Zmanim

These times appear in various contexts: Shabbat and Jewish holidays begin and end at specific times in the evening, while some rituals must be performed during the day or the night, or during specific hours of the day or night.

According to Rabbeinu Tam, nightfall occurs exactly 72 (or 90[8]) minutes after sundown (or, equivalently, when the sun falls 16.1° or 20° below the horizon).

[3] A third Talmudic passage (Shabbat 35b) states that nightfall occurs when three medium-sized stars become visible.

Until recently, all Jewish communities followed this passage, waiting for the observation of three stars to end Shabbat.

To reconcile the passages, various writers have proposed that halachic "sundown" (the beginning of bein hashemashot) is not when the sun crosses the horizon, but rather earlier (according to Rabbeinu Tam) or later (according to Geonim).

Some, following the interpretation of Rabbeinu Tam, wait until 72 (or 90) minutes after astronomical sunset; this is common practice in Chasidic and other Charedi communities.

For morning calculations, daybreak is normally held to be when the sun is 16.1° below the horizon, or else a fixed 72 (or 90) minutes before sunrise.

This is the earliest time to wear tzitzit and tefillin (though ex post facto, if one did so after Alot Hashachar, he fulfilled his obligation).

Misheyakir is generally calculated relative to season and place, and because there are no Talmudic or early sources as to when this time occurs, there are a wide range of opinions.

On Shabbat, and Jewish holidays, it is preferable to begin Mussaf by this time, because otherwise it is questionable whether they would be required to pray the more frequent prayer (Minchah) first.

If one prayed Minchah before this time, one may recite Maariv afterwards (at the conclusion of the Sabbath, this may only be done under extenuating circumstances).

[15] Sunset (שְׁקִיעַת הַחַמָּה‎, Shkiyat Hachamah, often referred to simply as ShKia/Sh'Kia/SheKia),[16] is the time at which the ball of the sun falls below the horizon.

Bein Hashemashot (בֵּין הַשְּׁמָשׁוֹת, literally between the suns) is the period between sunset and nightfall, and is considered a time of questionable status.

For example, if the fast of Tish'a Ba'av immediately follows the Sabbath, the intervening Bein Hashemashot is forbidden in eating, drinking, and working.

Some rise at this time and recite Tikkun Chatzot, a series of supplications for the rebuilding of the Temple.