Hi, (Not a teacher) We could also say "was blue" if we follow the sequence - of - tense rule and we won't be incorrect. With "was blue" we are taking the listener back in the past (1917). Now , we are just reporting that the sky was blue when he showed it .
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Anonymous What's the best ending? I prefer 'is blue', because the fact persists to this day.In his famous experiment of 1917, scientist Mad Max showed that the sky a) is blue. b) was blue.The general rule is that back-shifting is always correct and that widely known facts need not be back-shifted, so both choices are fine.