It is all about your Declination (time of year) and your Latitude for what happens. It is rather like taking your noon sight to get your latitude. It is all about the Dec.
Zenith Distance (ZD) = 90- Sun's altitude above horizon.
Tilt = ZD
Say at 49N, at 21 Jun when Dec is 23N, and Alt is 64 at high noon.
ZD = 90- 64 = 26. correct tilt at high noon is 26 degrees--except that is not optimum (for a fixed pointing South panel)which is less than that to get more shoulder hour time early morning and late afternoon.
ZD + DEC = LAT so in this case 26 + 23N = 49N Lat.
Later, on 21 Sep Equinox, your Dec is zero. So now Lat = ZD (twice a year like a stopped clock twice a day) and 0 + ZD is 49N so tilt is still ZD or 49, way up from back in June at 26.
Sun is lower, so tilt is up. That is for high noon.
If you have a tilted twirler, then in the morning and late afternoon with it pointing at the sun, which is lower then, you want a higher tilt. As the sun comes around (at 15 degrees an hour) and gets higher, you want to lower the tilt to the high noon amount and then start raising the panel again for the afternoon while still keeping it aimed.
