Reference of Measurements in Halacha
There are many different measurements used in the Torah and classical Rabbinic Seforim, which are important for daily living. An example is the specific amount of wine that Kiddush is made on: One Revi'it. If one doesn't know how much a Revi'it is, how can one fulfill the Mitzvah properly? The modern day equivalents for many Hebrew measurements are discussed below. In addition, discussed below are many Halachic times which also matter for everyday use.
- 1 Kezayit
- 2 Kedi Achilat Pras
- 3 KeBaytzah
- 4 Amah, Tefach, Etzbah
- 5 Mil
- 6 Revi'it
- 7 Kedi Shtiyat Reviyit
- 8 Prutah
- 9 Toch Kedi Dibbur
- 10 Kdei Aniva
- 11 Shaot Zmaniot
- 12 Olot HaShachar
- 13 MeSheyakir
- 14 HaNetz HaChama (Sunrise)
- 15 Chatzot
- 16 Mincha Gedola
- 17 Mincha Ketana
- 18 Shekiyah (Sunset)
- 19 Tzet HaCochavim (Emergence of the stars)
- 20 Bein HaShemashot
- 21 Sources
- According to Ashkenazim, all sizes are measured in volume and not weight. However, according to Sephardim the actual halacha is that measures are in volume but the minhag is to follow the measures in weight. 
- There’s a dispute in the Rishonim whether the Kezayit (olive size) is a half or third of a KeBaytzah (egg size), and regarding Bracha Achrona and Deoritta Mitzvot, the halacha follows the opinion that a Kezayit is half a KeBeytzah. 
- According to Rav Chaim Noeh, the Kezayit for Deoritta measures (eating Matzah) and Bracha Achrona is 27cc. The Chazon Ish’s opinion is that the Kezayit is 33.3 grams (1.3 oz). 
- (4) Rav Mordechai Willig (Pesach To-Go, Nisan 5771, p. 60) is of the opinion that the size in volume of a kezayis is 22.5 cubic centimeters
- The minhag of the world and many other authorities is to follow Rav Chaim Noeh’s opinion. 
- If one ate between a third and a half a KeBaytzah one should not make a Bracha achrona because of safek Brachot. Therefore, one should have less than a third and not make a Bracha achrona or more than a half and make a Bracha achrona. 
- On Mitzvot Deoritta such as eating Matzah one must be strict to hold a Kezayit is a half a KeBaytzah. However, regarding מצות Derabbanan it’s preferable to hold that a Kezayit is a half a kebaytzah, however, one may rely on the opinion that a Kezayit is a third a kebaytzah. 
- One needs to make sure that he does not count air in the food as part of the shiur of kezayis. If one has a food that is hollow, a kezayis of that food is considered when the food would be pressed.
Kedi Achilat Pras
- According to Rav Chaim Noeh, the KeBaytzah is 55cc, and according to the Chazon Ish, it is 100cc. 
Amah, Tefach, Etzbah
- According to Rav Moshe Feinstein, the Amah is 21.25 inches (53.98 centimeters), the Tefach is 3.54 inches (9.00 centimeters), and the Etzbah is 0.89 inches (2.25 centimeters). 
- According to Rav Chaim Noeh, the Amah is 18.90 inches (48 centimeters), the Tefach is 3.15 inches (8 centimeters), and the Etzbah is 0.79 inches (2 centimeters) 
- According to the Chazon Ish, the Amah is 24 inches (60.96 centimeters), the Tefach is 4 inches (10.16 centimeters), and the Etzbah is 1 inch (2.54 centimeters). 
- When Mil is given in terms of time, some consider it 18 minutes, some 22.5 minutes, and some 24 minutes. 
- When Mil is given in terms of distance it is equal to 2000 Amot. 
- A Reviyit is a KeBaytzah and a half. Therefore, Rav Chaim Noeh holds that the Reviyit is 86 grams (3 oz), the Chazon Ish holds that the Reviyit is 150 grams (5.3 oz)., and some quote Rav Moshe as saying the Reviyit is 3.3 oz.
- With regards to drinking the four cups of wine, see the Required Amount of Matzah and Wine for the Seder page.
Kedi Shtiyat Reviyit
- A prutah is 1/40 of a gram of silver (which is about a few cents). However, the prutah with regards to the mitzvah of returning a lost object is discussed here. 
Toch Kedi Dibbur
- Kedi Dibbur is the time it takes to say Shalom Alecha Rebbe. 
- Toch Kedi Dibbur is less than the time it takes to say Shalom Alecha Rebbe.
- There are several opinions about how long Kdei Aniva is, however, one doesn't need to be strict to consider it to be longer than 2 gudalin. 
- How long is the shiur of 2 godlin in practical terms? Chazon Ish : 5 cm, Rav Chaim Naeh: 4 cm. Rav Mordechai Eliyahu (comments to Kitzur 9:13) writes that Kedi Anivah is 4 cm.
- Shaot Zmaniot are halachic hours which are calculated by dividing the daytime hours into 12. There’s a dispute of how to measure the day; some count the day from Olot HaShachar until Tzet HaKochavim (Magen Avraham) , and some count it from HaNetz until Shekiah (Gra).
- According to Sephardim, regarding Deoritta laws (including Kriyat Shema) one should be strict for the opinion of the Magen Avraham. 
- Among Ashkenazim, some are strict to hold like the Magen Avraham, unless there is an serious need, in which case one may hold like the Gra. However, some are lenient to rely on the Gra. 
- Many rishonim hold that Olot HaShachar is 72 minutes before sunrise on a perfect day in Jerusalem which is equivalent to when the sun is 16 degrees below the horizon for everywhere in the world, however, others hold that Olot HaShachar is 90 minutes on a perfect day which is 19.78 degrees below the horizon. 
- There is a wide range of opinions precisely when Misheyakir occurs in Jerusalem on a perfect day, when there is exactly 12 daylight hours and every seasonal hour consists of 60 minutes. The three opinions include: 35 minutes before sunrise, 60 minutes before sunrise, and 66 minutes before sunrise.
- The various opinions would then have to be extrapolated according to the region of the world and time of year. Thus, according to the commonly accepted opinion that Meshiyakir is between 50 and 60 minutes before sunrise in Jerusalem - In New York, depending on the time of the year, it could be anywhere from 56-73 minutes before sunrise.
HaNetz HaChama (Sunrise)
- HaNetz HaChama is sunrise which, in general, is halachically the same as the astronomical calculation of sunrise.
- Mincha Gedola is six and a half halachic hours (Shaot Zmaniyot) after sunrise.
- Mincha Ketana is nine and a half halachic hours (Shaot Zmaniyot) after sunrise.
- Shekiyah is sunset which, in general, is halachically the same as the astronomical calculation of sunset. 
Tzet HaCochavim (Emergence of the stars)
- There's a major dispute when Tzet HaCochavim is: Rabbenu Tam assumes that it's about 72 minutes after Shekiyah and the Gra assumes that it's three quarters of a mil after Shekiyah, varying according to one's locations. 
- Bein HaShemashot is the time period between certain daytime and certain nighttime. In many halachot, it is considered a doubt whether it is considered day or night and has many practical ramifications. There is a major dispute how long this period is. 
- The Gemara rules that if one does a Melacha on Shabbos for the duration of Bein Hashemashos on Friday evening and Saturday evening, one has certainly violated Shabbos, and must bring a sin-offering 
- Therefore, one can do no Melacha during Bein Hashemashot .
- Mishna Brurah 456:3, Vezot HaBracha (pg 6)
- See Yalkut Yosef (Klalei Brachot pg 195-202) discusses this at length.
- Mishna Brurah 486:1 writes that the Tosfot holds a Kezayit is half of a KeBeitzah with the shell and the Rambam holds it is a third of a KeBeitzah (see Tosfot (Yoma 80b s.v. Agav), Rambam (Eiruvin 1:9)). S”A 486 rules like Tosfot. Rav Avraham Chaim Noeh (Shiurei Torah 3:12) argues that Tosfot holds half a KeBeitzah without the shell.
- What's the halacha? Mishna Brurah 486:1 rules that for Deoritta Mitzvot and Bracha Achrona, one should eat the size of half a KeBeitzah, but for Derabbanan Mitzvot, one third suffices. He adds that since one has to make a Bracha on maror, one should eat half a KeBeitzah.
- Chazon Ish 39:17 decided that we could be lenient like the Rambam. see below for an exact amount according to Chazon Ish
- The Tzlach (Pesachim 116b) holds that the modern day eggs are half the size of those in the days of Chazal. However, Rav Noeh (Shiurei Torah Shaar 3) argues that the modern day eggs have not changed from the days of Chazal. Mishna Brurah 486:1 writes that for the mitzvah deoraitta of matza, one should follow the stringent view of the Tzlach. see Rav Shlomo Wahrman (Orot Haesach 28) who explains why there is no need to say that the eggs have shrunk since the times of the Gemara. see also Maaseh Rav 74, where it says that the Gra accepted the position of the Tzlach.
- Practically, how much is a Kezayit?
- (1) Rav Chaim Noeh in Shiurei Torah (3:11, pg 191, 5707) writes that for Deoritta Mitzvot one should eat 28.8cc, however, in Shiurei Tzion (p. 70, 5709) he writes that it is 27cc. Vezot HaBracha (Birur Halacha 1) explains that Rav Chaim Noeh retracted from his original ruling.
- (2) Rav Chaim Kanievsky in Shiurin Shel Torah (p. 66) quotes the Chazon Ish that a Kezayit is 2/3 of a modern egg and an egg can possibly be up to 55cc. Therefore, Vezot HaBracha (Birur Halacha 1) writes that the Chazon Ish holds that 33.3cc is a Kezayit. The Chazon Ish's opinion of Kezayit is partially based on the opinion of the Tzlach. see also Chazon Ish in Hilchot Shabbat 39.
- (3) Vezot HaBracha (Birur Halacha 1) quotes Haggadat Kol Dodi which quotes Rav Moshe Feinstein as saying that a Kezayit is 31.2cc. [However, Rabbi Bodner in Halachos of K’zayis (p. 24) writes that Rav Dovid Feinstein told him that Rav Moshe never measured the eggs himself and it was his student who calculated 1.1oz, but that if his calculations were more precise he should use those. Therefore, Rabbi Bodner rules, based on his own calculations, that a Kezayit is 27.2cc.]
- Vezot HaBracha (pg 6, Birur 1, pg 221) quotes Rav Elyashiv and Rav Shlomo Zalman saying that for Bracha Achrona the widely accepted measurement of the Kezayit is according to Rav Chaim Noeh. Vezot HaBracha also mentions that such is the minhag of the world.
- Mishna Brurah 486:1
- Mishna Brurah 486:1
- Rama 486:1
- Vezot HaBracha pg 6
- Sh”t Igrot Moshe O”C 1:136. writes that the Amah is 21.25 inches and one can be strict to hold that it is 23 inches.
- Shiurei Torah (by Rav Chaim Noeh, Siman 3 Seif 25 pg 249)
- Shiurin shel Torah (pg 3) by the Steipler quoting the Chazon Ish but admitting that these are approximate measures because of the need to publicize the measures but not an accurate calculation.
- Shulchan Aruch 459:2 writes that the mil is 18 minutes. Rama 261:1 agrees. Beiur Halacha 459:2 s.v. Haviy quotes some who consider it 22.5 minutes and others who say that it is 24 minutes.
- Rashi Yoma 67a s.v. shivah and Tosfot Shabbat 34b s.v. safek write that a mil is 2000 amot. Shulchan Aruch 261:2 writes that 3/4 of a mil is equal to 1500 amot, implying a mil is 2000 amot. Rabbi Melamed on yeshiva.org.il writes that a mil is 2000 amot.
- Mishna Brurah 486:1
- Shiurei Tzion (p. 69)
- Rav Chaim Kanievsky in Shiurin Shel Torah (p. 66) according to the rulings of the Chazon Ish
- Vezot HaBracha (Birur Halacha 1) quotes Haggadat Kol Dodi which quotes Rav Moshe Feinstein as saying that a Reviyit is 3.3 oz.
- S”A CM 88:1 says a prutah is a half of a pearl of barley. Shiurei Torah (Rav Chaim Noeh pg 177) and Shiurei HaMitzvot (Chazon Ish pg 65) say a prutah is 1/40 of a gram of silver (which currently is about 2.3 cents). Halachos of Chanukah (Rabbi Shimon Eider, pg 38) quotes Rav Moshe Feinstein saying that a prutah is 2 or 3 cents and not just one penny. See Halachos of Other People’s Money (Rabbi Bodner pg 150) who quotes Rav Moshe regarding a prutah for the mitzvah of returning a lost object.
- Kitzur Shulchan Aruch (Klalim s.v. Kedi)
- Kitzur Shulchan Aruch (Klalim s.v. Kedi)
- How long is kdei aniva? There’s several opinions about this topic:
- Smag- long enough to loop around all the rest of the 7 strings.
- Chayei Adam, Kitzur Shulchan Aruch 9:13- 4 gudalin, Beiur Halacha: 2 gudalin, Eliya Rabba (paskening like Rif and Rosh)- long enough to loop around just 1 string (very short).
- The Beiur Halacha concludes that there’s no need to be strict for more than 2 gudalin. And he says if you have no choice, you can rely on the Eliya Rabba. see Peninei Halacha
- Many authorities holds that the halachic hours in the day are considered from Olot including: Sh”t Trumat HaDeshen 1, Levush 267, Minchat Cohen (Mevoh Shemesh 2:6) in name of Tosfot Ramban and Rashba, Bach (431), Taz 433, Pri Chadash 443, Magen Avraham 58:1, 433:3, Eliyah Raba 58:2, Mizbe’ach Adama 4a, Mikraeh Kodesh 158b, Mateh Yehuda 433, Sh”t Chaim Shal 2:38(70), Tov Ayin 18:38, Sh”t Teshuva MaAhava 1:25, Shalmei Tzibbur 93c, Chesed Alafim 58:5, Chaye Adam 21:3,27:1, Kitzur Shulchan Aruch 17:1, Magen Giborim (Shiltei Hagiborim 58:3), and Rav Poalim O”C 2:2.
- Many authorities hold that the halachic hours of the day are considered to begin from HaNetz incluing: Rambam’s Sh”t Pear Hadar 44 (as understood by Halacha Brurah (Shaar Tzion 58:17), Rav Chaim Drok (Noam 9 pg 235), and Orot Chaim 320 against the Yetsiat Mitzmayim (Sefaka Deyoa pg 115)), Siddur Rav Sadyah Goan pg 12, Minchat Cohen Mevoh Hashemesh 2:6 in name of Goanim, Rambam, Rabbenu Yonah, Hagahot Maimon, and Mordechai (Pri Chadash rejects his proofs), Shiltei Hagiborim on the Mordechai (Brachot 4:3) (as understood by Magen Avraham 233:3, Shaar Tzion 233:10, and Kaf HaChaim 233:7), Levush (233:1,267), Shaarei Knesset Hagedolah 58:8, Tosfot Yom Tov (Pesachim 2:3), and Biur HaGra 459:2.
- In conclusion, Minchat Cohen (Mevoh Hashemesh 2:9), Erech HaShulchan 433, Halichot Olam (vol 1, Vaera 3), and Yalkut Yosef (vol 1, pg 98) write that one should be strict for the opinion of the Magen Avraham for Deoritta issues, for example Kriyat Shema.
- Rabbi Yechiel Michel Tukachinsky in Sefer Eretz Yisrael (pg 18#3) writes that the minhag of Israel is to follow the magen avraham and only under pressure may one rely on the Gra.
- Rav Hershel Schachter on yutorah.org (min 77-79) says that many follow the opinion of the Gra in calculating the time for Sof Kriyat Shema.
- Sefer Zmanim KeHilchatam (Rabbi Berstein, pg 347) writes that 72 minutes on a perfect day in Jerusalem is equal to when the sun is 16 degrees below the horizon and 90 minutes is equal to 19.78 degrees below the horizon. He personally rules that one should be strict for both of these opinions for Deoritta halachas and those derabbanan halachas where safek is lechumra. He tries to prove that this is the opinion of the Mishna Brurah (92:3, Beiur Halacha 163:1 s.v. Berachok). However, he totally rejects the opinion that Olot HaShachar is 120 minutes before sunrise.
- He also quotes Rabbi Tukachinsky who held like the opinion of 90 minutes.
- Rav Hershel Schachter on yutorah.org (min 65-7) says that we assume that Olot HaShachar depends on degrees below horizon and the specific times vary according to one’s location. Similarly, Rav Yisrael Belsky (OU Kosher Webcast, 2011, min 19-21) mentioned in passing that Olot HaShachar is assumed to be 72 minutes before sunrise, varying according to the longitude and latitude.
- Rav Moshe Feinstein in Le-Torah ve-Hora'ah Vol. 3:7 and in Iggeros Moshe OC 4:6 holds of 35 minutes. Rav Yaakov Kamenetsky in Emes liyaakov OC 58:1 rules that it is 36 minutes before sunrise. Zmanei Halacha LeMaaseh (pg 19) holds of 40 minutes before sunrise.
- Rav Tukaczinsky in Sefer Eretz Yisrael 1:4 (pg 18), Kaf HaChaim 18:18, Sh"t Binyan Tzion 2:16 hold of 60 minutes before sunrise. Nivreshet (vol 1, pg 43) holds of 52 minutes.
- Rav Ovadya Yoseh in Yechave Daat 2:8
- There are two factors at work here: First, the processes of daybreak and nightfall occur faster at places near the equator than places further away from the equator. Second, the speed of these processes are also affected by the seasons.
- A common solution would be to use seasonal minutes. In order to calculate a seasonal hour, one would need to add up all the daytime minutes and then divide by 12 to get the number of minutes that are in each seasonal hour. For instance, if there were 11 hours of daylight, each seasonal hour would consist of 55 minutes. To further complicate matters, there are two ways of counting the daytime minutes: The Magen Avraham counts from dawn until nightfall with the emergence of 3 stars, whereas the Gra counts from sunrise to sunset.
- Another means for calculation, which has only recently been implemented, is to compare the level of brightness by using the relative position of the sun. For instance, it has been determined that 60 minutes before sunrise on a perfect day in Jerusalem the sun is 12.9 degrees below the horizon. Therefore, if one wanted to find the equivalent time anywhere in the world during any season, he would simply determine at what time the sun will be 12.9 degrees below the horizon in that locale on that day and that would be the extrapolated Meshiyakir.
- MyZmanim.com. These figures are determined using MyZmanim’s earliest published position of 11.5 degrees below the horizon. Unfortunately, MyZmanim.com only offers calculations based upon 10.2 degrees, 11 degrees, and 11.5 degrees but not 12.9 degrees. It is unfortunate because 12.9 degrees correlates to Rav Tukaczinsky’s 60 minutes before sunrise which, although is a very lenient position is also well collaborated and accepted.
- Rav Yisrael Belsky (OU Kosher Webcast, 2011, min 21-23) mentions in passing that MeSheYakir is about 40 or 50 minutes before sunrise.
- Rav Hershel Schachter on yutorah.org (min 68-70) says that Rav Moshe held MeSheYakir is between 35-40 minutes before sunrise and seems to agree with him, although he mentions that some say it’s 55 minutes before sunrise.
- Rav Hershel Schachter on yutorah.org (min 69-72) says that sunrise is the same as the sunrise calculated by the naval observatory which is when the sun begins to rise.
- Rav Hershel Schachter on yutorah.org (min 72-77) says that sunset should be determined as though the mountains to the east and west are at sea level and do not block the visibility of the sun. However, the elevation of the place one is currently located is taken into consideration and if one is in a valley the sunset is a little later (less than 5 minutes) than what would have been visible. Yet, he adds, that most opinions hold that regarding sunrise both one’s location’s elevation and surrounding mountains are taken into consideration.
- Mishna Brurah 1:9 writes that chatzot at night is always 12 hours after chatzot of day for any place in the world.
- Rashi (Pesachim 107a s.v. mincha gedola), Rambam (Tefillah 3:2)
- Rashi (Pesachim 107b s.v. oh), Rambam (Tefillah 3:2)
- Rav Hershel Schachter on yutorah.org (min 69-72) says that sunset is the same as the sunset calculated by the naval observatory which is when the sun has completely set.
- Rabbi Hershel Schachter on yutorah.org (min 43-5) explains that many practice like the Gra regarding Tzet HaCochavim on Motzei Shabbat. See Biur HaGra and Mishna Brurah 261. Pri Megadim assumes that according to Rabbenu Tam, 72 minutes is a fixed time, but Mishna Brurah writes that it varies according to one's location.
- (1) Gemara Shabbat 34b says Bein HaShemashot is considered as a doubt whether it is considered daytime or nighttime. When is Bein HaShemashot? Rabbi Yehuda says that Bein HaShemashot starts from Shekiyah and concludes when the entire ball of the sun has set. Rabbi Yose, however, says that Bein HaShemashot is as short as the blink of an eye, though R. Yose also agrees that there is a doubt (Tosfot 34b s.v. Rabbi Yose). R. Nechemya says that it is from Shekiyah and lasts the length of time it takes to walk a half mil. [Note, a mil is 2000 amot (see above).]
- Explanation of Rabbi Yehuda's opinion: Rabba explains Rabbi Yehuda as saying that Bein HaShemashot starts from Shekiyah even though the sky is still red, while Rav Yosef says Bein HaShemashot starts from when the sun already set. It follows, says the Gemara, that according to Rabba the span of Bein HaShemashot is 3/4 of a mil and according to Rav Yosef Bein HaShemashot is 2/3 of a mil.
- Who do we follow, Rabba or Rav Yosef, in explaining Rabbi Yehuda? The Rif (15a) writes that since it is unclear whether halacha follows Rav Yosef or Rabba, one should be strict to follow Rabba that Bein HaShemashot begins at Shekiyah. Then he says that it seems halacha should follow Rabba because halacha always follows Rabba with three exceptions and this isn't one of them. Similarly, Rambam (Shabbat 5:4) rules like Rabba that Bein HaShemashot begins at Shekiyah. Rabbenu Yonah (quoted by Rosh Shabbat 2:23) argues that since they're not arguing about which logic is correct but about what Rav Yehuda said the halacha follows both Rabba and Rav Yosef, whichever way will be a stringency. Therefore, in context of accepting Shabbat, Rama 261:1 rules like Rabba that Bein HaShemashot is 3/4 of a mil.
- Explanation of Rabbi Yose's opinion: Shmuel (Shabbat 35a) says that Rav Yose’s concept of Bein HaShemashot takes place after the Bein HaShemashot already finished. Tosfot (Shabbat 34b s.v. Rabbi Yose) explains that even according to Rabbi Yose there’s a window of time when there is an uncertainty whether it is day or night but it isn’t as long as it is for Rabbi Yehuda. Rosh (Shabbat 2:23) discusses whether Rabbi Yose's Bein HaShemashot begins immediately after Rabbi Yehuda's or a while afterwards and doesn't come to a clear conclusion. Rashi to 35a s.v. Bein HaShemashot DeRabbi Yose seems to hold that it is right after R. Yehuda's period.
- Who does the halacha follow? Rabbi Yehuda or Rabbi Yose The Briatta (Shabbat 34b) says that in regards to practical ramifications Bein HaShemashot is always considered day as a stringency and night as a stringency. Rif (Shabbat 14b) and Rambam (Shabbat 5:4) codify this Briatta. Shulchan Aruch 261:1 rules that during Bein HaShemashot one may not do anything one may not do on Shabbat with some exceptions (see there). Therefore, Rabbi Yochanan (Shabbat 35a) says that we follow Rabbi Yehuda regarding Shabbat as a stringency and Rabbi Yose regarding Trumah as a stringency. Rif (15a) and Rosh 2:23 codify Rabbi Yochanan as halacha.
- (2) Gemara Pesachim 94a records Rabbi Yochanan's statement that an average person can walk 30 mil from HaNetz HaChama until Shekiyah and 5 mil from Shekiyah until Tzet HaKochavim (according to Rashi s.v. Ovav). The Gemara challenges this and explains really an average person can walk 32 mil from HaNetz HaChama until Shekiyah and only 4 mil from Shekiyah until Tzet HaKochavim.
- The opinion of Rabbenu Tam: Tosfot (Pesachim 94a s.v. Rabbi Yehuda and Shabbat 35a s.v. Trei) is bothered how the Gemara Shabbat seems to say that from Shekiyah until Tzet HaKochavim a person can walk 3/4 of a mil (according to the Rabba) and the Gemara Pesachim says a person can walk 4 mil. Rabbenu Tam answers that the Gemara Shabbat was referring to the end of the Shekiyah and from then until Tzet HaKochavim is 3/4 of a mil, while the Gemara Pesachim is discussing the beginning of Shekiyah which takes 4 mil until Tzet HaKochavim. Ramban (cited by Maggid Mishna), Maggid Mishna (Shabbat 5:4), and Ran (Shabbat 15a) agree. The Ran (Shabbat 15a) explains that according to Rabbenu Tam there's two Shekiyot. After the first it is still considered daytime until 3.25 mil after the beginning of the Shekiyah. Bein HaShemashot, which is uncertain day or night, spans 3/4 of a mil and begins from the beginning of the second Shekiyah. See Beiur Halacha 261 s.v. Metechila who explains that the first Shekiyah is when the sun sets from our vision, and the second Shekiyah is when the light from the sun leaves the sky.
- Does halacha follow Rabbenu Tam? The Shulchan Aruch 261:2 rules in accordance with Rabbenu Tam and states that from the beginning of the Shekiyah until Bein HaShemashot there's 3.25 mil and then Bein HaShemashot itself is .75 of a mil, which is equivalent to the time it takes to walk 1500 amot. Beiur Halacha 261 s.v. Metechila writes that for sure one should be strict for the Gra not to do any melacha after sunset.
- (3) Shmuel's opinion: Then, Shmuel (Shabbat 35b) says that Bein HaShemashot is as long as two stars are seen in the sky, but if there’s only one star, it’s day, and if it’s three, then it’s night. The Gemara then clarifies that the stars which were discussed are medium size stars. Similarly, the Yerushalmi (Brachot, cited by Ran 15a) says that Shabbat concludes when three stars can be seen in one area and not spread out. The Rambam (Shabbat 5:4) rules like Shmuel that when three medium size stars are seen it is certain nighttime. The Kesef Mishna (Shabbat 5:4) is bothered why the Rambam said that Bein HaShemashot begins from Shekiyah and ends when three stars are seen, choosing the opinion of Rabba for the beginning of Bein HaShemashot and Shmuel for the end of Bein HaShemashot. He answers that really Rabba and Shmuel agree and just describe it differently. This may also be the intent of the Rif (15a) and Rosh 2:23 who say that halacha follows Rabba as a stringency and also quote Shmuel without anyone arguing. See Or Letzion Vol. 1 YD 1:10 who offers a novel interpretation of the opinion of the Rambam that he holds like Rabbi Yose (see there for the halachic implications). See also Beur Halacha 416 who paskens like Rabbi Yose MeIkar HaDin, as well, and Chazon Ovadia Shabbat Vol. 1 pp. 264.
- Shabbos 35b, Rambam (Hilchot Shabbos 5:4). This is because if one treats Bein Hashemashot as day, one broke Shabbos on Saturday evening. If one treats Bein Hashemashot as night, one violated Shabbos on Friday evening.
- Mishnah Shabbos 34b