Tool Tips and Practice
Measuring Tool

Graphics that show up with a ruler icon in the Tools area have been prepared for you to make distance measurements on them. Read and follow the tips that appear below. Practice measuring distances on this image until you feel confident that you are making reasonably accurate measurements with the tool.

  !   Follow the instructions below the ruler tool at left to measure the diameter of Earth and the moon on this image.



Tool Tips

  • Point the very tip of the cursor arrow at the starting and stopping points of the distance you want to measure. The very tip of the arrow's point on your cursor is the "hot spot" of the tool.
  • Measure each distance two or three times to make sure you get consistent results.
  • If you get around 3480 km for the moon's diameter, and around 12750 km for Earth's diameter, you're using the tool accurately.
  • Some measuring tools have multiple images from which you need to make measurements. After you have measured the first feature, click the "Continue" button at the bottom of the Instructions area to advance to the next image. Measure the next feature, then click "Continue" again until you have completed all measurements.
  • To measure curved lines, you can measure two or more straight line segments along the curve, and add their distances to get the total.

About measuring from images

All graphics that appear with a measuring tool on this site have been calibrated. That is, the actual distance represented by each pixel in an image is programmed into the tool. As you drag your cursor across pixels in an image, the tool converts the pixel distances to real world units. Precision of measurements is limited by the resolution of the image: the larger the distance represented by each pixel, the lower the precision of measurements.


  • If the image or ruler tool doesn't appear on a page where the instructions indicate they should be, your computer may have some issues with the Shockwave Plug-in. See Shockwave FAQ site.