Palm and Moon

Palm and MoonThis is going to be another of those things that everybody else knew intuitively but I had to learn from the math. The moon is really bright.

You see in photometry the measurement of irradiance of a source is in units of lux. Lux is equal to lumens per square meter. A typical sunny day on planet Earth will have illuminance levels on the order of about 10^5 lux. An overcast day, or in the late evening as the sun goes down: 10^3 lux. A moonlit night is usually about 10^-1 lux, much smaller. However, the moon itself, has about the same illuminance as the crepuscular daytime.

If it’s night time, you need to expose the moon much faster than you would other shots. If the moon happens to be up during the day, you can set your exposure to whatever you would use for taking pictures of the landscape around you and you will get a proper exposure.

This is a fascinating result — to me. This means that the moon reflects enough light that it is visible during the day (which is obvious to anybody who has sight and is older than two, but has interesting connotations, nonetheless).

As a young boy I would often look up at the night sky and dream of venturing across that grand expanse of nothingness to discover what lies between the darkness, to see what each of those little points of light might contain. That yearning for discovery and curiosity of the unknown is much of what drives me to this day.

The moon has become a kind of icon of that desire to move among the stars. When it is full I gaze at it and imagine standing on its surface, ready to leap off into space.

I can’t go to the moon — for reasons too numerous to count — but I can look at it, and I can take pictures of it. This one is a little bit different. I figured people needed something else to look at for once, I’ve shown off enough pictures of the moon, but I’m not nearly done taking them. I suspect I will be taking pictures of the moon until I can see cities on its surface — which would change the reflectivity of our friendly neighborhood natural satellite, and hence the irradiance level, — and then perhaps I will be taking pictures of the Earth instead.


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