Andromeda Constellation
Andromeda Galaxy, M31
The closest large galaxy to our own, at a distance of about 2.5 million light-years. It is a spiral galaxy that appears to be slightly larger but less massive than the Milky Way.
Anniversaries
Antarctic Astronomy
Astronomers and planetary scientists conduct extensive research in Antarctica. They operate telescopes at the south pole, launch instruments on balloons, and hunt for meteorites on the Antarctic ice. The balloon missions, launched in southern summer, take advantage of the 24-hour daylight to power solar cells and maintain a steady altitude without the need to drop ballast. They climb to about 130,000 feet, and can remain aloft for weeks or months. They follow circular wind currents that circle the continent, allowing the payload to drop back on the Antarctic ice at the end of a mission. Scientists from the United States and other countries conduct weeks-long searches for meteorites on the ice. Few terrestrial rocks are found atop the ice fields deep in Antarctica, so there's a good probability that any rocks on the ice fell from space.
Antares
The brightest star of the zodiacal constellation Scorpius. It is a binary system, consisting of an orange supergiant star that is many times more massive than the Sun, and a smaller companion that is hidden in the primary star’s glare. It is about 550 light-years away.
Aphelion and Perihelion
Aphelion is the greatest distance between a planet or other body and the Sun. Perihelion is the closest distance from the Sun. Objects move slowest in their orbits when they are aphelion and fastest when at perihelion.
Apogee and Perigee
Apogee is the point of greatest separation between Earth and the Moon or an orbiting satellite, while perigee is the point of closest approach.
Apollo Project
The NASA space program that landed the first humans on the Moon. From 1969 to 1972, it put six two-man crews on the lunar surface.
Aquarius, the Water Bearer
A constellation of the zodiac.
Aquila, the Eagle
A constellation best seen in the south in late summer.
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