Fast Fly


I am flying to Canada the night of Shiv'ah Asar BeTammuz, arriving in Toronto at 6 a.m. (same time zone as New York). At what point in the flight does my fast start?


The fast starts at amud hashachar, which is around 72 minutes before
sunrise. To be accurate, it really depends on the angle of the sun below the horizon
(16.1 degrees).

When you are in the air, you have to calculate the times as if you were on the
ground at those coordinates.[1]
Since your plane will be flying, the coordinates will be constantly changing and
will be difficult to determine. Furthermore, traveling from here to Canada at night
is particularly problematic, since the airplane curves north, where sunrise is earlier,
and it might get light and then dark again as the plane curves southward again.
You cannot estimate the start of the fast based on your expected arrival time; you
have to calculate it based on your position over the Atlantic Ocean and not according
to sunrise in Toronto. The only thing I can recommend is to do what all Jews did
for centuries before watches and timetables became popular. Look out the airplane
window. If the sky seems as black as midnight, then you can eat; if you see the
beginning of any light, it may be too late. Remember, just because someone saw light
doesn't mean that after that you will always be forbidden to eat. The airplane may
pass through an area that has light and then fly to a place that is still completely
dark. In other words, it may be halachically correct to start your fast, and then
break it when you fly south.[2]

Remember, people who are pregnant,[3]
sick, very weak or in great discomfort are exempt from fasting on Shiv’ah Asar BeTammuz.
(It is worth discussing each situation with a rav to be sure you are making a halachically
sound judgment.) When flying, it is particularly important to hydrate oneself since
the altitude changes the body’s physiology and one is more prone to dehydration.
If you feel that you really need a drink, are concerned about dehydrating and are
unsure about the time, you can be lenient.[5]
You may want to try to drink a lot at the beginning of the flight and sleep through
the night. If you feel dizzy or as if you are going to faint, break your fast.

BookID: 1 Chapter: 549


Rav Feinstein ruled that a person way up in the air calculates a third and a
quarter of the day regarding zeman tefilloh and zeman kerias Shema,
respectively, as if he were on the ground directly beneath him (see Iggros
Moshe, O.C. 3:96). Presumably, he would say the same for all other halachos,
including the start of a fast at amud hashachar and the end of the
fast at tzeis hakochavim. (A fast ends at tzeis hakochavim,
but in regard to fasts that are
midderabonon, one can rely on a lenient definition of tzeis
. Regarding the end of Shabbos and kerias shema,
tzeis hakochavim

is determined by three small stars whereas regarding the end of a fast
day [besides Yom Kippur], it is sufficient to see three medium sized stars.)

This approach can be helpful when one is flying from coast to coast in
Australia on a Sunday. According to the Chazon Ish, only the western edge of
Australia is beyond the halachic dateline. Nevertheless, he maintains that
the entire landmass has the same halachic day as the western edge. He uses a
called gereiroh
to explain this. However, a person traveling from the mainland to an
island off the eastern coast, or even taking a boat or swimming from the
land into the ocean on the eastern side would be going from Sunday into
Shabbos. What if one were to fly over the country? Is being in the air
similar to leaving the land and entering the previous day? If the air is not
considered part of the land mass, it may be Shabbos in the air over eastern
Australia while it is Sunday on the ground below! But according to Rav
Feinstein’s logic, which considers a person in the air to be governed by
time-related halachos as if he were on the ground at those coordinates, it
seems likely that as long as the airplane stays above land and does not fly
over the ocean, a passenger will never leave Sunday and enter Shabbos.


When the sun rises again, one would think that the traveler should start
his fast again since it is still the 17th of Tammuz. However, I
am not certain that this is required.

The Chazon Ish discusses flying over a dateline into and out of Shabbos.
He rules that Shabbos is fixed to the location and must be observed in the
air over the area that is Shabbos no matter how many times one enters and
leaves Shabbos.

This is not the case with regard to a mitzva that is specific to a
particular calendar date and comes around once a year. One only has to do
the mitzva once that year even if the day is repeated. For instance, if one
blew the shofar on the second day of Rosh Hashonoh and flew the next day
back into Rosh Hashono, he is not required to blow again. The same is true
of eating matzo and reading Megillas Esther. Therefore, if, after fasting
all day on Shiv’ah Asar BeTammuz, a person flew eastward on the 18th
of Tammuz and reentered the 17th
of Tammuz, he would not have to fast again, since he already performed
the mitzva of fasting on the 17th of Tammuz that year.

Similarly, if the sun rises and sets twice on the same day, it seems to
follow that the person should only have to fast once–from the first amud
until the first tzeis hakochavim. When the sun rises
again, he is exempt from fasting since he already fasted a complete fast on
the 17th of Tammuz that year. Therefore, in your case, it seems
logical to me that if you flew far enough north to have an amud hashachar
and then south soon enough to have a tzeis hakochavim, by fasting
during that time you have fulfilled your mitzva for that year. When the sun
rises again that day, you would be exempt from starting the fast again.

Because this suggestion is novel, I am reluctant to rely on my
conjecture in practice without the approval of
talmidei chachomim familiar with the topic. Rav Chisda commented
that any fast that has not lasted until nightfall cannot be considered a
fast (Taanis 12a). The implication is that once it is nightfall, it should
be considered a fast. Perhaps, if there are two nightfall’s in one day, it
is sufficient to reach the first tzeis hakochavim.

Reportedly, Rav S. Z. Auerbach
and Rav Moshe Feinstein z’’l ruled that if one experiences
two sunrises on the same day of the week, one is obligated in only one
tefilloh. The time cycle for tefilloh is once a day, regardless of how many
nightfall’s and sunrises there are on that day. Since they hold that
tefilloh is day-sensitive and not sunrise-sunset–sensitive, it follows that
someone who davens and then crosses the dateline westward (leading into the
following day) has to daven again even though he did not experience a sunset
or second sunrise. If it is the day that requires the tefilloh, then since
he is now in a new day, he must fulfill that day’s tefilloh obligation.

Rav Elyashiv is reported to hold the opposite opinion and to consider
tefilloh to be sunrise-sunset–sensitive. In that case, two sunrises in one
day obligate the person in two tefillos, whereas crossing the dateline in a
westward direction (effectively stepping into the next day without
nightfall) does not require a new tefilloh. These two views regarding
tefilloh are expounded upon in Rav Betzalel Stern’s Betzel Hachochmo,
and Rav Yechezkel Roth’s Emek Hateshuva.

Even if there is a dispute regarding tefilloh, it seems that all
would agree that the obligation to fast is day-sensitive, and if
you fasted from amud hashachar to tzeis hakochavim once you do
not have to fast again if you experience another sunrise on the same day.


The poskim commenting on the Shulchan Aruch consider a gestational age
of 12 weeks or more as a recognizable pregnancy exempting the woman from
fasting even if she feels perfectly fine. They also mention that a woman who
is less than 12 weeks pregnant does not need to fast if she experienced any
complications or is in pain. Any woman who feels nauseous, ill or weak at
any stage of her pregnancy (even before 40 days from conception) is advised
not to fast. The fetus is particularly sensitive in the first trimester and
sometimes the mother is also at risk (Mor U’kitzi’oh 550).


After delivery (even if there wasn’t a live birth), a woman is exempt
from fasting for at least 30 days, despite the fact that she is not nursing.


כשיש ספק אם עוד לא התחיל הצום או אולי כבר התחיל, נראה דיש להקל מדין
ספק דרבנן לקולא. ואף מצאנו שכ''כ המ''ב תרט''ז ס''ק י''ג לענין צום דרבנן באחד
שהגיע לכלל שנים ולא הביא ב' שערות וספק שמא נשרו, דפטור מלצום משום ספק דרבנן
להקל. ואע''פ דדברי המ''ב תמוהין דבשאר דברים מדרבנן אמרינן דהגיע לגדלות
וסמכינן אחזקה, ורק לענין דאורייתא חיישינן שמא אינו גדול לחומרא, אבל אי''ז
ממש כספק שנקל בו שלא צריך לצום. וראיתי בא''א לפמ''ג סימן תק''ן סק''ב דהביא
מש''כ הא''ר דכל שהגיעו לי''ב שנים לנקבה וי''ג שנים לזכר חייבים להתענות
תעניות מדרבנן אף שלא הביאו ב' שערות. וא''כ דברי המ''ב צ''ע.

עכ''פ היכא דבאמת איכא ספק בזה, כגון בנד''ד שאולי לא הגיע עה''ש, נראה דבמקום
צורך יש להקל.

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