I'm measuring the rise and set of the Pleiades on every day of october and saw an suddenly 1 hour difference. The input I use is rise Pleiades on first date = year 1507 - month 10 - day 26 AD and time = hour 8 - min 1 - sec 1 PM.
Next I increase the day 26 with 1 to 27 and then I get an date = year 1507 - month 10 - day 27 AD with time hour 7 - min 22 - sec 1 PM.
The difference between de two rises of the Pleiades may be max. 4 minutes, but I get this strange difference of 1 hour. Who can explain this strange effect?