The University of Chicago Yerkes Observatory

Sky Projects
Saturn

Hands-On Universe

Saturn's Rings and Moons

Saturn is quite a beautiful planet. It's rings surround its equatorial region and show off it's tilt toward Earth which is also toward the Sun when the planet is at opposition. This image was taken on November 18, 2002 with The University of Chicago Yerkes Observatory 24 inch telescope.

Challenge Questions: Look at how the planet is angled with respect to the Sun (which is behind the Earth - we are viewing Saturn from our night side). The sky of this picture shows North up and East to the left.

  1. Is Saturn's northern or southern hemisphere getting more sunlight?
  2. What season is it for the Northern hemisphere of Saturn.
  3. What season is it for the Southern hemisphere of Saturn?
  4. Explain your answers.

Saturn's brightest moon is Titan. In this image you can see three other moons in addition to Titan. Open the Hands-On Universe Image Processing software (HOU-IP). Then click on the box in front of the word, Log, on the toolbar. This tool logarithmically scales the display range so that you can see dim objects better.

Another way to see the moons is to just move the Min/Max red and blue tabs. If you still have the Log box checked, then click in the box to remove the check. To see dim parts of the image, like its moons, move the blue tab to the left or towards Min.

Try seeing the motions of any of Saturn's moons by adding three pictures of Saturn together. On November 18, 2002, we imaged Saturn three times in 4.5 hours. You can download the prepared Saturn images here. The images have been shifted so that the planet is in the same position. The images have also been normalized so the brightness is nearly the same for each. You can add these images together, or add, add, then subtract the images. This way you can see how much the moons moved from the first observation, to one 2 hours later, to the last image taken 2.5 hours after the second.

*To prepare the images yourself you can download the original images and follow this procedure. You will align the images to Saturn and normalize the images for adding together and seeing the motion of the moons. Make sure you have the images of Saturn. You can download the original Saturn images here.

  1. Open each image.
  2. Subtract the sky from each image. Data Tools, Sky. Manipulation, Subtract value of Sky.
  3. Shift images to match the best one, whcih will become your reference image. For the Saturn images, we thought that the middle image was the best, so we shifted the others to match it. First, use Data Tools, Axes, and make a rectangle around Saturn. In the Results Box you will see the (x,y) center values. Do this for each image. Then subtract the (x,y) values of each image from the (x,y) values for the image you want to be the reference, which in this case is saturn021118-0725.fts. Now click on the first image. Go, to Maniuplation, Shift and put in the amount of shift for the x and y. When you finish, check your work by going to Data Tools, Axes again. Do this for the third image as well.
  4. Normalize the images so they are all about the same brightness. Use the auto aperture tool on the brightest moon, Titan, on each image. Open the Calculator tool on your computer or use a hand held calculator, or just do the arithmetic with a paper and pencil! Divide the auto aperture value you got from Titan on the reference image by the value you got on each of the other images. Click on the first image to make it the active window. Now go to Manipulation, Multiply. Insert the result of the above division. Check your work with auto aperture again.
  5. Divide the counts so that you can add all the images without saturating the sum. For the images to add together the sum of the counts for any pixel should not exceed 32767. An easy way to accomplish this is to divide each image by the number of images in your set. So, in this case, you would divide by 3.
  6. Finally save each image by a new name, so you don't destroy your originals. You can call them by the name of the object, the date, and a,b,c, etc. .fts
  7. Now you are ready to add or add and subtract the images any way you like!

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Saturn's Ring System - NASA http://ringmaster.arc.nasa.gov/saturn/saturn.html

The Real Lord of the Rings - NASA http://kids.msfc.nasa.gov/news/2002/news-rings.asp

Saturn Fact Sheet - NASA JPL http://www.jpl.nasa.gov/saturn/fact.html

Other HOU images of Saturn.

Other Links http://www.handsonuniverse.org/activities/Explorations/Links/index.html

More images of Saturn.  These images are days, weeks, or months apart. Find ones that are relatively close together and see if you can shift and add or subtract them together to determine the period and the radius of Saturn's brightest and largest moon, Titan.  Once you know these values, find the Mass of Saturn.  Follow the same calculations as in the HOU curriculum activity: Mass of Jupiter.


Sky Projects
http://www.handsonuniverse.org/activities/Explorations/SkyProjects

Vivian Hoette, Education Outreach Coordinator
The University of Chicago Yerkes Observatory, Williams Bay, WI. 53191
vhoette@hale.yerkes.uchicago.edu 262-245-5555