In Back to the Future, there is a flyer indicating that the lightning struck the clock tower at 10:04pm on November 12, 1955. Based on that information, Doc calculates the distance, acceleration speed and time needed for the DeLorean to reach the cable.

But we all know that the minute has 60 seconds. And the lightning could strike any second within that minute. How could Doc predict that it would be at exactly 10:04:00? And thus he was risking. Should Marty arrive 2 seconds later or sooner, the experiment would fail.

  • You also assume the clock works perfectly - it may well be off 15 or 20 seconds, in which case.... the timespan is even longer. – TomTom May 25 '17 at 10:45
  • @TomTom - Ah, but Marty and the Doc are setting their watches by the time on the (working) Clock Tower. – Valorum May 25 '17 at 13:10
  • Yeah, which means it is relative time only, so a 60 second window. – TomTom May 25 '17 at 13:13

In the film's fourth draft script (the one that introduced the lightning-struck clock tower), the clock had a second hand.

It includes a photocopy of a NEWSPAPER ARTICLE, dated March 27,1955, with a picture of the clock tower stopped at 10:02." The headline: "CLOCK TOWER STRUCK BY LIGHTNING. CLOCK STOPPED AT 10:02."

BACK TO SHOT - Brown reads it, nodding. He's getting an idea.

BROWN: Kid, if this is true, we just might be able to get your ass back to the future! It's totally insane, but it's certainly no crazier than building a nuclear reactor onto the back of a car... According to this, we know the exact moment lightning will strike a specific spot — at 10:02 p.m. and 11 seconds on next Saturday. All I have to do is rig up a conducting system that'll channel the lightning directly into the T.F.C. As long as you're doing 88 miles an hour when it happens... See you later, alligator.

Although this was changed, the idea of hitting the clock at a very specific time doesn't seem to have been altered.

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    Now that explains everything. I wonder why was the second had removed from the final script. Its disappearance leads to confusion. – user2513149 May 24 '17 at 20:57
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    @user2513149 - Because the Doc needs to hang from the hour hand like Harold Lloyd. The second hand would conflict, making the stunt impossible. – Valorum May 24 '17 at 20:58

From the script:

Doc: A bolt of lightning, unfortunately, you never know when or where it's ever gonna strike.

Marty: We do now.

Doc: This is it. This is the answer. It says here that a bolt of lightning is gonna strike the clock tower precisely at 10:04 p.m. next Saturday night. If we could somehow harness this bolt of lightning, channel it into the flux capacitor, it just might work. Next Saturday night, we're sending you back to the future.

From this it looks like the lightning struck the tower at exactly 10:04:00. The rest is simple math.

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    Since the clock on the tower had no hand to display seconds, how could people know at what second the lightning had happened? It could have been on any of the 60 seconds. There were no witnesses on the street to see that lightning. People probably noticed that the clock had stopped working only in the morning. The flyer authors could not know the exact second since there was no "second" hand frozen on the clock. Doc could only assume it was at 00 seconds. But the chances of this event are 1/60. It's very unlikely that the lightning chose to strike precisely at 0 seconds. – user2513149 May 24 '17 at 20:39
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    Clocks still have mechanisms that represent seconds internally, even if they don't show them with a second hand. Someone probably inspected the clock after the storm and reported something like, "What a coincidence! The clock stopped at exactly 10:40!" – DCOPTimDowd May 24 '17 at 20:42
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    Those who had inspected the clock tower after the strike could have determined at what second it struck based on it's internal gearing state. We can assume this is what was done and published in the paper (the actual paper contains repeated gibberish iirc). Probability-wise, there's a 1/60 chance for it to have stopped in that state, which is certainly far less than the odds of him having traveled back with a newspaper chronicling such an event to a time that just happened to be reasonably before it enough so to as make use of it. – Mwr247 May 24 '17 at 20:42

They did not need to know the exact time, down to the millisecond, of the lightning strike - because they are about to initiate it themselves. By building a lightning rod that has an open circuit to the ground, that will only be triggered as the conducting car starts to pass under the suspended cable and initiate the lightning, which will then strike after a few microseconds.

All Brown had to know is when conditions for lightning is present (which he got from future data) and than design the hook so that it will be further back from the front of the car as to connect with the cable about 320uS after the car front pass under the cable, just in time for the lightning to strike.

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  • This doesn't sound like a bad idea, but is there anything to actually back this up? – Valorum Jun 5 '17 at 17:41
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    In the original timeline, when Marty has not been in the past, the lightning still struck the clock tower, even without any cables. – user2513149 Jun 5 '17 at 17:44
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    Had marty not gone to the past, the lightning may have struk at a different time or non at all. Since the pamphlet is from marty's timeline, it would have according to BTTF logic had self-updated to reprint the time of strike from marty's past, due to marty's actions in that past. – user84400 Jun 5 '17 at 17:57

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