Introduction to the The Vatigash Gam Leah Air Travel Times

(last revised the 28th of Iyar 5774,  5/28/2014)

It is difficult for airplane passengers to determine the correct time for prayers. This is especially true for eastbound cross-Atlantic night flights during the summer months. For those flights, it is not uncommon for sunrise to occur less than three hours after takeoff, quite shortly after the passengers have been fed and plan to go asleep for the "night". Sof z'man krias shma occurs, on the average, only one and a half hours after that! In the figure below, the time from departure to sunrise, and the time from sunrise to sof z'man krias shma for a typical flight between New York and Eretz Yisroel are plotted. Notice that during the winter there is only one hour between sunrise and sof z'man krias shma. Even in the summer, this time difference never gets larger than 2.2 hours. (This points out the necessity for all passengers on such flights to begin morning prayer at least shortly after sunrise.) Passengers on westbound flights from Eretz Yisroel often encounter alos haschahar (dawn) before the airline serves breakfast. This problem is compounded when traveling on a night before a taanis (fast).

It is clearly essential that the times of z'manei hayom become available to air travelers. The goal of the Chai Air Tables is to provide the best possible estimate of those times. (The proper source for the times of z'manei hayom is the airplane crew itself since they track the position of the airplane to a high precision. We should encourage the airlines to calculate these times for us.)

Sunrise and sof z'man krias shma occur so early in east bound flights because of the combination of two factors: (1) the eastbound velocity of the airplane, and (2) the initial northerly path of the airplane. It is easily understood how the airplane's eastbound velocity can make the day arrive earlier. However the second factor needs some explanation. The pictures below depict the

earth during the summer (left) and winter (right). Two typical flight paths to Eretz Yisroel originating in Los Angeles and New York are drawn. Notice that the flight paths are straight lines when viewed from space. Nevertheless, when these flight paths are projected on the globe, they pass through latitudes further north than the place of origin and the place of arrival. See the figure below. As can be seen from the figure below, the flight paths have parabolic shapes when drawn on a flat projection of the earth.

The flight path from New York to Tel Aviv climbs to more than 52 degrees north latitude. (New York is less than 41 degrees north latitude. Tel Aviv is approximately 32 degrees north latitude.) The flight path from Los Angeles to Tel Aviv passes over Greenland at latitudes greater than 70 degrees north latitude. However, Los Angeles is approximately 34 degrees north latitude.

The airplane's northerly route causes the passenger's sunlight hours to increase in the summer during the first half of the journey. Likewise, the passenger's sunlight hours decrease in the winter during the first half of the journey as the plane flies to more northern latitudes. Since the earth is tilted by 23.5 degrees with respect to the horizontal, places that are higher north than 66.5 degrees latitude (the line labeled "Arctic Circle" in the figure above) experience a perpetual day in the summer and a perpetual night in the winter. By the same token, places further north have longer days in the summer and shorter days in the winter. A passenger leaving on a (theoretical) summer night's non-stop flight from Los Angeles to Eretz Yisroel will encounter sunrise over Northern Canada. He will then pass the Arctic Circle to a region that is experiencing perpetual daylight. For that passenger there is no longer any sunrise or sunset, nor any z'manei hayom. A night passenger from New York in the summer will experience a less drastic effect. Nevertheless he will encounter sunrise much sooner than expected.

Due to changing air traffic and weather conditions along transatlantic routes, flights may be rerouted up to several hundred kilometers from the ideal paths depicted in the figures above. This is a source of inaccuracy for the predicted times. (For example, if the plane is diverted to the north in the summer, then sunrise will come even earlier.) Furthermore, the airplane's speed varies during its journey and between flights on the same route. (Airplanes can have different speeds due to the presence of or lack of tail winds. After takeoff, the airplane is travelling slower and lower than the average. This is also the case before landing. This causes any z'manei hayom that are supposed to occur close to takeoff and landing to be more inaccurate than what is to be expected on the average.) Since these variations are not known a priori, it is necessary to calculate the Chai Air Travel Tables using an average predicted speed throughout the route (the total flight distance divided by the difference of the arrival and departure times). In general the accuracy of these tables is expected to be better than a half hour. For this reason one should not wait to complete krias shma later than a half hour before the listed time. Even though Morning Prayer and krias shma can be said bedieved before the sunrise, krias shma must be completed no later than the third hour (shaos z'manios).

From the previous discussion it is clear that these tables only apply for non-circuitous airline routes. This is usually a valid assumption. However, there are striking exceptions, e.g., El Al flights to the Far East. For those flights, these tables cannot be used. In general, the passenger should clarify whether the airline flies a direct route before using these tables.  This can be accomplished by comparing the air route depicted in the "Ideal Air Route" previewer (a link is provided on the air times web page, or here) to the air routes shown in the airline's websites. 

Now, with the advent of the on-board Chai Air Times "Pro" program, the online program should be used with great caution or not at all in the following four situations:: (1) long flights lasting more than 14 hours (they often follow non-ideal routes to avoid headwinds, bad weather, etc.), (2) easterly flights over the North Atlantic during the winter months (they often encounter tail winds), (3) flights from North America to the Far East [they often follow non-ideal routes for the reasons given in (1)], (4) all El Al flights to eastern destinations (they are always circuitous due to restrictive routing).  The full version of the Chai Air Times "Pro" program now has a GPS device interface which makes it highly user friendly and gives the calculations unbeatable accuracy.  Furthermore, the majority of airlines including Continental, Delta, and United (with the notable exception of El Al) allows the onboard use of GPS devices and also provide 112 V plugs for laptops.  (The non GPS program option can be used for El Al flights.)  The onboard program with GPS is therefore the best option for anyone traveling with a PC laptop. The major disadvantage is that you must book a window seat in order to allow for reliable reception (the weak GPS signal can't penetrate far into the metallic shell of the airplane).  Use the link above for more information.

There is a dispute among the authorities whether the z'manim on the airplane are calculated according the airplane's height or according to the ground below the airplane. The difference between the two is approximately a half hour. That is, sunrise/sunset will appear about a half hour earlier/later for an observer on the airplane than for an observer on the ground. In fact, sunrise times for an observer at the height of the airplane are impossible to calculate to accuracy much better than a half hour. This is due to daily variations in the refraction of the light through an extremely long air path. Therefore, out of necessity, and for the sake of simplicity, the Chai Air Tables calculate all times according to an observer on the ground at the coordinates of the airplane.

The table below is an example of times given in the Chai Air Travel Tables.  The column titled  "Delay in departure"  lists how many hours the departure time was delayed from the predicted departure time listed on your ticket. Note that for most airlines the predicted "departure time" is actually the time when the airplane is expected to depart from the terminal.  Take-off often occurs only after a further lengthy taxiing period on the runway. The "Chai Tables" program assumes that "Departure Time" is synonymous with "take-off time" since the time for taxiing cannot not be surmised a priori.  Based on the author's experiences, this approximation appears adequate for lengthy international flights probably since it balances the comparable time that the airplane is flying slower than its cruising velocity.  Therefore, unless the flight is unduly delayed, it is sufficient to use the departure time from the gate as the "Departure Time".  In the case that the flight is unduly delayed on the runway, the added delay should be estimated and treated as a further "Delay in departure".  If the plane left the terminal on time, then the added delay on the runway is equal to the amount of time the plane is expected to be late in arriving at its destination.

All other times in the table are with respect to the true departure time.  I.e., they are in units of elapsed time after take-off.  Take note of your flight's delay from the predicted departure time.  Use only the row of values that most closely corresponds to that departure delay.  For example, a fictitious winter flight that was supposed to leave the terminal at JFK (New York) at 3:00 AM, arriving in Eretz Yisroel at 9:00 PM, was delayed by 30 minutes.  We therefore read the third row (highlighted) corresponding to a delay of 30 minutes.  Reading this row left-to-right provides you with the z'manim.  Namely, alos (dawn) will occur 1 hour and 30 minutes min. after take-off.  Sunrise will occur 2 hours and 33 minutes after take-off.  Sof z'man krias shma will be reached 3 hours 50 minutes after takeoff.  Minchah Gedolah will be reached 5 hours and 17 minutes after takeoff.  Sunset will be encountered 7 hours after 45 minutes take-off.  Finally, Zeis Hakochavim (corresponding to 8.5 degrees of solar depression) will be reached after 8 hours and 19 minutes. (Note, that these times are only estimates, since the exact air route is not known a priori.  The error can be as much as 1/2 hours, so these z'manim should only be used l'chumra.)

If the time entry is equal to "none", then that z'manei hayom was never encountered or can't be calculated.  For Alos, this can happen if the plane is traveling in the North Atlantic during summer months (i.e., the sun never sets below 16.1 degrees for those latitudes in the summer).  For z'manei hayom this happen if the plane passes the Arctic Circle in the summer or winter months.  In that latter case it is not clear how one behaves with respect to prayer.  A rabbi well versed in these matters must be consulted.

Advanced Formats: Starting with version 6, the user can calculate the air times as standard watch times (hours:minutes AM/PM, or hh:mm, i.e., twenty four hour time) with reference to any time zone on the planet.  The advanced formats should only be used by those travelers who are familiar with traveling through several time zones.  Otherwise, the default mode of time intervals should be used.  By clicking the "Show" button, a list of options are revealed.  The choices are (1) time differences (default -- shown in the figure above), (2) watch time according to the time zone of the departure airport, (3) watch time according to the time zone of the arrival airport, (4-53) watch time according to any time zone.  When picking a non standard format (choices 2-53), one must be careful to keep track of elapsed day and night periods.  If there is room for confusion, then the twenty-four hour format should be selected.  The example below corresponds to the same flight given in the example above.   However, in the example below the z'manim are calculated according to New York watch time (time zone of 5 hours west of Greenwich).  For example, 4:50 AM corresponds to 1 hr and 50 minutes.  Another example; when the flight is delayed by 30 minutes, it leaves at 3:30 AM.  We know that sunrise occurs 2 hours and 33 minutes afterwards (see the table above).  This corresponds to a watch time of 3:30 + 2 hrs. 33 min. = 6:03 AM as listed below in the table.)

Delay in departure	Alos Hashachar	Sunrise	Sof Zman Krias Shma	Minchoh Gedolah	Sunset	Zeis Hakochavim
0 min.	4:50 AM	5:53 AM	7:08 AM	8:34 AM	11:07 AM	11:39 AM
15 min.	4:55 AM	5:58 AM	7:15 AM	8:40 AM	11:10 AM	11:44 AM
30 min.	5:00 AM	6:03 AM	7:20 AM	8:47 AM	11:15 AM	11:49 AM

 

Crossing the Dateline = Traveling on Shabbos?

As business travel to the Far East has become more prevalent, there has been a concomitant increase in incidences of unintentional chilul Shabbos. Flights leaving from the Far East to North America on Friday often land on Friday in North America, hiding the fact that they travelled during Shabbos.  How does this happen? The plane often encounters sunset of Erev Shabbos before crossing the dateline. The plane continues flying during Shabbos until it crosses the dateline.  At that point,  the day reverts to either late Thursday night or early Friday morning.  Finally, the plane lands on Friday afternoon in North America.  Sunday flights from the Far East  also  inadvertently incur chilul Shabbos.  If the plane crosses the dateline before sunset, then the local time reverts from Sunday to Shabbos.  The plane will then be traveling during Shabbos until  the sun sets (which begins the night of Motzei Shabbos).  In the latter case, the plane lands on Sunday in North America.  This scenario can also occur for Sunday flights from Australia to North America.  If Shabbos will be encountered during such a flight, the Chai Air Travel Tables prints out the following warning: 

WARNING: You will be flying during Shabbos for one or more of the following flight plans!    You should consult an halochic authority before taking this flight.

As the warning suggests, such flights should be avoided.  At least, an halochic authority well versed in problems associated with the dateline should be consulted (see below).

Rabbi Dovid Heber of Star-K Kosher Certification in Baltimore, MD has kindly made the following links available for publication in this Introduction concerning halochic issues arising from travelling on an airplane.   He has also kindly agreed to answer shaylos concerning flights.  Please contact him using the link under his name.  Please

Daylight Saving Time?

In order to obtain a meaningful table for your flight you must know if Daylight Saving Time is in effect at your departure and arrival point.  The rules for most of the countries in the world that maintain a consistent practice for Daylight Saving Time can be obtained from the calculator below.  (Enter the year in the first edit box on the left, and the dates for DST appear in the other boxes.)  In Eretz Yisroel, starting in 2013, Daylight Saving Time begins on the Friday before the last Sunday in March, and ending on the first Sunday after October 1.  Russia no longer changes its clocks. Mexico follows the system of Daylight Saving Time that was practiced in the US until the year 2007, i.e., DST starts on the first Sunday in April and ends on the last Sunday of October. Brazil starts DST on the first Sunday in October and ends it on the last Sunday in February.  Chile starts DST on the second Saturday of October at midnight and ends it on the second Saturday of March at midnight.  Paraguay starts on the first Sunday in October and ends on the last Saturday in February.  Uruguay begins Daylight Saving Time at 2:00 AM on the first Sunday of October, and reverts back to Standard Time at 2:00 AM on the second Sunday in March.  Turkey no longer observes DST.  China does not practice DST nor do most countries in the tropics.  The rules for many other countries can be erratic and should be obtained from your travel agent or airline or by consulting, for example, the webexibits.org web site (use link in the calculator).  In all countries, clocks are changed some time between midnight and 3 am.

The rules constantly change!  Please help us stay up to date by sending us feedback.

Daylight Saving Time Calculator (based on calculator at:  webexhibits.org)
Enter the secular year	U.S. ( except Hawaii, Arizona), Canada (except Saskatchewan		Western Europe, Most of Eastern Europe,		Australia  (except Queensland, N. Ter., W. Au.)
Year	DST begins at 2 a.m.	DST ends at 2 a.m.	Summertime period begins at 1 a.m. UT	Summertime period ends at 1 a.m. UT	DST period begins	DST period ends

Tell your friends and travel agents that the Chai Air Tables for your flight can be downloaded free of charge at www.chaitables.com.  Help your fellow passenger by posting these times on the airplane in a conspicuous place. The author welcomes your feedback .