Explanation of Yeshiva.org.il's Hebrew calendar principles
The subject of times is a very complicated one in Halacha, there are few rabbis who are experts on the subject, and are familiar with both all Halachic opinions and various astronomic calculations by which the times are determined. In order to be as accurate as possible and to offer data the public can rely upon, we've approached Hagaon Rav Sraya Devlitsky Shlit"a of Bnei Brak, to guide us on this issue. For building sunrise charts we've had generous help from Rabbi Chaim Keller, who composed the "Bikurei Yosef" tables. All calculations are based on the "Chazon Shamaim" program, made by Rabbi Eitan Tzikuni (for details: http://www.sky-view.co.il).
It is recommended to consult a Halachic authority on border-line cases such as a Brit Milla etc.
Here are the principles upon which our calendar is based, according to the guidance we received from the rav:
The sunrise that appears on our site is the visible sunrise for the following places: Eilat, Be'er Sheva, Bat Yam, Holon, Haifa, Tiberias, Jerusalem, Safed ,Tel Aviv and Bet-El. For all other places the time is the astronomical sunrise. Rabbi Devlitsky instructed to follow these times for davening Vatikin, and Lechatchila for all Mitzvot done during the day. Visible sunrise is calculated by: A. Topographic data of the highest place in the area. B. The sun's appearance over the high mountains in the horizon. Data was collected from Rabbi Chaim Keller's "Bikurei Yosef" tables (for further information see http://184.108.40.206/webpub/chaiintro.html).
For seasonal hours on the site, we calculate the beginning of the day by the astronomical sunrise, meaning according to the sea-level horizon and not by the visible sunrise. This means that latest times for Shema etc. are calculated by the day starting from the astronomical sunrise.
Rabbi Devlitsky instructed that the correct time for sunset is the visible sunset. Here too, for Mitzvot done during the day he instructed to rule stringently by the visible sunset, and for calculating seasonal hours by the astronomical sunset. However, since some places have mountains hiding the sky which can cause the sunset to "move" a whole hour back, and this can have consequences in De'oritah issues, Rabbi alman Melamed instructed us to show only astronomical sunset times on the site.
Earliest Tzitzit and Tefillin Time
In Yerushalaim the Minhag was to wear them an hour before sunrise. People who were precise and careful were stringent and put Tefillin on only 52 minutes before sunrise.
In his Minyan, Rabbi Devlitsky lead a time of 45 minutes before visible sunrise, which is the time we put on the site.
Amud Hashachar should be calculated stringently. Concerning things that are done at night until Amud Hashachar, one should follow Rabbi Tikochinsky's opinion, which is 90 minutes calculated in the degree method before astronomical sunrise, as is the opinion of the GR"A. Regarding things done by day and begin at Amud Hashachar, the time is 72 regular minutes before astronomical sunrise.
The Amud Hashachar brought on the site is 72 regular minutes before astronomical sunrise.
Chatzot Hayom (Midday)
The calculation is astronomical. Chatzot Hayom is when the sun is at an azimuth of 90 degrees.
During the summer (when the day is longer than 12 hours), Mincha Gedola time is 30 seasonal minutes after Chatzot Hayom. During the winter (when the day is shorter than 12 hours), Mincha Gedola time is 30 regular minutes after Chatzot Hayom.
According to the Rosh Yeshiva, Rabbi Zalmin Baruch Melamed, the time of "Tzet Hakochavim" in the summer (when the day is longer than 12 hours), is 18 temporariness minutes ", Dakot Zmaniyot" after sunset. In the winter (when the day is shorter than 12 hours), the time is 18 regular minutes, "Dakot Pshutot" after sunset, there for the time of "Tzet Hakochavim" is never less than 18 minutes after sunset. This is the time of the end of the fast days, not including Tisha B'Av.
On Tisha B'Av, one should be stringent, Machmir since Chazal forbade on Tisha B'Av also it's "Ben Hashmahot", the time between sunset and "Tzet Hakochavim", Therefor the end of the fast is 6.45 degrees underneath the horizon. (see the book "Ben Hashmashot" by Rabbi Yeciel Michel Tuktzinsky page 74)
Regarding Yom Kipur, the end of the fast is the same time as the end of Shabbat.
According to Rabbi Dablitzky, the end of the "easy fasts" including "Yom Kipur Katan", is 15 minutes after sunset.
Regarding Yom Kipur and the rest of these days Rabbi Zablitzky goes by "Rabenu tam's" times.
Shabbat starts a set number of minutes before astronomical sunset. This number varies by place. For example, the Minhag in Yerushalaim and Petach Tiqva is 40 minutes before sunset. The Minhag in Safed, Tiberias and Haifa is 30 minutes before sunset. In Tel Aviv and Gush Dan they hold by 21 minutes before sunset, and in Be'er Sheva and Ashdod 22 minutes.
In other places where we don't know of a specific Minhad, the time for Knissat Shabbat will be 30 minutes before sunset, as ruled by the Mishna Brura.
Shabbat ends according to when the small stars appear in the sky. On the site we brought the usual Tzet Hakochavim, and according to Rabbi Tikochinsky's calendar - when the sun is 8.5 degrees under the horizon. This is between 30 to 38 minutes after sunset.
According to Rabbenu Tam, Shabbat ends 72 regular minutes after level sunset.
In a large city where there are different height coordinates the calculation is according to the average of the high places in the city, that affect visible sunrise and sunset. In Israel it may be a difference of a few seconds either way.
Visible sunrise times are rounded down to five seconds, and visible sunset rounded up to five seconds. Daily times are rounded to the closest minute by stringency (for instance, Alot Hashachar to the later minute, but latest time for Shema to the earlier minute).