Stargazers have a natural tendency to draw lines between random collections of dots. The patterns and pictures that they see are known as constellations. The ancient Chinese, the Babylonians, the Greeks and the Australian aborigines have all seen these shapes. Ptolemy named forty-eight of the constellations that can be recognised in the northern night sky.
Modern stargazers can have fun identifying the classical constellations. To do so requires a star chart and a leap of imagination. The early astronomers were very inventive in their use of names and it can be quite hard to identify the shape from the name. Start with the easy to recognise constellations, such as the Big Dipper, Orion and Casseopia. It takes patience to find all forty-eight.
One of the most practical ways to identify the constellations is to join an astronomical club. Go out with a seasoned astronomer on a cloudless night. If this is not possible, purchase a planisphere.
A planisphere is a very simple device composed of two plastic discs. It displays the night sky which is visible at a certain latitude at a specific time and date. The plansphere takes into account the Earth’s rotation about the sun and the Earth’s rotation about its own axis. A star chart is printed on the lower wheel with the constellations marked. These charts are printed for specific latitudes and are only valid for latitudes that differ by several degrees. Be sure to buy the correct planisphere for your latitude. The upper wheel contains an oval window and is free to rotate. Align the upper wheel with the correct time of day and see the stars that are currently visible in the window. The planisphere shows the rising and the setting of the stars.
For practical use align the east and west setting on the planisphere with geographic east and west. Some observers like to place the planisphere over their heads. It is then quite simple to identify the constellations from their true position in the sky.
Remember that the constellations are fictitious groupings. The stars that form a pattern on Earth rarely have any physical proximity or resemblance to one another.
A working knowledge of the constellations is extremely useful to identify other items in the night sky. The International Astronomical Union supplies maps of the features that can be found in each constellation. Finding them greatly enhances the experience of stargazing.