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I seem to recall learning that it is permissible to use a cell phone as a watch on Shabbos as long as the time is always displayed on the front and no buttons would need to be pressed in order to observe the time. Of course, in order to carry the phone, there would need to be an eruv. The argument was that the phone is not muktzeh if it is being used specifically for the purpose of time-telling.

Has anybody else ever heard this, and is this idea sourced anywhere?

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Perhaps the problem would not be with muktzeh, but with moras ayin? –  gt6989b Apr 18 '13 at 18:42
    
@gt6989b, what if you just did it in your house when nobody was around? –  Daniel Apr 18 '13 at 18:45
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I don't think that works for moras ayin, but there are much more competent people on the site than me - perhaps someone will offer an opinion. Thanks for an interesting question. –  gt6989b Apr 18 '13 at 18:48
    
I was told it wouldn't work since it's a 'trick' and the real purpose of the phone is to call –  JNF Apr 19 '13 at 9:54
    
But isn't it still considered usage, even without doing tiltul (moving), which would make it asur due to muktzeh? –  Adam Mosheh Nov 14 '13 at 2:12
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up vote 5 down vote accepted

I would like to add that the mishna brura in 'סימן שח' סעיף קטן יב says that one should not use an item that is a kli shemelachto lissur if a kli that is muttar is available. So this question can only be relevant if one doesn't have a watch.

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Thanks for the information! And welcome to Mi Yodeya. I hope you stick around and enjoy the site. Do you think you could edit into your answer a more precise citation to the Mishna B'rura? (It's a long siman!) –  msh210 Mar 21 at 5:38
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So this is actually several different questions. In order from more to fewer poskim would say "yes":

1) Is it permissible to look at a cell phone that happens to be displaying the time on Shabbos?

2) Is it permissible to pick up your cell phone within your house and move it around for the purpose of having the time displayed elsewhere?

3) Is it permissible to press a button on your cell phone to cause the time to appear?

4) Is it permissible to carry your cell phone (where there's an eruv) and look at the time on it where other people can see you?

The answer to the first question is for sure "yes." Questions 2-3 depend on why you think using electricity on Shabbos is forbidden. Most cell phones today, especially so-called "smartphones," have gyroscope sensors in them and you will be causing circuits to change state merely by jostling them around.

As for question 4, it seems to me that if your cell phone is displaying the time, then it is also on, and so, if a non-Jew calls you, then it will ring, and that would be zilzul shabbos for sure, all other issues aside. Many phones have "airplane mode," which complicates the factors even more.

These kinds of issues are so complicated that only ravs who are also electrical engineers are really qualified to poskin on them, and how many of those are there? Also, each phone would have to be individually certified for "clock use on Shabbos" since the factors are sufficiently complicated that a general ruling would not cut it. In this case, I cannot see how one could justify a "yes" answer to number four under ordinary circumstances.

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This is written in way too poskin-y a way. I should remind you that I'm not a rabbi of any kind, and of course any issue should be discussed with your local rabbi. –  Tatpurusha Mar 20 at 7:17
    
Sources for your claims would improve your answer's value to its readers. –  msh210 Mar 21 at 5:36
    
Unfortunately, I'm essentially illiterate and the halakha I do know are the result of someone telling it to me in English. I try to provide my reasoning based on extremely generally-accepted principles in lieu of a citation. –  Tatpurusha Mar 21 at 7:14
    
Regarding 1): Most smartphones have ambient light sensors (adjusting brightness according to the environment), and all have proximity sensors (turning off the screen when held to the ear). It would therefore be very questionable to have a smartphone anything but off over shabbos. Even flight-mode will not help on these two issues. –  NBZ Jul 2 at 16:08
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