Name ___________________________________

Prof. Jon Satko

 

Astronomy 101

LAB: Using the Planisphere

 

The rotation of the Earth on its axis causes the stars to rise and set each evening.  In addition, the orbit of the Earth around the Sun places different regions of the sky in our night-time view.  A planisphere may be used to solve the following problems:

  1. How will the sky appear at a particular date and time?
  2. What time of year will a specified object be visible at a specified time?

 

Procedure

Hold the planisphere so that the side with the title “The Night Sky” is facing you.  Notice that the names of the constellations are given in all capital letters and that the names of the stars are given in smaller letters.  Notice, too, that the different stars have different sizes of circles representing them.  The larger the circle, the brighter the star looks in the sky.

 

Notice the outside of the star wheel with the months and days indicated.  Set the wheel to today’s month, day, and time.  To do this, line up the date (month & day) with the time.  Times are only written on the wheel for nighttime hours (6 pm to 6 am), so for daytime hours, just count the appropriate number of tick marks.  Now the planisphere shows a picture of the sky as it is right now!

 

As you look at the planisphere, does it seem as if east and west are reversed (the words WEST, NORTH, and EAST written on the blue part are directions on the horizon)?  The planisphere represents a map of the sky.  The planisphere should be held above your head when using it to observe the sky.

 

Now, flip your planisphere over, and look at the part of the sky map that is shaped like a crescent.  This is what the sky looks like to the south.  Notice how this side and the front side, if put together, make a complete circle.

 

Exercises

  1. Define “horizon”.  Name two constellations that are currently near the horizon.

 

 

 

  1. Define “meridian”. Name three stars or constellations that are currently near the meridian.

 

 

 

  1. Define “zenith”.  Name a star or constellation that is currently near the zenith.  Note that the zenith is NOT the center of the wheel’s rotation; the zenith is actually about 1 ½ inches above the center hole.

 

 

 

 

 

  1. Your planisphere should still be set to the current time and date.  Notice, though, that every date of the year is also lined up with some time.  For example, what date is lined up with 2 am? 

 

 

What date is lined up with 4 pm?

 

 

Not only does the sky appear as shown on the planisphere right now, but also during all the other corresponding dates and times.

 

Your planisphere shows the sky as it looks at this moment.  On what date will it look like this exactly one hour earlier?  (For example, if the current time set on the planisphere is 8 pm, what date is aligned with 7 pm?)

           

 

 

 

  1. What star or constellation is rising in the eastern sky right now?  To answer this question, just look to the part of the sky map right above the horizon where it is labeled “EAST”.

 

 

 

  1. What star or constellation is on or near the northern horizon?

 

 

 

  1. Now, slowly turn the wheel counter-clockwise just enough (15 degrees) to advance today’s date to the next hour.  Notice how much (or how little) the sky changed.  Locate the stars/constellations that you identified in question #2 as being on the meridian.  Where are they now?  This is what the sky should look like one hour from now.

 

 

 

 

Now, slowly turn the wheel counter-clockwise, and notice how the sky will change as the hours go by tonight, and into the morning.

 

  1. Find the hole in the center of the wheel.  It actually represents a special star.  What is the name of that star? 

 

 

  1. This star is part of what constellation?

 

 

 

Notice that this star, also known as the “North Star,” never moves its position, no matter what the time or day is.  It is always due north (azimuth of 0°), and its altitude is always the same as the observer’s latitude on the Earth.  Since Porterville is at latitude of 36°, the North Star will be at an altitude of 36° in the sky. 

 

  1. Suppose you were to travel to Seattle (latitude of 47°).  Would the North Star’s azimuth be different than in Porterville?  Explain why or why not.

 

 

 

 

 

  1. In Seattle, would the North Star’s altitude be HIGHER, LOWER, or THE SAME, relative to its altitude in Porterville?  Explain your answer.

 

 

 

 

 

  1. If you were at the North Pole, where in the sky would you find the North Star?  (Hint: What is the latitude at the North Pole?)

 

 

 

 

  1. If you were at the equator, where in the sky would you find the North Star?

 

 

 

 

  1. Name three constellations that are always above the horizon (never rise or set).  (Keep in mind that this is only true for your planisphere’s specified latitudes, 30° - 40° North.)

 

 

 

  1. Find the dashed line (labeled “ecliptic”) that goes through the map of the sky.  This is the path that the Sun, Moon and planets will always travel on or near.  Name four constellations that lie on the ecliptic.