100 million years. The restriction limited the improvement of the almanac accuracy. There were 19 almanacs used between 960 and 1279, which means that on average a new almanac was created every 17 years. This shows, on one hand, the improvement in measurement precision and that errors in almanacs were easy to detect. On the other hand, it shows that there was little improvement in the accuracy of the almanacs over these 319 years. The main reason was probably because the almanacs were restricted by the limi