If you were look up at the position of the stars on any particular day of the year from the same location on Earth, and compared them to the same day a year earlier, you would see the same stars in the same exact positions relative to one another. That is why the night sky can be likened to a giant celestial clock with the appearance of certain star constellations heralding the various seasons, such as from the northern hemisphere the constellation of Orion dominating the winter sky, Leo most visible in spring, Scorpius a familiar sight in the summer time, and Pegasus associated with the autumn sky.
Perspective During Annual Orbit Around Sun
The reason for this is because the Earth completes one orbit around the Sun every year, eventually returning to the same position at the end of a 365.256 day (1 sidereal year) period. Similar to riding a fairground carousel, we are therefore exposed to a different region of space and background stars at each stage of our orbital journey around the Sun until we return to our starting point after one full revolution. Incidentally, this annual merry-go-round is 584 million miles (940 million kms) long, during which time the Earth travels around the Sun at a speed of 18.5 miles/second (30 km/s).
Stars Start One Degree Further West
At this point, it is worth noting that this process means more stars appear in the eastern sky each night, while the stars we have seen the night before start their journeys around 1 degree further west each night before making their way across the sky and eventually disappearing off the western horizon. This can be explained by simple mathematics, as the Earth takes 365.256 days to complete a 360 degree orbit around the Sun, therefore after one day a star has changed its apparent position by 360 ÷ 365.25 = 0.99 degrees, or roughly 1 degree.
Taking this calculation a step further, we can that any individual star that has just appeared on the eastern horizon will have traveled 30 degrees higher westward after a 30 day period. In the same way, a circumpolar asterism like the Big Dipper in the constellation of Ursa Major would go from looking like a horizontal saucepan that is able to hold water in autumn, to being turned upside down 180 days later and emptying its liquid content in spring.
Planets and Moon Have a Different Schedule
While the stars and constellations looks exactly the same on any one night as they did a year earlier, the same cannot be said about the Moon and planets which have their own schedules and do not return to their same night sky positions on an annual basis. In ancient cultures, the “fixed” stars were therefore considered different to the few strange “stars” which seemed to vary in brightness and wander freely from one zodiac constellation to the next along an apparent path called the ecliptic, whilst obeying a different set of rules all together. These subsequently became known as planets (“wanderers”), and were associated with the gods, namely the five naked-eye planets of Mercury, Venus, Mars, Jupiter, and Saturn.
How Long Does a Planet Stay in a Constellation?
Of course, the closer a planet is to the Sun the more quickly it appears to move in the sky, with Mercury and Venus changing their positions relative to the stars over just a few days, while the outer planets, such as Jupiter and Saturn, appear to drift slowly from one constellation to another every year or two. To be more precise, the Sun spends around 30 days in each zodiac sign, while the Moon spends around 2.5 days, and visits all 12 signs every 28 days. In terms of the planets, Mercury and Venus take around one year to visit all 12 zodiac signs, while Mars takes around 2 years, Jupiter about 12 years, Saturn around 29.5 years, Uranus around 84 years, Neptune about 165 years, while Pluto visits all 12 zodiac constellations once every 248 years or so.
And Then There is Precession..
While stars maintain their same relative positions and configuration from one year to the next, over a period of centuries they do not. This is due to precession, or the wobble motion of the Earth which causes the direction of its axis to change over longer periods of time. Greek astronomer Hipparchus is generally credited with discovering precession in 127 BC, having noticed that the equinoxes occurred in a different position among the stars than depicted on comparison charts of 150 years earlier. Likewise, star positions have changed since ancient Greek times at a rate of roughly 1 degree every 71.6 years, corresponding to a cycle period lasting around 25,772 years. The North Star around 3000 BC, for instance, was Thuban in the constellation of Draco, while today it is Polaris in Ursa Minor, and in 14,000 AD will become Vega in Lyra, before becoming Thuban again sometime around 23,000 AD.