Computing halachic times ("zmanim")
With the help of this program you can find out the daily schedule of Jewish prayers, candlelighting and the end of Shabbat or a holiday. Below is a short description of the times computed. By default we compute all relevant moments of the day according to the opinion of the Vilna Gaon (GR"O), which is the most widely accepted, with the exception of the time for nightfall and end of Shabbat and holidays. See below about other opinions.
Currently this feature is available only for US locations, by postal zip code. This program is not yet very well tested, so preciseness of the times is not guaranteed. Please let us know of any errors if you find them. We are working on an extended international version, so that international locations will be added soon.
Description of zmanim.
1. 'Sunrise' is the time of appearance of the sun above the horizon.
2. According to the Talmud and commentaries, dawn is the time when an average person can recognize colours (see Tractate Brachot for the discussion and various opinions on which colours, at what distance, etc.). Since there is no objective criterion for this, two methods are being used: a certain angle at which the sun should be below the horizon, or fixed 72 astronomical minutes.
3. Sunset is the time when the sun disappears completely below the horizon.
4. The time of nightfall has many implications in Jewish law. The most widely accepted is the opinion that night (and so the new Jewish day) begins with the appearance of three medium-sized stars. Since there is, again, no objective criterion for this, and the stars are not always visible, most authorities use a certain angle at which the sun should descend below the horizon. The most widely accepted opinion on this is, in turn, that of the gaonim, and we use it here. The angle use is approximately 8 degrees, which corresponds to the angle of descent of the sun 40 minutes after sunset at equinox in Jerusalem.
Regarding this the Vilna Gaon had an opinion that 18 minutes, and 42 should be used, normalized according to equinox at the latitude of Jerusalem.
According to Rabbeinu Tam, one of the well-known Tosafot (commentators on the Talmud), night falls 72 astronomical minutes after sunset, which is the time it took an average person to walk 4 mil (a unit of distance used in the Talmud). Most of the time, unless you live in very high latitude, this occurs much later than the nightfall according to other opinion. Some communities have a custom to wait until this time before ending Shabbat and holidays.
5. A halachic hour is 1/12th part of the daytime, and not the astronomical hour. At night, 1/12 of the night is used. According to most widely accepted opinion of the GR"O, "daytime" is from sunrise to sunset. In some communities it is customary to also take into account the opinion of Mogen Avrohom for the purpose of determining the proper times for prayers. According to the latter opinion, daytime should be considered to span the period between dawn and nightfall. For that purpose we use fixed 72 minutes (Rabbeinu Tam's opinion) here to compute dawn and nightfall, which is for all other ends a less accepted opinion. Usually, unless you live on unusually high latitude, the difference should be negligible.
6. Halachic noon is just that - the middle of the day (which is the same to both GR"O and Mogen Avrohom).
7. The following times of the day determine proper times to pray minchah (afternoon service) according to different customs, the time on Fridays when it becomes possible to accept Shabbat.
1) Minchah Gedolah (earliest minchah) - 1/2 hour after the halachic noon.
2) Minchah Ktanah - 3.5 hours after (halachic) noon.
3) Plag Minchah - 4.75 hours after noon.
8. Midnight - middle of the night.
9. For the proper time to put on Tefillin, the commentaries prescribe the time when an average person can recognize an acquaintance in the street at a certain distance. In practice, we use a certain angle (about 6 degrees) below the horizon.
Your commentaries and notes on possible bugs, errors in this text etc. are appreciated.