbsolute magnitude: The apparent brightness-or magnitude-a star would have if it were 10 parsecs (32.6 light-years) from Earth. ![]()
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