Although most of the stars change position during the year, one of the most
interesting effects is seen in the stars that lie in the plane of the Earth's
orbit around the Sun. Stars in the plane of the Earth's orbit will be obscured
by the Sun for part of the year. Stars near the pole are never obscured.
Constellations that lie in the plane of the Earth's orbit are called zodiacal
constellations and the period of the year that each zodiac sign relates to
the period of the year that the Sun travels through that constellation, or
at least it did 2500 years ago when the star signs were first named. Due to
the movement of the Earth through the years the stars are now some 30 degrees
out of sink, meaning that the constellation that the Sun was in during the
month you where born is no longer the same as your star sign in fact it is
one or two signs further round.
In the above diagram the Sun is shown at the centre of the diagram with the Earth moving around it, anticlockwise viewed from the top, all surrounded by the distant stars in their star signs. The Sun is blocking the Earths view of the constellation Sagittarius. As the Earth moves around in its orbit the Sun will appear to move through Sagittarius and into Capricorn.