Monday, March 5, 2018

The Kezayis Post

With Pesach approaching, it's kezayis season again. The monograph that I wrote on the evolution of the kezayis, from the size of an olive to a matzah ten times that size, seems to be the most popular piece that I have ever published. If you haven't read it, you can download it at this link. This is the latest version, updated with the evidence from the Mishnah that Chazal's kezayis was much smaller than the sizes popularly stated today (namely, the Mishnah showing that a kezayis is less than three by three finger-breadths of fabric). There is no charge for the monograph, but if you feel that you (or others) have benefited from it, please make a donation to the Biblical Museum of Natural History.

Here is a list of other posts relating to this topic:

Matzah/Maror Chart for Rationalists - so that you, too, can have a chart!

The Popularity of Olives - exploring why this paper is so popular and yet hated by some.

Why On Earth Would One Eat A Kezayis?  - discussing the strange notion that one should aim to eat a kezayis of matzah on Seder night. (Added as a postscript to the monograph)

The Riddle of the Giant Kezayis Defense - wondering why many people would not accept that a kezayis is the size of an olive.

Maniacal Dishonesty About Olives - exposing an error-ridden critique that appeared in the charedi polemical journal Dialogue.

It's Krazy Kezayis Time! - discussing the view that one should eat a huge amount of matzah in a very short time in order to fulfill all opinions.

The Kezayis Revolution - announcing the fabulous sefer by Rabbi Hadar Margolin, which presents the same arguments that I brought but in a more yeshivish manner. He also brings an astonishing array of evidence that many recent charedi gedolim likewise held that a kezayis is very small, including even the Chazon Ish! Best of all, the entire sefer can be freely downloaded.

Finally, two notes regarding The Biblical Museum of Natural History:

First, there are lots of tours over the next few weeks, including before Pesach as well as Chol HaMoed. But they are rapidly filling up, so if you'd like to come, book your tour as early as you can!

Second, we are really looking for people who support our goals of educating the entire spectrum of society about the relationship between Torah and the natural world, and who want to be part of our mission. To join the museum as a patron, please see for details. We can now arrange tax-deductible donations in Israel, the UK and Canada, as well as the US. For easy online donations, please click this link. Thank you for supporting our mission!

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  1. " most popular piece" And probably your most dangerous.

    1. With a moniker like "Dynamic Weight Loss", you should be more positive about this.


    2. Dangerous because it makes way too much sense !

  2. Thanks to past articles such as these from Rabbi Slifkin, Rabbi David Bar Hayim, and Rav Uri Cherki, we have for the past several years been m'kayim mitzvat achilat matzah without the prior agony and overnight side effects.

  3. This may have been covered during the first iteration of this post, but it bears repeating. Not only is the Ashkenazy 'kazayit' unrealistic, i.e. 1/3 or 1/2 of a 'beitza' - with 1/2 being more prevalent view, but the volume of the 'beitza' is also made unrelated to actual eggs in these circles. I refer to the view that our egg volumes must be doubled to allegedly make them agree with corresponding measurements involving length (etzba). This notion stems from a measurement of egg volumes made by the sage, Rav Yechezkel Landau, who found them to be 1/2 the volume corresponding to his measurement of an etzba. The means of the measurement of an etzbais is unstated (it's not an easy measurement and requires measuring many people to get a reasonable average). Furthermore, the eggs in 18th century Prague, of which he was the rav, may have been quite small relative to modern eggs. The author of the renowned halachic seifer, the Aruch Hashulchan - a commentary on the entire Shulchan Aruch at the turn of the 20th century, noted that egg volumes had doubled in Lita since the introduction of a new breed of hen. Our large eggs are certainly no smaller than the eggs in Lita, and agree with the corresponding volume based on a more realistic etzba measurement. Rav Moshe Feinstein measured the volume of commercial large (or very large) eggs and found that 1.5 eggs (a reviit) corresponded to the calculated volume of 10.8 cubic etzbaot(the alternative Talmudic measure of a reviit based on his measurement of many people in the lower East Side of NYC. So much for realism. Unfortunately, the prior 'doubling' view still holds force in some circles, having been accepted by the Hazon Ish and some other such authoritative figures. It's a question of choice of authority figures vs. knowledge of reality.

    Y. Aharon

  4. See Aruch HaShulchan in hilchot Challah where he schools the world in grand style on this issue.

  5. Can someone please do the math for me?

    Tachlis, how much flour is needed to trigger making a bracha on dough for Hafrashat Challah?

  6. what page on the monograph does it say "the Mishnah showing that a kezayis is less than three by three finger-breadths of fabric" ?

  7. Jewish Israel, I wouldn't presume to judge the issue of hafrashat challah - only to provide some guidelines. The volume of flour that requires making a bracha is at least 43.2 egg volumes. An egg volume according to Rav Chaim Naeh (a prevalent view) is 2 fl. oz. According to Rav Moshe Feinstein it is 2.2 fl.oz. Thus the required minimum flour volume is either 86.4 or 95 fl. oz. Of course, the shiur is essentially doubled according to the Hazon Ish. Translating the volume into weights gets us into the question of the bulk density of flours (mass per unit volume of flour strained through a sifter and into a measuring bowl, with minimal or no shaking, to get the volume of a given weight. I have seen different values cited on the internet. If one takes one cited value of 0.67 kg/L, one gets a bit more (4.2) or a bit less (3.8) than 4 lbs of flour for the above volumes (86.4 and 95 fl.oz., respectively). I have also seen a figure of 2.7 lbs flour. More research or more expert opinion is needed to provide a practical determination, i.e., either measure the volume yourself using the above provisos or ask a qualified posek (ask also what is his measure of an egg volume). In case of doubt, separate challah without a beracha.

    Y. Aharon

    1. Y. Aharon, the question is what constitutes the volume of an egg. Our current day chicken eggs? Eggs used to be smaller. R' Chayim Na'eh's and R' Moshes' weights coincide with today's average US L/XL egg sizes.

  8. There are many other proofs from Chazal that a kezayis is indeed the size of a regular olive. See this kuntres from an avreich in Merkaz Harav (the haskamos are interesting too):

    See also R. Shmuel Tal's comprehensive discussion of the shiurei torah:

    R. Tal also agrees that a kezayis is the size of a contemporary olive, which he places at 6.5cc. Interestingly, he dissents slightly from R. Hadar Margolin's 75cc reviis (and associated length measurements). He ends up in between R. Margolin and R. Chaim Naeh, with an 80cc reviis and a 46.7cm amah. To my mind, R. Tal's approach has the fewest difficulties of any I've seen.

  9. I was led here by an article which appeared in the Jewish Press before Pesach (found online at )

    I checked and found the exact same thing online as in the paper.

    That article is a bit garbled. It has, for instance, Rav Hai Gaon as the father instead of the son of Rav Sherira Gaon.

    It also says "a vastly expanded version of this essay, with extensive discussion and sources, is available for download at" [this URL] but that is wrong.

    After following a few links I got to:

    ...which is probably the link you had in mind.

    After so many years, you must be getting tired out writing this and errors are creeping in somehow with the editing.

  10. I have read and printed out some of the posts associated with this, and have a few comments to start off with.

    Theer are some things left out, like the name of that Rishon who saw olives in Jerusalem and Eretz Yisroel except that he lived in the generation of the Rosh. That needs to be included. I think the online article has the additional fact not in the printed paper that this is found in Piskei Rabboseinu SheBeAshkenaz, Moriah 2:3.

    I see, with tyhe help of Google, that that source is also included in the Sunday, April 10, 2011 post entitled

    "The Riddle of the Giant Kezayis Defense."

    Without an explanation of what it says, though.

    There is a little bit of a contradiction between olive pits from different types being found at Masada and in the Judean Desert dating from the Bar-Kochba revolt AND olives not being available in northern Euro=oe circa 1200 but I think I have an explanation. In ancient timesw it wass olives that was transported, but in the Middle Ages, it was olive oil. And I think I can explain why. Olives were transported till maybe teh time of teh GAomnim because liquid can spill and because theer is a difference in the quality of the olive oil between the first drop or three drops maybe and later drops - in fact there are different grades of olive oil. I think mentioned in the Talmud. But in the Middle Ages 1) this was forgotten and 2) olive oil was transported because it was less heavy.

    Also olives could be eaten but the process made them non kosher.


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