You've touched on a subject which is discussed at great length in various sources in Jewish literature. There are differing opinions on the halachic definition of sundown, and the 12th century French Talmudic scholar Rabbeinu Tam is renowned for his opinion in this matter. The following is a short synopsis of the debate:

According to Jewish law, "shkiah" (sunset) is the cut-off time for all mitzvot associated with daytime hours (click here for more on this topic), and nightfall is the starting point for all mitzvot associated with the following day.1 However, as aforementioned, the exact definitions of sunset and nightfall are the subject of debate amongst the greatest halachic authorities.

The commonly held custom follows the opinion that sunset occurs when the sun totally sinks beneath the horizon, and nightfall is when three medium sized stars appear in the sky. This is shortly after sunset, but the exact timing depends on the location and time of year.

Rabbeinu Tam was of the opinion, however, that sunset is actually at a later time. He based his opinion on what appears to be contradictory statements in the Talmud regarding the length of the period between sunset and nightfall. In one place2 Rabbi Judah says that the length of this period is the same as the time it would take an average person to walk four mil3 (approximately 72 minutes), whereas Rabbi Judah is elsewhere quoted4 saying that the time between sunset and nightfall is the same amount of time as takes to walk ¾ of a mil (13.5 minutes). Rabbeinu Tam reconciles these two statements as follows: Nightfall is 72 minutes after the sun disappears beneath the horizon; however, the halachic sunset (which marks the end of the day) is only 13.5 minutes before nightfall. Although the sun sets beneath the horizon earlier, in Rabeinu Tam's opinion there is still some light visible for another 58.5 minutes, therefore rendering it day and not night.

According to this opinion, those areas in halachah that depend on nightfall, such as the end-time of Shabbat and holidays, must wait until 72 minutes after the sun sets below the horizon (approximately half an hour later than the commonly observed "nightfall"). And those mitzvot that may be done until sunset can be done until 13.5 minutes before this nightfall, (58.5 minutes later than what is commonly considered sunset).

Many other great authorities reconciled this seeming contradiction quite differently, and therefore maintained that the end of the day is marked by the sun's complete disappearance beneath the horizon, and night fall when three medium-sized stars appear.

Although in pre-war Europe there were many communities who followed the opinion of Rabeinu Tam in all instances, today it commonly accepted to follow the latter opinion. However, some people extend the Shabbat at its conclusion on Saturday night in order to be sure that Shabbat is over even according to the opinion of Rabeinu Tam. In some communities his opinion is also relied upon with regard to the latest time for praying mincha, the afternoon service.

Best wishes for a sweet new year,

Rabbi Baruch S. Davidson

P.S. The sunset and nightfall times on our Halachic Times page follows the commonly accepted opinion and custom, not Rabbeinu Tam's opinion.