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## Activity: The Planisphere

### Introduction

This handout was adapted from the web pages of Alan M. MacRobert at http://www.astro-tom.com/ and describes some basic information about planispheres. This handout gives some basic information about Astronomy that everyone should know. It was published by Starry Knights Foothill High School Astronomy Club, Santa Ana, California, "Basic Rules of Stargazing(v.6)," ©1998 by Ben Balmages and is published online at http://www.astro-tom.com/getting_started/astro_essentials.htm.

### Part I

#### The Planisphere as a Model of the Sky

The planisphere can be used to model the motion of stars across the sky as the Earth turns and revolves around the Sun. The following questions are designed to enable you to use your planisphere to do just that. You will be able to answer all questions simply by gently rotating the disk inside the planisphere and carefully noting what happens. Always rotate the disk counter-clockwise when you want to model the way the stars move through the sky. Use a separate piece of paper to answer the following questions. Some questions require you to draw a picture.

1. What are the names of the constellations that never set? Draw labelled pictures of them on your paper. These constellations are called circumpolar: can you explain why?
2. What famous star is located under the pivot in the center of the planisphere? What is the importance of that star?
3. Find five constellations which rise in the east and set in the west and draw labelled pictures of them on your paper.
4. Draw a picture of one circumpolar constellation at three different times of night: 8 pm, 12 am, and 4 am. Draw the constellation as if you were facing North. At each time the constellation will have a different orientation: make sure your drawing clearly shows this.
5. Draw a picture of one non-circumpolar constellation at three different times of night: 8 pm, 12 am, and 4 am. Draw the constellation as if you were facing South. At each time the constellation will have a different orientation: make sure your drawing clearly shows this.
6. What is the difference between the drawings from the previous two problems? Why do the constellations in different parts of the sky move differently?
7. Set the planisphere for 9 pm Sept 14. At what other times of the year does the sky look just as it does now that you have set it? In order for the sky to look just as it does now (9 pm, 9/14) when would you have to look at the sky in July? In May?
8. Set the planisphere for 9 pm Sept 14. Slowly rotate the planisphere counter-clockwise and watch the date as it slides by the different times of night. When you do this you are modeling the motion of the stars over the course of a single night.
1. Turn back to the starting date and time. What constellations are about to set in the West?
2. What constellations are rising?
3. If you turn through a whole night what constellations are rising when the Sun is coming up (the sun rises at about 6:20 am in mid-September in this part of Maine)? Explain what you observe.
4. Which constellation(s) could not be seen after sunset or before sunrise? Are any of those familiar (say from astrology, perhaps)? What is the significance of this?
5. What explains this motion over the course of one night?
6. Draw a picture showing constellations, the Sun, and the Earth which can be used to explain this motion of the stars through the sky during one night.
9. The stars you can see shortly after dusk changes throughout the year. Set the time and date for 9 pm Sept 14 again. Now rotate the disk counter-clockwise and watch the time as different dates slide past. When you do this you are modeling the motion of the stars as the Earth moves around the Sun.
1. How far, in degrees, does the sky move each night and in which direction? (Hint: January 1 is 180° from July 9 and 90° from April 3).
2. What explains this motion from night to night?
3. Draw a picture showing constellations, the Sun, and the Earth which can be used to explain this motion of the stars through the sky over the course of a year.
Part II of this activity
Last updated: Sep 06, 2007       Home