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Scientists put the Moon to work daily as a calibration source for space-based cameras that use the brightness and colors of sunlight reflecting off our planet to track weather patterns, trends in crop health, the locations of harmful algal blooms in oceans and much more. The information sent from Earth-facing imagers allows researchers to predict famines and floods and can help communities plan emergency response and disaster relief.

To make sure that one satellite camera’s “green” isn’t another’s “yellow,” each camera is calibrated – in space – against a common source. The Moon makes a convenient target because, unlike Earth, it has no atmosphere and its surface changes very little.

The trouble is that, for all the songs written about the light of the silvery Moon, it’s still not understood exactly how bright the Moon’s reflected light is, at all times and from all angles. Today’s best measurements allow researchers to calculate the Moon’s brightness with uncertainties of a few percent — not quite good enough for the most sensitive measurement needs, says NIST’s Stephen Maxwell. To make up for these shortcomings, scientists have developed complicated workarounds. For example, they must periodically check the accuracy of their satellite images by making the same measurements multiple ways — from space, from the air and from the ground — simultaneously.

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Or, if they want to compare images taken at different times by different satellites, they have to ensure that there is some overlap during their time in space so that the imagers have the chance to measure the same part of the planet at roughly the same time. But what happens if a research team can’t get a new camera into space before an old one is retired? “You get what’s called a data gap, and you lose the ability to stitch together measurements from different satellites to determine long-term trends,” Maxwell says.

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Really knowing how bright the Moon is – with uncertainties of much less than 1 percent — would reduce the need for these logistically challenging solutions and ultimately save money.

So NIST is setting out to take new measurements of the Moon’s brightness. Researchers hope they will be the best measurements to date. (1)

We want to accurately measure the brightness of the Moon. In order to perform other measurements based on that measurements. And the measurement of the telescope that will be used for the measurements of the brightness of the moon will again need to be calibrated vis-a-vis other measurements.

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But what is the initial measure of all measurements? What is the measurement which gave meaning to other measurements in the first place?

Space. Monads. Light. Time. Darkness.

Tiny specks of (a priori) knowledge shaping our minds…

We know we can measure.

Because of something that is immeasurable…

The moon is bright.

Shining tonight.

I stroll on my own.

And still, I feel not alone…

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