The Earth is tilted on its axis from the plane of the ecliptic by 23.5 degrees. That tilt causes the North Pole to be currently pointed towards Polaris. As the Earth moves around the sun its pole stays pointed at Polaris. This is the cause of the seasons we experience. Note. This tilt varies back and forth from 21.6 degrees to 24.5 degrees approximately every 41,000 years.
There is also a precession of our pole and it sweeps a complete circle in the sky (think of the Earth as a top wobbling as it rotates) about every 26,000 years. (Hard to explain without a diagram)
There are also a number of other motions that must be taken into effect over the years such as the precession of the aphelion. Our Earth’s orbit around the Sun is not a perfect circle. It is an ellipse with the closest point of the orbit called the perihelion and the furthest point the aphelion. Currently the aphelion falls on the fourth of July. However, this is not always the case. The aphelion and perihelion change over the centuries and sweeps thru the calendar year with a periodicity of around 22,000 years. The amount of “squishing” (LOL now that’s a scientific term) of an ellipse is called its eccentricity. Note; the Earth's eccentricity is very small. However, even this changes over time. Its eccentricity varies periodically about every 100,000 years.
There are also other motions caused by the Moon, Jupiter and the Sun called Nutations. One of the major nutations has a period of 18.6 years. (There are others that must be computed as well when flying a spacecraft)
Now that we have that out of the way, we will now describe the Celestial Sphere. If we look at the stars in the night sky they appear to be stationary relative to each other. Even with the Earth moving from one side of the Sun to the other, the displacement due to parallax is less than one second of arc even for the closest star. One way of looking at this, is a fixed sphere of stars surrounding the Earth/Sun system. This is often referred to as the Celestial Sphere.
However, over time the stars do move relative to each other and relative to the Earth. This is why the right ascension and declination (star location) changes over the years. If you look at a star catalogue based on the epoch B1950 and one base on the epoch J2000, you will find a difference.
Another interesting item of note is that the constellations we see are made up of the brightest stars. Even in the same constellation these stars are at different distances from the Earth. Some may be dimmer than the others, however, being closer they are just as bright as a larger one further away. The brightness of a star is called its magnitude. There are two ways astronomers measure magnitude. Apparent Magnitude and Absolute Magnitude.
The Apparent Magnitude is how bright a star appears to us hear on the Earth.
The Absolute Magnitude is how bright a star would appear if it were exactly ten parsecs away from the Earth. (Close to 33 light years).
Two notes:
1) Apparent magnitude is usually denoted with a small “m” and absolute magnitude uses a capital “M”.
2) The magnitude scale is backwards of what you might think; the larger the number the fainter the object.
WOW, now that we got thru all of that, we see that the stellar positions and our relationships to them vary over the centuries.