Daily Rotation of the Sky
Goals of the Lab
To observe and analyze the apparent motion of the stars across the sky over the course of a few hours.
Requirements: paper, colored pencils or pens, a ruler, a protractor, and a compass (to draw a circle).
Part I: Observations
Draw the stars and constellations visible over the horizon in two directions: North and East. (Make two different drawings.) Each drawing should include a sketch of the horizon in that direction. Each drawing should include at least 7-8 stars. Drawing more than 7-8 stars will only help you see the pattern of movement more easily. If you can identify a couple of constellations with bright stars, draw as much of the constellation as possible. Throughout the evening it will be easier to identify the movement of a group of stars, compared to a single star. Label at least two stars.
Measure the angular distance of each star off of the horizon, and include it either on your drawing or in a table off to the side (see How To Measure Angular Distances). Also provide a horizontal angular scale for your drawing. Label each drawing with the time of the drawing and the direction in which you look.
Draw stars up to at least 45° off of the horizon; in the South, you may wish to draw stars up to 60-70° off of the horizon, so as to include the Celestial Equator. Your drawing of the stars to the North should include Polaris, and (if possible) at least one star which is higher in the sky than Polaris.
Between 45 and 60 minutes later, repeat the two drawings from step 1. Draw as many of the same stars as you can still see in each drawing. Draw these stars on the same drawing from step 2 in each direction; use a different color from the color you used for the first drawing, so you can keep track of what you're doing. Clearly label the time of the second observation in the second color.
Try to include as many of the stars from your first drawing as possible; you will need to have at least 3-4 stars common between the two drawings to successfully complete this lab. If more have moved into the region you are drawing, also draw those.
Two hours after the first drawing, repeat the drawings from step 1 again, in a third color on the same four drawings. You will need at least 3-4 stars in common between the drawing of steps 2 and 3 to successfully complete the lab. Label the time of the third observation on each drawing in the third color.
Part II: Analysis
Describe the motion of the stars in each of the direction you observed. Did the stars move up or down? to the right or left? in a circular motion?
Are stars in the East rising "straight up"? If not, use a protractor to measure the angle at which the stars are rising. Compare this with Nashville's latitude, and comment on what you find.
Use your drawings to estimate the speed (in degrees per hour) of at least two stars in each direction. (Hint: think about angular distance travelled by a star in a given amount of time)
Use your observations of the northern horizon (i.e. the motion of the stars with respect to Polaris) to estimate the length of a day. Your observations should show the stars in the north moving in a circular motion around Polaris. Use a compass to draw a complete the circle. What fraction of the circle do your observations represent? Using this, calculate how long it would take the stars to complete the circle. How does your answer compare to the known length of a day? Show your calculations.
Give a brief physical explanation of the star movement you observed.
Are some stars moving at different speeds? If so, can you predict how fast a star is moving based on where it is in the sky? Where would you expect to find the slowest stars? the fastest stars? Give a brief explanation of your response.