Science & Technology

Bytes & PCs: Finding God at Thirty-five Thousand Feet

Where to find God was never much of a concern to Jews, armed as we are with the Torah’s very specific maps and directions. When to find Him is a different matter.

As Western Civilization spread to more exotic places, Torah scholars took note of the halachic issues that arose. A comment by Rabbi Yisrael Lifshitz (1782-1860) in his Tiferet Yisrael commentary to the Mishnah1 became a classic locus of discussion. How does one figure out the proper times to pray at the North Pole, where the sun does not set for months during the summer, or rise during the winter? How does one count the days toward Shabbat, when daylight never declines into darkness? The Tiferet Yisrael admits that he cannot solve all the halachic questions, but leans toward having travelers set their clocks according to their points of origin. He concedes that two pole-dwellers, one hailing from North America and the other from Europe, might observe Shabbat at completely different times.

Much Torah thought has gone into these and related questions since then.2 You needn’t be on an Alaskan cruise above the Arctic Circle for these to be relevant. Most people need to deal with these issues at elevated heights, not elevated latitudes on the ground. Say your flight leaves in the evening from the East Coast, heading for Israel; just when are you supposed to daven Shacharit? Clock time “speeds up” as you race toward longitudes “ahead” of your point of departure. I have seen people try to get a few hours of sleep, and wake up so many hours into daylight that when they start putting on tefillin and davening Shacharit, other passengers are getting together for Minchah! I have kept watch for a “sunrise” of sorts, and then sat with other befuddled passengers wondering how much time we had before the statutory few hours of Keriyat Shema recitation evaporated. We knew that we had far less time available than we have on the ground, but just how long did we have? When do passengers on a daytime flight on a fast day break their fasts?

Chaim Keller has solved these problems for the observant air traveler with his Chai Air Travel Tables. Dr. Keller has lots of background in physics and astronomy, and holds a PhD in the former from the University of Washington. After learning in Torah Ohr Kollel for many years, and also davening regularly at a vatikin minyan, he noticed that the calculated times for sunrise just didn’t match his observations. Dr. Keller looked into both the way they were routinely calculated (he found them wanting) and to state-of-the-art methods of computing elevation-appropriate times based on satellite surveys of the earth’s terrain. It led him to write some pretty esoteric scholarly papers on the matter, and to develop a keen interest in all methods of halachic time calculation, including the in-flight issues. After taking his findings and his methods to the Torah luminaries of Jerusalem, he has his celestial turf very well staked out.

Go to the main page at, and find the link to “Zemanim for the Airplane.” Once there, read the introduction and then find the “Enter details of your flight” link to take you to the dialogue boxes where you can enter information specific to your journey. You will enter times and places of departure and arrival. The program will generate a table of important halachic times, including daybreak, sunrise, the end of Shema recitation time and the beginning of Minchah time. Anticipating the realities of contemporary travel, the chart gives you alternate times in case of delayed departure. Daylight savings time is taken into account as well. You can print out the tables (and post them, if possible, for other observant passengers) or you can download them to your PDA.

Various factors make calculation difficult. The tables average the projected travel time over the entire distance. In fact, an individual flight will speed up and slow down along the way. (For this reason, the site is particularly less accurate on short flights, as the aircraft will be spending much of its time at slower velocities after takeoff and before landing.) Weather conditions will often take the flight off the projected Great Circle Route. So will political conditions, especially on El Al flights from Israel to the Far East, which must avoid Arab countries. Tail winds and air corridor congestion can move flights 500 miles from their expected trajectory. The Tiferet Yisrael returns to complicate matters. We have all become polar travelers without knowing it, as many transoceanic flights fly over the Arctic Circle, making it impossible to determine halachic times to the satisfaction of Dr. Keller. The bottom line is that times are accurate only to within a half hour, but this is a huge improvement over the uniformly shrugged shoulders of passengers without the benefit of the Chai Air Travel Times Tables.

One halachic factor is ignored by the Tables, as per the advice of one of the world’s most important posekim, Rav Shalom Yosef Elyashiv, shlit”a. Apparent sunrise is much earlier in a plane than on the ground below; sunset is much later. Which times should the air traveler use? I was able to confirm that Rav Elyashiv holds that times should be calculated according to the ground position below the aircraft. Those who disagree will have to adjust the times further. Good luck.

Dr. Keller’s hope is to generate enough consumer interest that airlines will provide more accurate information about exact routes for every flight. Better yet, he would love to integrate his software with theirs, so that observant travelers on El Al would be given far more precise readings from the flight deck, taking into account the actual route followed, and real-time velocity. In the meantime, his program will allow you to set your alarm clock and fall asleep, confident that you will not miss Shacharit at the proper time.

Once a traveler reaches his or her destination, matters become much more manageable, especially if said traveler’s PDA has a halachic calendar that will compute local halachic times. For those who forget to take them along, relief is never further than the next computer terminal. will do all the calculations for any location on the globe you can think of. Within the United States you can input by city or zip code; in other parts of the world, you need a place name. Myzmanim offers more countries and places than any program I have seen! The display is user friendly, offering not only key times and halachic parameters, but summaries for the following days.

Best of all, however, is a real life saver for travelers within the United States who are not near a computer. You can receive a short summary sheet of daily halachic times as a text message on all text-enabled cell phones anywhere in the United States. Just call 516-7-ZMANIM (516-796-2646) and enter the zip code for the location you request. Within seconds, you will receive, free of charge, an SMS Zmanim text message. Requests are limited to one per day and work for all major service providers. This is a phone number you should be sure to add to your contact list.

While much progress has been made for the observant traveler, some halachic issues remain intractable. Before providing real advice on prayer times for observant astronauts, the article cited in note 2 offers that “ideally, one should not travel to outer space.” This remains good advice, even to halachah– and computer-savvy readers of this column.


Rabbi Adlerstein tries to daven at the right time between jobs while juggling his responsibilities at the Simon Wiesenthal Center, Loyola Law School and Jewish Action, where he serves on the editorial board.



  1. Berachot, following the first chapter.
  2. A particularly good survey of computing halachic times in northern and southern latitudes can be found in Rabbi Dovid Heber, “When Does One Pray When There Is No Day: A Guide to Shabbos Observance and Prayer Times in Alaska and Other Arctic Regions,” Kashrus Kurrents (autumn 2002), which can be found at
This article was featured in the Summer 2006 issue of Jewish Action.
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