What is the Sod HaIbbur (secret of impregnation)? I have seen it discussed in many places including the Pirkeu D'Rebbi Eliezer (8th chapter?) in reference to the intercalation for the lunar months of the year and applying a leap month (which creates an "impregnated" year) so that the lunar calendar doesn't continually fall behind the solar one.
If all this is, is the mathematics behind the lunar calendar, why is it a secret? Does this secret continue to exist now that we have a set calendar? Is there another interpretation of Sod HaIbbur?