But Kepler also decided to track the movements of each planet's orbit and form ratios using the same period of other planets. For instance, Kepler tracked how far Jupiter moved per day as it rounded the sun at perigee (when the planet is closest to the sun and actually speeds up) and divided this value by the same movement of, say, Venus. To his astonishment, the ratio was nearly a perfect musical interval of some sort. Then he would try tracking one planet at perigee and divide it by another at apogee (when the planet is farthest away from the sun and moving slower) and found, again, the ratio formed a shockingly close approximation of a musical interval. When astronomers tried this using precise modern data measured by computers, they found to their astonishment that the ratios were even closer to the musical intervals than Kepler's less precise data.