Of jewish dating
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 sunrise.
Until the Tannaitic period (approximately 10–220 CE), the calendar employed a new crescent moon, with an additional month normally added every two or three years to correct for the difference between twelve lunar months and the solar year.
When to add it was based on observation of natural agriculture-related events in Israel.
Maimonides' work also replaced counting "years since the destruction of the Temple" with the modern creation-era Anno Mundi.
The Hebrew lunar year is about eleven days shorter than the solar year and uses the 19-year Metonic cycle to bring it into line with the solar year, with the addition of an intercalary month every two or three years, for a total of seven times per 19 years. or AD), the words or abbreviation for Anno Mundi (A. or AM) for the era should properly precede the date rather than follow it.
(In most populated parts of the world this is always approximately 24 standard hours; but, depending on the season of the year, it can be slightly less or slightly more.) The time between sunset and the time when three stars are visible (known as 'tzait ha'kochavim') is known as 'bein hashmashot' and for some uses it is debated as what day it is.
In Judaism, an hour is defined as 1/12 of the time from sunrise to sunset, so, during the winter, an hour can be much less than 60 minutes, and during the summer, it can be much more than 60 minutes.
This proportional hour is known as a 'sha'ah z'manit' (lit. Instead of the international date line convention, there are varying opinions as to where the day changes. (Jerusalem is 35°13’ east of the prime meridian, so the antimeridian is at 144°47' W, passing through eastern Alaska.) Other opinions exist as well.
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.
While calculations of days, months and years are based on fixed hours equal to of a day, the beginning of each halachic day is based on the local time of sunset.