If you ever have looked up in the sky at different times during a night, you might have noticed that the stars seem to move. This movement of the sky is called diurnal motion. Diurnal motion is caused by the rotation of the Earth around its axis. Since it takes the Earth 24 hours to complete one rotation, the diurnal motion caused by this rotation will also take 24 hours to repeat. The direction the sky moves is also related to the rotation of the Earth. If you were looking down on the Earth from space, you would see it rotating from west to east. This is the true motion of the Earth. But, for a person on the Earth, the Earth appears to be stationary. Therefore, for that person, the sky seems to rotate from east to west. Therefore, stars, planets, and moons appear to rise in the east and set in the west. If you watch the sky for an entire night, you will see that most stars rise and set in the course of the night. But you will also notice there are some stars that do not rise or set, but seem to make circles in the sky around a point. These stars are referred to as circumpolar stars. The point these stars circle around are the celestial poles.
The number of circumpolar stars that can be seen depend on the latitude. At the North or South poles, all stars would be circumpolar. At the equator, no stars would be circumpolar. The further you are from the equator, the more circumpolar stars. In the Northern Hemisphere, there is one star that does not seem to move at all during the night. This star is Polaris the North or Pole star. It lies less than one degree from the North celestial pole. Polaris is one of the most famous stars in the northern skies because of its position at the North celestial pole. This position means that if you can find Polaris, you know which way North is, making Polaris a great navigational tool.
In the Southern Hemisphere, there are no visible stars close enough to the South celestial pole to be used as a South star. There is one problem with pole stars, which is the celestial poles move. The celestial poles make a circle in the sky which takes 26,000 years to complete. This is caused by the complex interactions of the Sun's and the Moon's gravity. They both tug on the Earth as it rotates, causing it to wobble. This wobble, called precession, causes the celestial poles to trace the circles in the sky. This means Polaris was not and will not always be the Pole star. In the distant past, the bright star Vega was the Pole star, and in approximately 13,000 years, Vega will again be the Pole star. Another thing to note is that the motion in the sky is not all a result of diurnal motion. If you look at the sky over a year, you would notice different stars would appear at different times during the year. This slower motion in the sky is caused by the Earth orbiting the Sun. As the Earth moves in its orbit, it faces different sections of the sky, causing this slower motion. One final note. The stars themselves are not stationary. They do move around in space. But this movement is too slow and too small for us to notice with our naked eyes. So while stars do move in space, the motion in the sky is not a result of this movement.
Below there are several questions about diurnal motion. Select the answer that you think is correct. Along with the written answers, there are movies and images from the Night Sky Live project that help to demonstrate the answer.
1. Are the stars really moving?
A Yes
B No
2. Do the stars appear to rise in the North, South, East, or West?
A North
B South
C East
D West
3. After how long will diurnal motion repeat?
A 1 hour
B 6 hours
C 12 hours
D 24 hours
4. Why do some stars stay up all night?
A They are circumpolar stars
B They are frozen stars
C They move at the same speed the Earth rotates
D There are no stars that do this
5. Which star is the Pole(or North)star?
A The Sun
B Polaris
C Venus
D There is no such star
6. If you looked out one night and then looked out again six months later at the same time, would the sky look exactly the same, in terms of stars?
A Yes
B No
7. Will Polaris always be the Pole(North)star?
A Yes
B No