Your Eddington Number is the largest number that you have run/biked/etc at least that many miles on at least that many different days.
For example, if a runner over the course of a week runs 3, 4, 5, 4, 3, 7, 3 mile days, her counts would look like
5+, 6+, and 7+ mile days don't meet the criteria. 4+ is the highest number that meets the criteria, so her Eddington Number for the week is 4.
While Eddington Numbers can be calculated for any period of time, they are often thought of as lifetime numbers.
For more about Eddington Numbers, see this article and this section of the Arthur Eddington Wikipedia article.
Also check out this other Eddington Calculator.
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