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Eddington Number E is the largest value where a cyclist has ridden E days of at least E miles. For example over your lifetime you might have ridden over 120 miles in a day on 120 separate occasions. One of its characteristics is that larger Eddington numbers are much much harder to achieve than slightly smaller ones (moving from E=120 to E=121 isn't just about riding one extra day at 121 miles, but ensuring you have 121 days where you have ridden at least 121 miles). For the OYTT challengers, I have been recording not the lifetime Eddington numbers, but their annual ones, for which larger numbers are harder still to achieve.

The Eddington chart shows the frequency of ride lengths for each rider. So for example, following Amanda's purple line from left to right shows that she has ridden at least 5, 10, 15...50 miles on 118 occasions (the vertical position of the line). It dips down by one ride at the 55 mile mark indicating that on 117 days she has ridden at least 55, 60, 65...160 miles. It then dips downward indicating that she had ridden progressively fewer days with longer rides with only a couple of days when she has ridden 260 miles or so.

The Eddington number can be found on the chart by looking along the diagonal line where the value on the horizontal axis (distance) is equal to the value on the vertical axis (number of days). Where the frequency line intersects that diagonal, the Eddington number is highlighted with a small circle.

The shape of the line for each rider gives an indication of how their Eddington number is likely to change in the future. Amanda's purple line shows that her E is limited simply by the number of days she has ridden so far. For every extra day she rides at the current rate she is likely to increase her E by 1 until she hits around 230 days (because she has been riding at least 230 miles almost every day). In contrast, the steep fall off for Kajsa means that her rate of increase will be much less day by day (because she has already ridden more days than the number of miles in a typical day). She has to ride longer distances than she has been to move up that diagonal line. In fact that is precisely what she is doing this month, aiming to ride at least 100 miles per day, so we can expect her to hit E=100 in a couple of weeks.