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People in many cultures have imagined that the brightest stars form constellations, which are apparent pictures in the sky seeming to be persistent, being deemed also as fixed. That way, constellations have been used for centuries, and still are today, to identify regions of the night sky by both professional and amateur astronomers.
Objects on hyperbolic orbits that do not show cometary activity also receive an A/ designation (example: A/2018 C2, which became C/2018 C2 (Lemmon) when cometary activity was detected). [34] Periodic comets also have a number indicating the order of their discovery. Thus Bennett's Comet has the systematic designation C/1969 Y1.
A general list containing 48 constellations likely began to develop with the astronomer Hipparchus (c. 190 – c. 120 BCE). As constellations were considered to be composed only of the stars that constituted the figure, it was always possible to use any leftover stars to create and squeeze in a new grouping among the established constellations.
Because of precession, the positions of the constellations slowly change over time. By comparing the positions of the 41 constellations against the grid circles, an accurate determination can be made of the epoch when the original observations were performed. Based upon this information, the constellations were catalogued at 125 ± 55 BC.
Such measurements show no shifts in the stars' positions. Unlike the Sun, Moon, and planets, they do not change position with respect to one another over human lifetimes; the shapes of the constellations are constant. This makes them a convenient reference background for determining the shape of Earth.
The constellation Crux as it can be seen by the naked eye Crux with clouds, from Cape Town. Within the constellation's borders, there are 49 stars brighter than or equal to apparent magnitude 6.5. [c] [18] The four main stars that form the asterism are Alpha, Beta, Gamma, and Delta Crucis. α Crucis or Acrux is a triple star 321 light-years ...
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A circumpolar constellation is a constellation (group of stars) that never sets below the horizon, as viewed from a location on Earth. As viewed from the North Pole , all fully visible constellations north of the celestial equator are circumpolar, and likewise for constellations south of the celestial equator as viewed from the South Pole .