Determining the Diameters of Pluto and Charon

The purpose of this lab is to teach you how astronomers measure the size of distant objects when fortunate events called occultations occur. An occultation is an event when one celestial body passes in front of another. If we know the distance to an object and that object's velocity in space, and that object covers up a more distant object (an occultation), then by measuring the amount of time for the more distant object to reappear will give us a measure of the closer objects diameter. In this case, the alignment of Pluto and its moon Charon was edge on relative to our line of sight. Pluto and Charon repeatedly occulted each other and we were able to measure the diameters of both.

To your left is the typical arrangement of Pluto and Charon as viewed from Earth. You can clearly see the objects spinning around each other. This image was taken on June 5, 2002 from the Gemini Telescope in Chile. In July 3, 2002 I was standing atop Mauna Kea inside the Gemini Telescope twin. These are identical telescopes with 8.1 meter mirrors. It was extremely impressive to stand inside a dome that housed a telescope 7 stories high and weighed over 620 tons. However, the instrumentation is so finely balanced that when it is doing an observing run, you could push the entire scope with one hand.








Okay ... on to the lab. In this webpage are a series of scanned pages from my "Activities in Astronomy" lab book. Since I have blatantly copied these pages for this exercise, I want to take time to give credit to Darrel B. Hoff, the chief author and editor. Kendall/Hunt Publishing Company in Dubuque, Iowa, 1996. Since I am using these pages entirely for the purposes of education and not for resale or profit in my part or that of Hopkins On-line Academy, I believe that the "Fair Use" rules of the Internet apply, and that I am fully within those guidelines. If I were in a classroom setting, I would make copies of these pages for those students as well.

Lab Instructions

Please read the following three pages of instructions and look closely at the data from the Pluto-Charon occultations. Then print out the graph paper to construct your light curve from the values you see on page three. Then, answer the questions on the final page of this lab. I will not be asking you to respond to discussion question #4, but everything else. Once you have finished this work, then move to the mirror Pluto-Diameter webpage where you can put your numbers and responses into those little boxes that will come to me over the internet ... provided you press the submit button.

When you have completed the data sheet on your own, please move to a mirror Pluto-Diameter site where your answers can readily be placed in a file that I can receive via the internet.

Return Planet Pluto, the Introduction to Icy Worlds, the Introduction to Planets, or to Syllabus. If you want to wait on this lab until a later time and move forward to Comets, well, who am I to stop you, but I prefer that you finish this first:)


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