Your Fingertip to the Moon

I’m reading Simon Singh’s The Big Bang at the moment (only because I’ve already devoured our current Skepchick Book). In the beginning, he discusses how the Greeks first began to scientifically determine the nature of our solar system using little besides logic, math, a stick, and a fingertip. For instance, if you go outside tonight and point your finger at the moon, you’ll see that your fingertip just covers it. That means the distance between your eye and your fingertip is a little scale model of the distance between Earth and the moon. If you know how big the moon and your fingertip are, and the distance between your eye and your fingertip, you can figure out the distance between Earth and the moon.

Of course, you should go out tonight and try this mini-experiment, mostly because at approximately 10:30 pm ET there will be a full lunar eclipse, when the shadow of Earth will fall over the moon. Here in Boston, the skies should be clear enough to see the moon turn orangey red, so I’ll be out in the freezing cold stretching my fingertip upwards and thinking about the distance.

Rebecca Watson

Rebecca is a writer, speaker, YouTube personality, and unrepentant science nerd. In addition to founding and continuing to run Skepchick, she hosts Quiz-o-Tron, a monthly science-themed quiz show and podcast that pits comedians against nerds. There is an asteroid named in her honor. Twitter @rebeccawatson Mastodon Instagram @actuallyrebeccawatson TikTok @actuallyrebeccawatson YouTube @rebeccawatson BlueSky

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  1. Did the Greeks know how big the moon was?

    They had a pretty good idea, and how appropriate that one simple way to find out is using (drumroll…) a lunar eclipse! It takes about 50 minutes for the moon to go from touching Earth's shadow to being full covered, and about 200 minutes to cross through the entire shadow. This tells you that the diameter of the Earth is about four times the diameter of the moon.

  2. Those clever Greeks… Too bad science was set back so much during the later Roman era and especially after the fall. I want my rocket pack and flying car already.

  3. The Big Bang is a wonderful book! Quite apart from anything else. That and "Quantum- A guide for the Perplexed" are two of my favorites of the last 12 months.

  4. Okay! It cleared up, and I have been running out to the back yard (then back into the house) and so on, and so on– Watching the eclipse!


  5. Yeah– the moon is coming back now.

    It's just amazing. Events like this help me wrap my mind around the scale of the solar system.

  6. Sadly, I couldn't justify staying up until 4AM, on a cold, cloudy, misty thursdaymorning.

    And apparently there won't be another one until 2015.

    Then again, I caught the last two, so that makes up for it.

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