# Affect of leap seconds on the prediction of an eclipse

There seem to be irregularities in the motion of the earth, so we add "leap seconds" to adjust our time accordingly. Since 1972 there have been 26 leap seconds I think. If we extrapolate that backwards 0.5 leap seconds per year, it would be 1 leap minute in 120 years and 5 leap minutes in 600 years.

Would this be enough to throw off a prediction of an eclipse? In other words, could someone in 1416 predict an eclipse right now given the unpredictable errors caused by leap seconds?

• How much time difference would you consider "thrown off"? 1 second? 5 minutes? 3 days? It's a good question, but maybe you should ask "roughly how far off..." – uhoh Aug 30 '16 at 9:26
• @uhoh It may be a complex question because the affect of variations in the earth (and moons) orbits might have an effect on an eclipse which is not proportional. One way to describe the error would be differences in the path of totality. So the path of totality could differ by a certain number of miles. Since the width of the path of totality is about 75 miles, if the error was greater than that, then the prediction would be totally wrong in the sense that none of the places predicted to see a total eclipse would actually see one. If the prediction was 38 miles off, then it would be half right – Tyler Durden Aug 30 '16 at 12:55
• Sure I know it's complex. But you need to make sure you ask the question in such a way that it is likely to get a high quality answers. if you askL Would this be enough to throw off a prediction of an eclipse? how does one say "yes" or "no"? By what criteria? – uhoh Aug 30 '16 at 13:05
• An interesting line of questioning to have here is what sort of eclipse prediction would someone have made in 1416? Would they have elected to phrase it down to the minute? If they did, what clock would they have chosen to reference in their prediction, knowing that no man-made clock of that era had sufficient accuracy. Surely they would have referenced the time with respect to the motion of the earth and sun, which may have interesting variances, but they're not the same as what we need leap seconds for. – Cort Ammon Aug 30 '16 at 21:26
• As noted in a comment to one of the answers, the Earth's somewhat irregular rotation will have only a very small impact on the Moon's orbit over the course of hundreds to thousands of years. That somewhat irregular rotation does however mean that variations in $\Delta T$ has a cumulative impact on where the path of the Moon's shadow intersects the the surface of the rotating Earth. – David Hammen Dec 30 '17 at 20:16