Measuring the Motion of A Close Approach Comet

Astronomers had a spectacular object to observe in the March skies called C/1996 B2, better known as Comet Hyakutake. This comet, first seen on Jan. 30th, and named after its discoverer, Yuji Hyakutake, was only a mere 9.3 million miles from Earth, only 1/10 th of the distance from Earth to the Sun, on March 25th, 1996. NOAO Outreach Advisory Board member Suzanne Maly developed the following classroom exercise appropriate for middle and high school students using images of Hyakutake taken at Kitt Peak National Observatory and the NIH Image software. The exercise, which has students measure the apparent motion of Hyakutake through the night sky, was presented at the July 1996 Image Processing for Teaching Conference in Orlando, FL.

About the Data:

The images that you need for the following activity were taken by astronomer Beatrice Muller with the Case Western Reserve University (CWRU) Burrell Schmidt telescope at Kitt Peak National Observatory (KPNO) near Tucson, Arizona. To do this exercise yourself, you will need to download three TIFF format images of Comet Hyakutake:

KPNO C/1996 B2 (Hyakutake) R
KPNO C/1996 B2 (Hyakutake) V
KPNO C/1996 B2 (Hyakutake) B

The last letter in each image name indicates which filter was used for the image: R (red), V (visual), and B (blue), respectively. The images were all taken on April 1st, 1996, one week after Hyakutake's closest approach to Earth. The R filter image was taken first at 4:23, the V filter image at 4:31. and the B filter image at 4:43. All times are given in Universal Time (U.T.), the local mean time of the prime meridian that passes through Greenwich, England.

Schmidt telescopes are designed for observing relatively large amounts of sky at once. The CWRU Burrell Schmidt telescope on Kitt Peak can record images that extend over 69 (arc minutes) on a side, just over one square degree of sky. Each pixel on the images used with this exercise is 8.1 arc seconds on a side. Note that 60 arc seconds = 1 arc minute and 60 arc minutes = 1. For comparison, a full moon and the Sun each subtend about 1/2 degree of sky.

Try this classroom activity with the CWRU Schmidt images and NIH Image:

Download the TIFF images on to your system using your preferred software. Save the images in a file on your desktop, hard drive, or disk. File/Open NIH Image File/Import the CWRU Schmidt Images that you saved, one at a time, starting with 1, 2, & 3 in that order because the first image taken was with an R filter, the second with, a V filter and the third with, a B filter. Stack/Windows to Stack the individual images. Here you may want to Stacks/Animate slowing down the blinking by using the 1 key. Notice the motion of the background stars from frame to frame. Notice that the comet does not look exactly the same in the three different frames. The comets tail is more prominent in some filters than others and even the background stars appear brighter or dimmer in the different images. Go to Stacks/Register. Choose three or 4 bright stars that surround the comet nucleus and can be seen well in all three frames. Some suggested x and y values for stars you may choose are given before, as measured in the first (R band) image:

		X(292) Y(485) value=243 
		X( 16) Y(192) value=243 
		X(492) Y(153) value=243
		X(192) Y(135) value=231

You may choose your own stars or use the ones suggested in the printed picture of the comet that comes with this lesson. Center the cursor on each star in an exact order and click once except double click on the last star. Double clicking on the last star will complete the process for the top image and advance the stack to the next image. Then, click on the same stars, in the second and third images in your stack, in exactly the same order. When the frames have been successfully registered, animate them again. Now you should notice that the background stars do not move from frame to frame. With the series of images registered using the background stars, you should now see only the motion of the comet relative to the stars as the frames move sequentially.

We now want to measure the motion of the comet as it moves along the pattern of background stars. In each of the three frames, measure the position of the comet nucleus.

Find the center of the comet by using the magnify tool, magnify 3 or 4 times, and watch results in the Info box to check intensity (Value) for the nucleus. Refer to your LUT to see intensity scale numbers and use this information to select the brightest pixel.

Record the center of the comet nucleus in each image in the table below:

			x                       y
Image 1
Image 2
Image 3
  1. How much time passed between the first and last image? _________________.
  2. How far has the comet moved in pixels in this time? ( Most of the comets motion in these images is to the north, or right; it is okay to simplify the exercise by considering motion only in the x direction.) ______________.
  3. In these images, each pixel covers 8.1 arc seconds on the sky. With that information, how many arc seconds has the comet moved between the first and last image? ___________________________.
  4. Now, what is the rate of motion of the comet across the pattern of background stars in arc seconds per minute? Divide the number of arc seconds calculated above by the number of minutes of time. _________________________________.
  5. From Earth, the apparent size of the full moon is about 1/2 degree or 30 arc minutes. At the rate you measured above, how long would it take for the comet to move a distance equal to the apparent size of the full moon?_______________

Most of all, have fun viewing Comet Hyakutake!!

Exercise developed by Suzanne Maly, Safford Magnet Middle School, Tucson, AZ
Page created and maintained by Suzanne H. Jacoby (
Last Updated: 24 July 1996
Artwork by students of the Satori School and Miles Exploratory Learning Center, Tucson, Arizona

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