While calculations of days, months and years are based on fixed hours equal to 1/24 of a day, the beginning of each day is based on the local time of . The end of the Shabbat and other is based on nightfall () which occurs some amount of time, typically 42 to 72 minutes, after sunset. According to Maimonides, nightfall occurs when three medium-sized stars become visible after sunset. By the 17th century this had become three second-magnitude stars. The modern definition is when the center of the sun is 7° below the geometric (airless) horizon, somewhat later than civil twilight at 6°. The beginning of the daytime portion of each day is determined both by dawn and . Most times are based on some combination of these four times and vary from day to day throughout the year and also vary significantly depending on location. The daytime hours are often divided into or "Halachic hours" by taking the time between sunrise and sunset or between dawn and nightfall and dividing it into 12 equal hours. The nighttime hours are similarly divided into 12 equal portions, albeit a different amount of time than the "hours" of the daytime. The earliest and latest times for , the latest time to eat on the day before and many other rules are based on . For convenience, the modern day using is often discussed as if sunset were at 6:00pm, sunrise at 6:00am and each hour were equal to a fixed hour. For example, noon may be after 1:00pm in some areas during . Within the , however, the numbering of the hours starts with the "first" hour after the start of the day.
Another memory aid notes that intervals of the follow the same pattern as do Jewish leap years, with corresponding to year 19 (or 0): a in the scale corresponds to two common years between consecutive leap years, and a to one common year between two leap years. This connection with the major scale is more plain in the context of : counting the tonic as 0, the notes of the major scale in 19 equal temperament are numbers 0 (or 19), 3, 6, 8, 11, 14, 17, the same numbers as the leap years in the Hebrew calendar.
When teaching the months of the year, talk about the four seasons and how they fall in the same months every year. To help further the connection, break out your photo albums (or your smartphone!) and show your preschooler pictures of your friends and family in action during the various months, calling attention to the particular season and the month in which it falls.
In calculating the number of months that will have passed since the known molad that one uses as the starting point, one must remember to include any leap month(s) that falls within the elapsed interval, according to the cycle of leap years. A 19-year cycle of 235 synodic months has 991 weeks 2 days 16 hours 595 parts, a common year of 12 synodic months has 50 weeks 4 days 8 hours 876 parts, while a leap year of 13 synodic months has 54 weeks 5 days 21 hours 589 parts.
I hope you enjoyed learning all about Days, Weeks, Months and Years.
Now I must return home. Bye for now!