An astronomy introduction for the benefit of those unfamiliar with the Universe: we face now themes are more familiar to amateur astronomers such as the nature of celestial objects, their names and recognition in firmament. In a pause to observe starry night sky from a place with little light pollution, we can see hundreds of stars. Most are real stars, some are planets like Jupiter or Saturn. Moon, the only natural satellite of Earth, can even be annoying with its intense light, but majestic and fascinating because of the details that can be observed with naked eye. If we have a pair of binoculars or a telescope and we know where to look, we can discover nebulae, star clusters and galaxies of various shapes, sizes and brightness.
For our astronomy introduction, take a camera and let's take a series of photographs at regular intervals, for example, ten minutes, keeping it pointed towards same direction in the sky. Let's spend two or three hours and look at images in sequence: you can notice that stars are moving. This is the result of the Earth movement that we can decompose in several complex motions:
Rotation of Earth around its axis passing through the north pole (near the North Star) and the south celestial. This motion alternates day and night every twenty-four hours.
Precession of Earth: Imagine a top that rotates rapidly on its axis. We will notice that the axis is not stationary but rotates on itself as a cone. Earth does this rotation every 26,000 years. Earth's axis is therefore not always pointed towards North Star. For example, in 12,000 years will be at the star Vega.
Revolution of Earth around the Sun that happens in 365 days and that is one of the causes of seasons alternation.
Visibility of objects in the sky varies depending on date, time and point on Earth from which we observe. So we can say that, in the same place, you can see different objects each month. To recognize a star and its constellation (ie, group of stars that form a note shape) is therefore important.