FAQ: Zmanim and Leap Seconds

Leap Second



  • Yes
  • Yes, but infinitesimally

Before delving into the answer, I would like to note that zmanim accuracy down to the second that are supported by the API are really nonsense, because variations in refraction make this accuracy pointless. The second (or millisecond) zmanim accuracy subject will hopefully have its own article in the future.
Since 1972, 27 leap seconds have been added to UTC time VS International Atomic Time (TAI). This is in addition to the initial 10 seconds added in 1972 to reflect pre-1972 corrections. The reason for adding leap seconds is to correct our clocks for incorrect original estimates of the speed of earth’s rotation. Assuming that you keep your clock time correct (and add the leap seconds), no change in any zmanim calculations by either the KosherJava zmanim library, or any other zmanim programs or APIs are needed. However, if you have a very accurate clock, and did not change it since 1972, your zmanim calculations will be off by 37 seconds. This is not something any zmanim APIs, apps or programs should have to deal with. Had we added any corrections for leap seconds, your zmanim calculations would be inaccurate.
As far as the Tōhoku earthquake in Japan, this is really the same question as the first question. Yes, that earthquake slowed down earth’s rotation by 1.8 microseconds or 1.8 millionth of a second (meaningless for zmanim). This as well as the impact of other earthquakes that impact earth’s rotation are factored into the leap second calculations that “correct” the time of our clocks and ensure that your zmanim are correct.

Technical Details

The leap second corrections address incorrect clock time that result from a slower earth rotation than expected. To get a drop more technical (thank you Pinny Markowitz), when the rate or the angle of rotation changes (due to earthquakes or other natural or man-made changes such as building of the Three Gorges Dam), the changes are minuscule. These small changes as well as the original incorrect calculation of the length of the day add up over time. On occasion (about every 2 years) the accumulated difference is significant enough to warrant introducing (or theoretically removing, something that has yet to happen) a leap second to reconcile our clocks with the natural one. On any given day, UTC time always considers 86,400 (60 * 60 * 24) seconds as a day. On the day with a leap second, UTC time still considers the day as having only 86,400 seconds, but a second is added to account for each of the seconds over a year or more being a drop too short. There are various ways of making system clocks conform with the UTC standard, but the net effect is always the same, an extra second is added. UTC doesn’t view a second the same way that the atomic clocks (International Atomic Time) do. UTC views a second as 1/86,400 of a relative day (that is actually 86,400.002 seconds long on average), while, atomic clocks count a day as 86,400 fixed length seconds. Fortunately, the approach of adding leap seconds is exactly what we need when calculating zmanim. By ignoring leap second events in zmanim code, and instead focusing on the measurements of a relative day, we can yield calculations that will be correct in the context of a relative day (assuming that you properly adjust your clock).

9 thoughts on “FAQ: Zmanim and Leap Seconds”

    1. There is no conflict in saying both statements are true, it simply depends on which context is to be imagined as stationary. In truth, neither the Earth or the Sun are stationary, and there is no difference, Halachically or mathematically, which point is actually used as the stationary one for any given context. In fact, the NOAA algorithm used in KosherJava presumes the Earth to be the stationary center for calculating a Sun position. This is done purely for mathematical simplicity, and not because of any inherent truth of any particular body being immobile.

      I would add that Chazal were quite aware of this, and that throwing ‘kefira’ accusations without any basis doesn’t give your argument much credibility.

    2. Reb Yid,

      Shalom Aleichem. Would you care to provide some citations please that the Torah confirms the geocentric view?

      Oddly, you mention the theory of relativity as a “proof” to the Torah. Your use of the word “Moreover” further reinforces the precociousness of your logic. “Moreover” means “besides,” not “additionally.” Your use of the word “moreover” translates to something like this:

      “Regardless of what the Torah says, the famous Einstein said it!”
      Who do you think you are? Does the Torah need proof from a German scientist in the early 1900s?

      Allow me to demonstrate an appropriate use of the word moreover, so we can all learn from this silly error: “Moreover, the theory of relativity has little or nothing to do with heliocentrism or geocentrism.”

      Also, if I may, spelling words correctly adds credibility to one’s assertions about advanced scientific theory. (In this particular circumstance: “oppesite,” are “relotivity” misspellings.)

      The point is: Don’t be a kof 🐒.

  1. I would like to point you to a few articles from charedi Rabbanim supporting the Heliocentric model. There is an article from Harav Yonah Mertzbuch written in Ohr Yisrael vol 59 ״והארץ לעולם עומדת״ כפשוטו של מקרא או רק לפנים. Also see Rav Moshe Sternbuch’s אמונה ותורה on the רמב״ם הלכות יסודי התורה פ״ג ה״ד. I would like to point out to the first commenter Reb Yid, that while cute, the theory of relativity in this case would only be applicable in a two body problem (if the universe only had an earth and sun). With multiple stars, planets and moons, the concept is not applicable and does not hold water.

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