To create a diagram like this, you can make use of Dashed Strokes in the Stroke panel. However, if your diagram needs dashes of different lengths, this method can only adjust up to 3 segment sizes.
Step 1: Draw an ellipse with the Circle tool and add a stroke.
Step 2: Using the Dashed Stroke feature, start by setting the gap size you desire. Note: This gap size will remain consistent regardless of circle size provided your scale strokes settings haven't been changed from default.
Step 3: You can now use the first dash size box to adjust the size of the dashes. This will decide how many segments you want in your diagram.
Note: If you resize your circle after setting the gap size, you can readjust your segments with the dash size box until your segments are divided consistently.