No, it's no secret. Do an internet search for "obliquity of the ecliptic" and you'll find plenty of references. However, think about it - the total change is only 2° and the half cycle is about 20,000 years. This means that the obliquity changes about 1° every 10,000 years - a tenth of a degree every thousand years, or a hundredth of a degree every century.
For the purposes of navigation, this is not too much of a big deal, and therefore most aviators don't get very excited about it. But it is quite well recorded in astronomical literature. The obliquity is now about 23.44°, as mentioned. In the Pyramid Age, it was about 24.02°. It has been decreasing from a peak of about 24° some 8,000 years ago toward a low of about 22° some 13,000 years hence. The obliquity goes through cycles of varying amplitudes with a period of about 41,000 years. The rate varies-currently it is about 0.47 arcseconds per year.
It is caused by the interaction of gravity from other planets' orbits. The limits of the tropics are whatever the obliquity happens to be at the time - currently 23.44°. This means that the tropics also vary between about 24° N/S to about 22° N/S over 20,000 years.
To calculate the difference in the length of daylight precisely would require some spherical trigonometry which I can't be bothered to work out precisely just to answer a casual internet query. But you can get an approximate but pretty accurate answer by saying that 2° difference in the position of the tropics would be like the difference between 50N and 52N - ie, not very much. Work it out from the Air Almanac if you want to - I haven't got one with me right now.
Hope this helps