May one run a program or machine that will operate on Shabbos, either by setting a timer to activate once Shabbos has started, or by leaving it running continually? What if the action is one that is only meaningful on Shabbos (i.e., it couldn't be accomplished on any day other than Shabbos?) If yes, could one then benefit from that operation on Shabbos, e.g. a coffee-maker or a Roomba?

I read this response (update: the link is dead, but the suggestion was to write a script to visit SE on Shabbos in order to earn the enthusiast and fanatic badges) and while I think that that suggestion may be dishonest, it is an interesting question.

In summary then, there are several questions:

  • May one leave a device, mechanical or computerized, operating continually from Friday into Shabbos? (This is asked in another question (below), but the answer doesn't bring any contemporary sources)
  • What if it doesn't run continually, but rather uses a timer to activate itself once Shabbos has started?
  • What if one benefits from said device on Shabbos itself? (Lights, coffee, TV, Robotic vacuum cleaner?)
  • What if the function performed can only be performed on Shabbos specifically? (Fanatic badge)
  • Is Maaras Ayin* the only concern?

Please include contemporary sources where applicable.

Related: Computer working on Shabbos

*Maaras Ayin is the concern that others will think you are doing/have done something that actually is forbidden, but they will think that because you have done it, it must be alright. In this case, people will think that I have visted the site on Shabbbos (which is forbidden), and will conclude that visiting the site on Shabbos isn't forbidden at all, "after all", they'll say, "HodofHod did it!".

Update: While YDK's answer is very good, I'd like to see someone even more contemporary than R' Moshe address this question, especially regarding computers, which could be different than timers. Also, R' Moshe's teshuva doesn't cover a case where there is no benefit/interaction with the device until after Shabbos.

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    @IsaacMoses Did you read the edit history? Terrible. – Hod - Monica's Army Sep 28 '11 at 16:11
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    Oh, I see that the tone came from a snarky and persistent editor. I still think that the underlying question is pretty silly, as it concerns a meaningless electronic bauble, but I apologize for my previous comment, whose harshness shouldn't have been directed at him. I'm disappointed that they locked the question in its snarky, rather than original, form. – Isaac Moses Sep 28 '11 at 16:15
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    Agreed on all points, but look on the bright side: it led to a great Judaism.se question! – Hod - Monica's Army Sep 28 '11 at 16:21
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    Another bright side: this answer. – Isaac Moses Sep 28 '11 at 16:22
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    Yep, a good answer, that. When we graduate, can we make a custom badge? "Seventh Day Rest" or something like that? To be earned by those that visit the site only 6 days a week, consistently. Just kidding. ;-) – Hod - Monica's Army Sep 28 '11 at 16:27

Rav Moshe Feinstein (O.C. IV 60) strongly disagreed with the use of timers. His strongest definite reason is that

  • its usage is a disgrace to shabbos, since through their use one can run a business and circumvent shabbos. This is a Torah violation of "honoring shabbos". (Contrast this with placing a pot on the stove right before shabbos, where that is the final action.)

Other reasons to not permit timers is that the sages certainly would have forbid its use

  • because of its ability to circumvent shabbos

  • because it is effectively similar (and worse) than requesting a non-jew to perform the act

Since historically there had been those who permitted lighting lamps through a non-jew, Rav Moshe allowed timers on lights as a continuation of that tradition.


Lights: Yes

Coffee: No

TV: No, plus it makes noise, so it would be forbidden under a type of "maris ayin"

Other devices: This teshuva doesn't cover devices that were actually started before shabbos, don't make noise and don't really do anything on shabbos. For example, if one runs his computer over shabbos, but it's not really doing anything, that is certainly different than setting it to go on on shabbos. Setting updates would be a good question.

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    +1, Thanks for your answer. While I try to decipher that teshuvah, maybe you could explain a couple things. Would the leniency on lights apply to heating/cooling as well? Does this teshuva refer only to things that are actually benefited from on Shabbos as opposed to things which one would benefit from after Shabbos? – Hod - Monica's Army Oct 24 '11 at 1:24
  • Although R' Moshe doesn't mention heating and cooling, I would assume he would be more lenient than lights since amira l'akum, his main comparison to forbid timers, is widely accepted by heating and cooling. I don't know of any distinction based on the timing of the benefit. – YDK Oct 24 '11 at 4:32
  • So, I suppose the concern of disgracing Shabbos would apply even if one doesn't benefit from, or even see, it on Shabbos? – Hod - Monica's Army Oct 28 '11 at 20:44
  • Small update: I asked a rav about this, and he said that most poskim do not hold like the R' Moshe on this. I'm going to try to find the tshuvos he referenced. – Hod - Monica's Army Nov 22 '12 at 5:20

You can use an electric timer to turn your lights on and off, where manipulating the lights directly would not be permitted. A computer seems like just a specialized case of this, so long as you are not interacting with the computer on Shabbat/Yom Tov.

As @tom smith notes in a comment, if the effect of your programmed computer use would lead people to think you had done, rather than programmed, the action on Shabbat/Yom Tov, that's problematic. Using a computer to control your coffee maker is one thing; using it to post to J.SE is another. :-)

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  • Do you suppose that can be extended to things that can only be done on Shabbos, e.g. that badge? – Hod - Monica's Army Sep 28 '11 at 17:19
  • It would be worthwhile to include a source for the timer for lights, since a) I believe that there are both past and contemporary authorities who disagree with this view, and b) if I recall correctly, this permission may (at least according to some) be specifically for particular quality-of-life elements such as lighting and heating. – Isaac Moses Sep 28 '11 at 17:21
  • @IsaacMoses, interesting -- since "everybody" I know does it, it hadn't occurred to me that it was controversial. Thank you. – Monica Cellio Sep 28 '11 at 17:42
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    @IsaacMoses ASAIR, R Moshe says that lighting is unique because everyone knows of the concept of a timer. In other cases there could be problems of Maaras Ayin. – Shmuel Brin Sep 28 '11 at 17:49
  • @HodofHod That badge can be earned without violating the shabbos, and still doing it manually if your timezone is such that your day, and the stackexchange day don't overlap by at least an hour or two. This would not work for yomtov though. – Ariel May 15 '12 at 6:46

From daat.ac.il:

(Non-quoted paragraphs are ones I have summarized from that page.)
The discussion seems to center around the following Halacha (SA 252:1).

It is permitted to start an action on Friday near darkness even though the work cannot be completed on Friday and can only be finished on Shabbat.

The Gemara (Shabbos 18a) records a dispute between Rava and R' Yosef about whether this applies to noisy [i.e., public and obvious] things, like leaving grain in a watermill on Friday. The Mechaber rules leniently, like R' Yosef, while the Rema is stringent.

Based on this stricture of the Ramo, there are some who claim that using a timer on Shabbat should be prohibited when it creates audible or visible action. For example, absent some significant need, both Rabbi Feinstein and Rabbi Auerbach (Minchat Shlomo pp. 68-69) agree that this rule prohibits one from playing a radio on Shabbat even if it is left on for all of Shabbat. Placing a radio on a timer is analogous to putting wheat into a water mill. Both cause noise on Shabbat and arouse suspicion that its owner has violated the laws of Shabbat. Hence, they rule that it is rabbinically prohibited to set a radio on a timer or to let it run the entire Shabbat.

Other rabbonim have given other reasons to forbid it. Some compare it to placing a dish around a sparking flame, which is forbidden due to the concern that one might forget and adjust it, accidentally extinguishing the flame. Others say that one may begin an activity on Friday that continues on Shabbos, provided one does not benefit from it on Shabbos.

The consensus of the achronim as well as the accepted practice is not in harmony with any of the opinions which prohibit timers [emphasis mine]. As the Encyclopedia Talmudit ("Electricity" 18:679) states:

Many of the achronim permit one to set a Shabbat clock on Friday - and this is the common practice - even those who prohibit creating a sound permit the use of timers... since all know that these timers are set before Shabbat. Essentially, since it has become common practice to use timers, there is no appearance of impropriety when timers are used. This approach has been accepted by most contemporary decisors such as Rabbi Waldenberg, Rabbi Breisch, Rabbi Henkin, Rabbi Auerbach, Rabbi Ovadia Yosef, the Chazon Ish, Maharam Schick and many others.

There are many relevent footnotes and sources there.

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