From the Northern Hemisphere, 30 constellations can be observed throughout the year, five of which are visible year around while the others appear seasonally. In 150 A.D., Ptolemy, a Greek astronomer, observed and created a list of constellations that, as of 2011, is still used. Ptolemy's list originally contained 21 visible constellations for the Northern Hemisphere, but others have been added since. All of the constellations are named after characters in Greek mythology.
The seven Northern Hemisphere winter constellations include: Canis Major, Cetus, Eridanus, Gemini, Orion, Perseus and Taurus. Each constellation contains star patterns that abstractly resemble the character it is named after. Because it is the brightest and easiest to recognise, Orion, or the Hunter, is thought to be the most famous of the winter constellations. Canis Major, or the Great Dog, is named after one of Orion's hunting dogs and contains Sirius, the brightest star in the sky. Only the moon, Venus, Mars and Jupiter appear brighter than Sirius, which is 8.7 light years away from Earth.
The six constellations visible from the Northern Hemisphere during spring include: Bootes, Cancer, Crater, Hydra, Leo and Virgo. Bootes, the Herdsman, contains the supergiant red star Arcturus, which is 37 light years from Earth and is 20 times larger than our sun. Hydra is the longest constellation in the sky and, in terms of area, is the largest. In Greek mythology, Hydra was a multi-headed serpent that grew its heads back immediately after they were cut off. As one of his 12 labours, Hercules slew Hydra. Virgo, or the Maiden, contains Spica as one of its stars. Spica is 260 light years away from Earth and is 100 times brighter than the sun. Scientists believe Spica is actually two stars that are orbiting each other very closely.
Aquila, Cygnus, Hercules, Lyra, Ophiuchus, Sagittarius and Scorpius make up the Northern Hemisphere's summer constellation line-up. In Greek mythology, Sagittarius is a centaur, with a man's torso on the body of a horse. This constellation features several celestial objects, including globular clusters and a recently-discovered galaxy that is crashing through the Milky Way. Lyra, or the Lyre, contains the star Vega, which is 26 light years away and is more than twice the size of the sun. The annual Lyrid meteor shower features meteors that appear to shoot out from the constellation Lyra.
Fall is the Northern Hemisphere's season with the fewest constellations. The autumnal constellation line-up includes: Andromeda, Aquarius, Capricornus, Pegasus and Pisces. Aquarius is one of the Zodiac constellations, which the sun passes through each year. This constellation is home to several globular clusters and the planetary nebula called the Saturn Nebula. Pegasus is symbolic of the winged white horse of Greek mythology and contains several galaxies and a bright globular cluster.
Constellations that can be seen year around from the Northern Hemisphere include: Cassiopeia, Cepheus, Draco, Ursa Major and Ursa Minor. Each of these constellations appear to rotate around the North Pole Star.