In-Flight Zmanim FAQ

Airline Zmanim
This is an FAQ about El Al’s recently introduced in-flight zmanim system that uses the KosherJava zmanim code. This FAQ is almost identical to what you will see on El Al flights, but it is slightly expanded and includes links to articles that are not available on the in-flight system.

  • Q: What is the Prayer Room / Mesivta Derakia?
    A: The Prayer Room / Mesivta Derakia is an in-flight system app built by FlightPath3D that shows zmanim (Jewish religious prayer and observance times) and directions for Jewish prayers.
  • Q: What is the difference between the options of Aircraft Location, Origin and Destination?
    A: These options change the time zone that is used for displaying times. For origin and destination, everything is in local time, but for aircraft location, times are relative, (i.e tzais is in 2h 13min). For example, if you are flying from New York to Tel Aviv, the “Origin” option will show zmanim based on NY time, while “Destination” will show the zmanim in Israel time. The “Aircraft Location” option shows the time interval to upcoming zmanim, as opposed to clock time. Use whichever option works best for you. These variations ultimately provide the same accurate zmanim times, but with a different frame of reference.
  • Q: Are these calculations accurate?
    A: Yes. The In-flight zmanim calculations are based on Rabbinically reviewed zmanim times and accurate flight path information. Your current location and flight path are sourced from the airline’s navigation system and are thus extremely precise. The times calculated in this way are much more accurate than pre-calculated times, since they are produced by real-time calculations using the flight’s exact current and future locations and the actual flight path of this flight. The times are updated regularly to account for the fact that winds can cause significant changes in the velocity of the flight as compared to initial projections. Furthermore, the flight path itself may change mid-flight. While land-based calculations usually suggest applying a 2-minute stringency to account for various weather-related factors, in the case of in-flight zmanim a 10-minute stringency is recommended, since aircraft speed and recalculation intervals can vary. See below for information about the source of the calculations used to calculate the zmanim.
  • Q: How are zmanim times on a flight different from regular zmanim?
    A: In a fixed position, zmanim calculations are straightforward (as straightforward as spherical trigonometry is concerned 😊) but when on a flight we must calculate when and where the prayer time will be reached based on your current location, the flight speed, and projected flight path. See the article Inflight Zmanim Calculations – Why So Complex? to understand some of the complexities involved without a solution like this one.
  • Q: Why do the zmanim seem to change over time?
    A: The times are calculated on the basis of the aircraft’s projected speed and flight path. Due to changes of speed caused by headwinds or tailwinds, and possible deviations from the originally planned flight path, the times need to be recalculated on a regular basis. This is similar to the way the “time to destination” on the flight map might change during the course of the flight.
  • Q: What is the Compass?
    A: This shows the prayer direction to Jerusalem. Typically, it will be toward the nose of the aircraft on eastbound flights from the USA to Israel and facing the tail when flying westbound from Israel to the USA, but the exact direction can vary for part of the flight. It is recalculated on a regular basis (as often as every 10 seconds). The great circle direction is used for this calculation. For more information on great circle versus rhumb line directions, see the various articles on this site dedicated to the subject.
  • Q: Are sunrise and sunset calculated at ground level or at the plane’s altitude?
    A: The calculations are done at sea level. According to most Rabbinic opinions, when flying one should use the times on the ground directly below for sunrise. All opinions agree that for sunset, the times at ground level should be used. Aside from how this affects regular zmanim, it also impacts the start time of Shabbos on late Friday flights (where Shabbos begins even if the sun is still visible from elevation). Additional points to note are that at airline elevation, extreme atmospheric refraction can make elevation-based calculations inaccurate; furthermore, it is hard to predict what the plane’s elevation will be later in the flight. If you would like to follow the opinion of the Yisroel Vehazmanim that elevation is taken into account with regards to the start time for Shacharis (in order to fulfill ייראוך עם שמש for Vasikin), try finding the sun through a window (something that can be very hard with electrochromic windows). Based on your location and time of year, this will be anywhere from 14 minutes (at the equator) to as much as two hours prior to ground level sunrise near the Arctic. Elevation based sunset time should never be used unless it is for a stringency. Note that due to the complexity of gathering ground level elevation along the flight path (that is mostly over the sea in any case), sea level is used for calculations as opposed to ground level. This will have a minor impact on sunset times according to many opinions.
  • Q: What is the earliest time for Shacharis (the morning prayers)?
    A: Ideally, one should pray the Shacharis Shemoneh Esrei at sunrise (ground level). This is very difficult to achieve on a flight constantly in motion. Ideally, begin putting on Tallis and Tefillin about 15-20 minutes before the projected time of sunrise. This will ensure that you say Shemoneh Esrei after sunrise. In a time of need (e.g., the flight is landing soon with a short time before a connecting flight), one may begin Shacharis at the time of Alos HaShachar and wait until Misheyakir to recite the blessing on Tallis and Tefillin. One may recite Shema until the Sof Zman Krias Shema time indicated in the system. If you are unable to pray right after sunrise, we recommend that you at least recite Shema at that point to ensure that you don’t miss the proper time. Make sure to finish Shacharis Shemoneh Esrei well before the latest time noted. In far north (or south) regions during the winter, such zmanim pass very quickly due to the very short days, so one should begin prayers before sunrise (but not before Alos HaShachar).
  • Q: Why are there two different times for Alos HaShachar?
    A: Alos HaShachar (dawn) is the instant when the night ends and the day begins. There are differing opinions as to the time of Alos Hashachar. To fulfill both, use the one that is more stringent in each case. The 16.1° Alos is the earlier one, when the sun is 16.1 degrees below the horizon. This should be the latest time that one may pray Maariv and is the time that one must stop eating prior to Jewish fast days that begin in the morning. The 72-minute Alos is 72 minutes before sunrise. This should be the earliest time that one may perform daytime mitzvos in case of a great need (bisha’as hadchak), otherwise one should wait until sunrise. Note that in locations near the equator (such as during flights between Israel and South Africa), the 72-minute Alos may be earlier than the 16.1° Alos, and the logic above should be reversed in those scenarios.
  • Q: What is the earliest time for Mincha (afternoon prayers)?
    A: One half hour after Chatzos (solar noon, the halfway point between sunrise and sunset). When the day is more than 12 hours long, a half hour is calculated as 1/24th of the day, or half of a sha’ah zmanis (variable hour). When the day is less than 12 hours long, a stringency of using 30 clock minutes is used.
  • Q: When can I start Maariv (evening prayers)?
    A: Ideally wait until the time of Tzais HaKochavim (appearance of stars) as indicated on the system. If this is not possible, one may pray Maariv immediately after sunset, or if there is a great need, under certain conditions, even after Plag HaMincha. In these two cases, Shema should be repeated after Tzais HaKochavim. When praying at Plag HaMincha, wait a few minutes after Plag HaMincha to start Maariv.
  • Q: When does a fast start?
    A: At Alos HaShachar (except for Yom Kippur and Tisha B’Av, which start at the previous day’s sunset). See above concerning the two different times given for Alos HaShachar.
  • Q: When does a fast end?
    A: At Tzais HaKochavim. The earlier Tzais HaKochavim of 8.5° is used by most. Some are stringent to wait for the 72-minute Tzais.
  • Q: How are the zmanim calculated, and is there any rabbinic supervision on these zmanim?
    A: Zmanim are calculated using Eliyahu Hershfeld’s KosherJava zmanim API. This has been reviewed by Rabbi Yisroel Dovid Harfenes, Rabbi Dovid Heber and Rabbi Dovid Braunfeld. The flight path calculations used to project when the zmanim will be reached use R’ Chaim Keller’s famed Chai Tables code.
  • Q: Why are some zmanim times blank when over the Arctic Circle, and how does this impact prayer time and zmanim?
    A: In the arctic there are periods when there is no day in the winter and no night in the summer. A rabbi should be consulted if you know that you will be flying over this area. For details, please read Rabbi Dovid Heber’s article When Does One Pray When There is No Day on the Star-K’s website. An example of where this issue can arise is if a flight from Los Angeles to Tel Aviv leaves in the late morning and arrives late the next morning in Tel Aviv. The route will sometimes fly north of the Arctic Circle (e.g., if the screen shows the plane flying north of Iceland) during the evening hours, but since one is so far north, in the summertime the sun never sets. In this case, one could not properly fulfill his obligation to recite the nighttime Shema, because the sun never goes down that night. A technical article on the subject can be seen in the article Why Some Zmanim Never Occur (Developers Beware) on this site.
  • Q: Why does the system display “No zmanim applicable on this flight”?
    A: On some short flights, there are no zmanim events. An example of this is a flight from Tel Aviv to Rome leaving Tel Aviv at 5:55 pm and arriving in Rome at 8:45 pm. In the winter, this flight takes off after tzais and lands before the next zman, chatzos halayla. In situations like this, the system is indicating that no relevant zmanim will be reached during the flight.
  • Q: What are the halachic dateline times and how are they used?
    A: According to most opinions, the Civil International Dateline has no bearing on the halachic dateline. There are various opinions as to where the halachic dateline is located. In the opinion of the Chazon Ish, the dateline is 90° east of Jerusalem at a longitude of 125.2°. This line cuts through Korea, eastern China and the eastern part of Russia. According to the Chazon Ish, the line does not split a landmass, and extends to the eastern coast when it crosses a landmass. The opinion of Rav Yechiel Michel Tucazinsky and others is that the dateline is 180° east/west of Jerusalem along the longitude of -144.7° that cuts across the eastern side of Alaska near the Canadian border and between Hawaii and California. There are varying opinions as to whether this line extends westward to the edge of the Alaskan coast. On flights that will cross these lines, the time of the crossing appears in the list of zmanim. The halachic date will change when crossing the line. For example, an eastbound flight from Hong Kong to New York will cross the Chazon Ish line about two and a half hours into the flight. At this point according to the Chazon Ish, the day will roll back to the previous day. According to Rav Tucazinsky, the day will not roll back to the previous day until the flight approaches the Alaska / Canada border. This subject is much too complex to be properly covered here, and a competent rabbi should be consulted before taking a flight crossing these lines. For more information in English, see R. Zalman (Solly) Tropper’s book Kitzur Taarich Yisrael. A free electronic version of the book, and additional information, is available on his site Dateline in Halacha. Also see Rabbi Dovid Heber’s excellent article A Traveler’s Guide to the International Dateline. This site also has various articles on the subject of the halachic dateline.
  • Q: Were can I find answers to additional questions that are not covered in this FAQ?
    A: See Rabbi Heber’s article Time Flies about general halachos related to flying. If you have comments or addition questions, you can contact the author via the information in the contact page.

לז״נ אבי מורי הרב יצחק אריה בן ר׳ ברוך הירשפלד ז״ל הכ״מ. נפ׳ י׳ אדר ב׳ תשפ״ב.
In memory of my father R’ Yitzchok Hershfeld (Montreal).

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