The definition of the term "constellation" has changed throughout the ages. In the current day, there are 88 constellations officially recognized by the International Astronomical Union
#. As astronomers currently use the term, every spot on the night sky falls within the boundaries of some constellation--the constellation is not just the bright stars, nor the lines or pictures that sometimes connect them in fanciful drawings. Every new star, molecular cloud, or galaxy that is discovered is already in a known and assigned constellation--much as if a biologist were to discover a new species of plant, that plant is already living in some pre-existing country.
This scientific division of the sky into regions called constellations is not the way the term has always been used, nor is it what most people think of when using the word in common English. The human brain searches for pattens, and the sky is no exception. Most of us today, as well as during Greek times, grouped the stars visible to the naked eye into more-or-less recognizeable forms, such as heroic mortals, gods and goddesses, fanciful monsters, and faithful companion animals.
Ptolemy recognized twelve zodiacal constellations and some 36 to 38 other constellations.
# These other constellations can be further broken down into circumpolar, other northern, and southern constellations. (Terms to be explained later.) Not visible from the Mediterranian are additional south circumpolar constellations. These were not discovered by Westerners until the boom of European exploration and navigation in the 1400's. The search for accurate navigation out of sight of land led to mapping of the southern constellations in the 1600's.
Every day the Sun rises and sets. Every night the stars rise and set. To get a clear picture of what the sky looks like, imagine freezing the dark sky above our heads one night--you can only see half the stars, because half are below the horizon. The other half can be found if you travelled to Australia and that same night froze its stars. Put the two together, and you appear to have a black crystal sphere with bright points of light surrounding the Earth. This image or model of how the stars are seen from the Earth is what astronomers call the celestial sphere. It has a north celestial pole (near Polaris, the Pole Star) directly overhead when standing at the Earth's North Pole, a south celestial pole (there are no bright stars nearby) directly overhead at the Earth's South Pole, and a celestial equator directly overhead at the Earth's equator.
We will be returning to the model of the celestial sphere in the discussion of the "zones" of the sky that the constellations fall into. For further details, see the (pending) appendix on celestial coordinate systems.
Zodiac
The ancient Greeks discovered that among all the fixed stars in the sky, there were some bright lights that seemed to meander through the rest during the course of a year. These "wanderers" were named "planets," and by a strict interpretation of the word, even the Sun and Moon would be considered planets. The apparent path of Sun through the sky is today called the ecliptic, and makes a Great Circle. The Moon and planets follow paths that are not nearly as perfect; some of the planets even do loop-the-loops through the sky. But despite these acrobatics, they stay close to the path of the ecliptic. The region they pass through is known as the zodiac, and traditionally contains twelve constellations.
The ecliptic intersects the celestial equator in two locations, and hence some of the zodiacal constellations are in the northern celestial hemisphere, and some are Southern.
Do I want more about retrograde motion, and why they're all in a plane?
North Circumpolar Constellations
If you go out on any clear night, unless you live too far South, and you will see the Big Dipper (Ursa Major). In the Spring, its bowl appears to be pointing downwards, as if pouring out the Spring rains. Other times of the year the Big Dipper will be pointing in different directions, sometimes above the Pole Star (Polaris), sometimes below, but it is always above the horizon. In fact, if you stood outside all night and watched the Big Dipper, and if you could also see the stars during the day, you would see it circle around and around Polaris and never set below the horizon. It is as though when the Big Dipper tries to set, it misses the horizon and instead swings around below Polaris and above the horizon.
This behavior of "missing the horizon" and never setting marks Ursa Major as a circumpolar ("around the pole") constellation. If you live near the equator, there are few or no circumpolar constellations. For someone living at the North or South Pole of the Earth, all constellations are circumpolar--think about the "midnight sun," when even in the middle of the night the sun is still above the horizon.
Other North Constellations
In short, these are the other north celestial hemisphere constellations visible to the ancients.
Southern Visible Constellations
Although these constellations are south of the celestial equator, they are visible part of the year from the Mediterranian.
South Circumpolar Constellations
These constellations were never visible to the Greeks and Romans, and are only visible as the observer travels further south.
