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 ZT"L 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/EN/Default.aspx).
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://www.chaitables.com/chai_eng.php).
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 Zalman Melamed instructed us to show only astronomical sunset times on the site.
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, therefore in Israel this is the time we put on the site. Outside of Israel, the time is measured in degrees according to the angle between the sun and the earth.
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. In Israel, the time of "Alot Hashachar" in the site is 72 minutes before the astronomical Netz by the ruling of Rabbi Zaman Melamed. This is because in the Talmud the time between the two is measured by the time it takes to walk a Mill (aprox. 980 meters). Seeing as this measurement doesnt change between winter and summer so to the time between Alot Hashachar and Netz don't change.
In Israel, the Amud HaShachar brought on the site is 72 regular minutes before astronomical sunrise. Outside of Israel, Amud HaShachar is measured in degrees according to the angle between the sun and the earth. Regarding the serious fasts (Yom Kippur and The 9th of Av) the start of the fast on the website is according to 90 degrees. Regarding the light fasts, the start of the fast on the website is according to 72 degrees. Those wishing to be strict can use the time for alot hashachar 90 degrees on the website.
According to the MG"A the calculation of the seasonal hours is from Amud Hashachar until Tzet Hakochavim. As was mentioned earlier, they are different opinions how to calculate Amud Hashachar. In our calendar in Israel, we followed the opinion of most of the Poskim, as was explained previously, which is 72 minutes before sunrise. Outside of Israel, Amud HaShachar is measured in degrees according to the angle between the sun and the earth.
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 Zalman 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 Devlitsky, 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 Devlitsky 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, Haifa and the Shomron is 30 minutes before sunset. In Tel Aviv and Gush Dan they hold by 21 minutes before sunset, in Be'er Sheva and Ashdod 22 minutes, and in ra'anana 20 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.
The 10th hour displayed on the site is the beginning of the tenth hour and the end of the 9th hour
The Molad times are based on the local time in Jerusalem (with changes according to daylight saving time), according to the ruling of Rabbi Zalman Melamed. Since the Molad times are based on Jerusalem, and the times are the same throughout the world, it is irrelevant to present an incorrect time (winter time during DST), in a way that might confuse people
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).
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