We Don't Know How Bright the Moon Is — But We Need To

A crescent moon is an ideal time to think about the brightness of the lunar surface. (Photo: GreenSprocket/Shutterstock)

Even though millions of us gaze at the moon every night, we don't know exactly how bright it is — which seems strange. I mean, we've already walked on it, tested moon rocks and NASA is planning a return to the lunar surface in 2024.

But there's a more practical reason: It's also incredibly important to know this detail with precision to keep our satellites running properly. That's because the way satellites work includes an important process of calibration.

To ensure they're getting the most accurate images, they regularly take photos at the same place and compare the brightness of the images to one other. The moon is a useful source as it's bright but not as harsh as the sun. If one of the images is off, the satellite will reset itself to make sure its sensitivity is correct. All of this is done automatically.

This ensures that information from those satellites — which track weather, wildfires, floods, agricultural crops, and sometimes even wildlife (like mass migrations) — is accurate. To work properly, they need to have measurements of the moon that are 99 percent accurate, but we can't get that accuracy here on Earth.

The best we can do from the planet's surface is about 95-97 percent accuracy due to atmospheric influences, from everything from clouds and weather to smoke, natural haze, and air pollution. Scientists at Hawaii's Moana Loa Observatory are taking the best Earth-based measurements they can and trying to calculate out the interference.

But another team from the University of Guelph in Ontario, NASA, the United States Geological Survey (USGS) and the National Institute of Standards and Technology (NIST) have another plan to get hyper accurate moon-brightness measurements, as the video above explains.

The air-LUSI telescope during a calibration
The air-LUSI telescope during a calibration. The light on the other side of the room is an 'artificial moon,' a stable source of light that has already been well-characterized. (Photo: Ken Ulbrich/NASA)

Flying up into the stratosphere, above all visual interference, the air-LUSI (Airborne Lunar Spectral Irradiance Mission) is a NASA-sponsored mission that will take a set of measurements to determine lunar brightness from closer to the moon's surface. The 500-pound airborne telescope "uses optical and robotic equipment to autonomously capture radiometric measurements of the Moon from within the science pod of an ER-2 aircraft while flying at an altitude of 70,000 feet," according to a journal article that describes it in detail. Air-LUSI can provide 99 percent accurate measurements.

A number of flights are needed, since the moon's brightness has to be measured during the various phases of the moon. And these are detailed measurements — the camera captured light from 380 to 1,000 nanometers, which is across the visible spectrum and includes the near-infrared.

The first flights began in November, and so far, so good. "The data we collected looks really nice," John Woodward, NIST physicist said in a NIST statement. "The whole team has done a great job getting this instrument to fly, and the ER-2 team at Armstrong has been a great partner in making this a success."

The full survey will probably take three to vive years to get all the needed data.

Then and only then will we really know how bright the moon truly is.