The minor fasts last from dawn (alot ha-shaĥar) until tzeit ha-kokhavim (when three medium-sized stars are visible in the sky). There are different opinions as to when exactly alot ha-shaĥar occurs: either when the first light begins to appear in the east (when the sun is 17.5 degrees below the horizon) or a short time later, when the eastern part of the sky is illuminated (and the sun is 16.1 degrees below the horizon).

Likewise, there are also two primary opinions about when tzeit ha-kokhavim occurs: either when experts or those with excellent eyesight can see three stars (and the sun is 4.8 degrees below the horizon) or when average people can see three stars (and the sun is 6.2 degrees below the horizon).

It is a common misconception that there is a fixed interval between alot ha-shaĥar and hanetz ha-ĥama (sunrise) and between shki’at ha-ĥama (sunset) and tzeit ha-kokhavim. This is not the case: these intervals depend on the time of year and the place. Therefore, one should use a precise calendar to determine these times.[4]

Technically, halakha follows the more lenient opinion, because these fasts are rabbinic enactments. However, it is best to be stringent. Since we are already fasting all day long, it is preferable to add a few extra minutes in order to fulfill our obligation according to all opinions.

When Asara Be-Tevet falls out on a Friday, one must fast until after tzeit ha-kokhavim, even though Shabbat has already begun by then (sa 249:4).

One who flies from America to Israel will experience a shorter fast, as he is flying against the direction of Earth’s rotation. Every hour in flight shortens the fast by more than half an hour. One who flies from Israel to America extends his fast, as he is flying in the same direction as Earth’s rotation. Every hour in flight adds more than half an hour to the fast. As a rule, the fast starts at alot ha-shaĥar and ends at tzeit ha-kokhavim, of wherever one happens to be at the time (see Igrot Moshe, oĥ 3:96).