Thursday, July 16, 2009

A Civil Dawn

We headed out the door at 4:29 am to catch our flight to the West Coast. After less than 3 hours sleep I crawled out of bed at 3:45 am. As we headed to the airport, Barnacle Boy and Crash Kid were bantering excitedly, as well complaining good naturedly about having to arise before dawn. Math Man wondered, if out of compassion for the parents, they might be a bit more civil. I said I thought civil dawn was later than this.

All this led to a conversation about the different ways to define dawn. My phone reports the time of civil dawn, nautical dawn and astronomical dawn. (Today that would be 3:49 am, 4:34 am and 5:14 am, respectively!). Crash wanted to know how the dawns were determined - and other than astronomical dawn, I only had some vague idea the decision hinged on whether or not you could read text outside.

Should you care - astronomical dawn is when the sun is 18o below the horizon and the sky begins to lighten enough to obscure astronomical features. Nautical dawn is when the sun is 12o below the horizon, but objects can be distinguished. Civil dawn is when the sun is 6o below the horizon and the ground is visible enough to begin outdoor activities. And of course, none of these times should be confused with sunrise!

I propose we redefine civil dawn as the hour in the day when you can be civil to (a) strangers and (b) siblings. Experience tells me that (a) would be significantly earlier than (b).

2 comments:

  1. cool stuff, and I would say there is enough data to make your final assessment.

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  2. Very enlightening, thanks -- but when is the butt crack of dawn?

    Hope you enjoy your trip!

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