Derivation of the Shabbat Times

Halachic sources contain differing and sometimes conflicting criteria for determining the boundaries of day and night and of twilight - that period of transition between the two which is of doubtful status halachically. Moreover, the duration of twilight is not only specified in (deliberately) imprecise terms, it is also the subject of differences of opinion in both the primary halachic sources and amongst later authorities who offer varying interpretations of those sources.

It is well known that the duration of twilight varies both seasonally and with the latitude of the observer. The concensus of halachic opinion is that the criteria defining its duration in halachah are to be understood as referring to the latitude of Jerusalem at the time of the equinoxes. In order to apply them to other times and places it is necessary to redefine those criteria in terms that are independent of time and place. These criteria are relevant not only to determining when Shabbat begins and ends but also to other mitzvot which apply during either daytime or nighttime only, or at certain times of the day or night.

All of this gives rise to a rather specialised area of study which might be termed Jewish archeo-chrononomy (a term which I have borrowed from the expression "archeo-astronomy.") I include in this the science of redefining the halachic boundaries of day and night in terms consonant with spherical astronomy, using the criteria mentioned in halachic sources as a guide. These terms are independent of time and place and we can also apply computational methods used in astronomy to them to calculate the times of their occurrence. All this requires not only an understanding of the halachic process as well as specific knowledge of the relevant areas of halachah, but also some understanding of positional astronomy.

There is, by and large, general agreement nowadays on at least the general principles of the above procedure. The calculation of the times is no trivial task, but the basic method is similar, in the case of termination of Shabbat, to the method used in astronomy for calculating the times of the end of twilight. However, just as astronomy differentiates between Civil, Nautical and Astronomical Twilight (which begin in the morning and end in the evening when the sun is 6, 12 and 18 degrees below the horizon, respectively), differing standards may legitimately be followed for defining halachic twilight. The choice will depend on the halachic authority whose definitions in these matters one wishes to be guided by. Some opinions enjoy more widespread acceptance than others, and some will, inevitably, seem more credible than others in the light of our understanding of related areas in the natural sciences.

The Shabbat times shown in these tables are the same as those appearing in the J.N.F. pocket diary, distributed throughout Australia and New Zealand. I have been supplying these times to the J.N.F. for some twenty years.

Prior to 1997, the times given in that diary for the termination of Shabbat were calculated on the basis that, halachically, nightfall occurs when the sun is 7 degrees and 5 arc-minutes below the horizon. Three minutes were added to the calculated times of this event as a provision for tosefet Shabbat (Shulchan Aruch ch. 293.) There is no intrinsic minimum laid down in halachah for this (see Daggul Mervavah, loc. cit.), but three minutes was used so as to incorporate in this provision an allowance for the error factor to which these computations are unavoidably subject for the reasons described below.

In mid 1996, the Rabbinical Council of New South Wales was asked to consider the anomalous situation where differing Shabbat termination times were being given for Sydney in various communal publications. They recommended to the Sydney Beth Din that it move to standardise these times.

The Sydney Beth Din responded with a ruling that communally published times for that city should follow the halachic doctrine (shitah) that termination of Shabbat corresponds to a solar depression angle of 8.5 degrees (with no addition.) This is based on the view of Rabbi Y. M. Tucazinski (Bein Hashmashot, Jer., 5689), quoted by Rabbi Meir Posen (in Or Meir, London, 1973) and by Leo Levi (in Halachic Times, the revised edition of his Jewish Chrononomy, Jer. 1992.) The adoption of this standard by the Sydney Beth Din follows an increasing trend amongst calendar makers throughout the Jewish world to adopt this standard for the termination of Shabbat.

For termination of rabbinically ordained fast days, the Sydney Beth Din have adopted the more lenient doctrine of Rabbi Schneur Zalman, which, according to Leo Levi, equates nightfall with a solar depression angle of six degrees. This corresponds to the astronomical definition of Civil Twilight.

The Sydney Beth Din wrote to me at that time advising me of their ruling and requesting that it thenceforth be followed in the times published for Sydney. After consultation with Melbourne rabbis, I considered it preferrable to standardise the times for all the cities on that basis, rather than publish times for Sydney calculated according to a different criterion from that used for all other cities.

It is important to point out that this is not to say that the doctrine previously followed, which equates nightfall with a solar depression angle of 7 degrees 5 minutes, is an erroneous one. It has the rabbinic endorsement of Rabbi David Tzvi Hoffman (Melamed Lehoil) and there are good arguments, both halachic and scientific, in its favour (ibid, responsum 30, and see, also, Zemanim by Edgar Frank.) It was also, I believe, (though I have been unable to obtain confirmation of this) the standard adopted in a calendar that was, many years ago, published by a New York Va'ad Harabanim (Board of Rabbis).

Any congregation that wishes to keep following that doctrine is welcome to request tables calculated according to that doctrine, or, for that matter, any other doctrine. You may e-mail such requests to me.

It must be understood that the ruling of the Sydney Beth Din applies only to "communally published times for that city", and is not necessarily binding on other congregations, who may, justifiably, consider it an unwarranted chumra (stricter interpretation of the law) to have the new standard arbitrarily imposed upon them, especially in more southerly latitudes. Nevertheless, since a consistent standard must be chosen, and the Sydney Beth Din is the only communally based rabbinic body to have lent its authority to the endorsement of any one standard, that is the standard now adopted for these tables.

Candle-lighting times are (as always) 18 minutes before the calculated times of observed sunset. This conforms to widespread and long-standing Jewish practice throughout the world. (For possible origins of this practice, see Leo Levi, op. cit., footnotes to chapter 4 of the Hebrew section.)

All these times vary seasonally and with geographic location. They have been calculated individually for each date and location, assuming an even horizon at an elevation equal to sea level.

One must assume an error factor of plus or minus two minutes for these times. This is because of the effects of (a) arithmetic rounding of fractions of minutes, and (b) the fact that it is, in some cases, impossible and, in others, impracticable to make precise allowances for certain variables. In the former category are such factors as atmospheric refraction, which is affected by local meteorological conditions. In the latter category are the minor perturbations to which the earth's rotational and orbital motions are subject.


For their assistance in providing technical advice and related information, I wish to thank:

- Prof. Dan Censor, Ben Gurion University, (author of the Zodiac program),

- Prof. Leo Levi, Jerusalem College of Technology, (author of Jewish Chrononomy),

- Ed Hyduke, head of computing section, Astronomical Society of Victoria,

- Peter Caldwell, former lecturer in astronomy at Deakin University,

- Geoff Luton, Australian Surveying and Land Information Group, Geodesy Division.

Mottel Gutnick, 1996.

Copyright © Mottel Gutnick