In answer to the first headline question, we can provide an approximate answer to the question of the moon’s brightness: It has an apparent magnitude of -12.9 when in the full phase. But this value changes depending on the viewing angle and phase of the moon—it drops to -2.5 when it is a new moon. This makes the full moon the second brightest object in the sky after the sun, which itself has an apparent magnitude of -26.24, meaning that the sun is approximately 400,000 times brighter than the full moon when viewed from Earth.
Confused by the apparent magnitude scale? Don’t be. It’s a logarithmic scale that means small changes on the numerical scale represent big changes in real-life brightness. The smaller or more negative a number, the brighter the object is.
Now to address the second question: Why should we care?
Scientists and engineers operating satellites in space use the moon as a calibration tool to ensure the satellite’s cameras are reading colors correctly. What may appear as blue to one camera may register as a shade of green to another. It’s important to verify they are all seeing the same thing in terms of color. The moon is perfect for this job because it has no weather or noticeable atmosphere that can alter the colors and brightness of the lunar surface. The moon is a stable point of reference. We should care because satellites are used for all manner of useful tasks such as weather monitoring, climate change, precision farming and other critical remote sensing jobs.
By using the moon as a reliable reference for calibration, engineers can measure any drift in the instrument. When it comes to observing the Earth, the satellite operators can be certain that it is the satellite instrument changing and not the Earth itself. They can compensate for this drift, sort of.
This minor ambiguity causes problems. At the moment, the best methods of correction and compensation can only reduce the error in reading the brightness down to a few percent. This just isn’t good enough for modern instruments.
Luckily, engineers and scientists have noticed that the angles between the sun and moon vary over a repeating cycle period of 20 years. In principle, this should allow them to predict the brightness of the moon according to where we are on that particular cycle.
Researchers at The National Institute of Standards and Technology in Gaithersburg, Md., have devised a method to measure the brightness changes over this cycle and will use the information to make predictions, which will reduce the need for the costly workarounds they previously relied upon. The best part is that it won’t take the full 20 years to achieve this data collection—more than 95 percent of the angles required can be measured within three to five years, and the data will begin to be useful almost immediately.
The experiment will start in 2018 at the Mauna Loa Observatory in Hawaii, which is ideally situated at an altitude of 3,300 meters. It will allow the most accurate data to be collected without interference from the Earth’s atmosphere. Atmospheric gases and dust distort astronomical observations due to an effect known as the astronomer’s see. This effect is what makes stars have a twinkling appearance.
Researchers will use a small telescope designed to collect light over frequencies ranging from ultraviolet radiation (about 350 nanometers wavelength), through the visible spectrum and into the short-wave infrared (2.5 micrometers wavelength).
The lens of the telescope is manufactured from a material called calcium fluoride, which can focus the moonlight from this range of wavelengths into a detector.
Similar to how satellites use the moon to calibrate before observing the Earth, the telescope in Hawaii will use an Earth-based broadband light calibration source before each measurement of the moon’s brightness. To validate the broadband source, researchers will use a secondary light source emitting a much narrower band that can be tuned as per the experimental requirements.
Over time, these highly calibrated readings will help determine the lunar brightness with respect to the 20-yearly cycle and reduce the error to way below 1 percent.
And with that, they will finally be able to answer exactly how bright the moon is.
This video from NASA explains the process of on-orbit lunar calibration, as performed by Lands at engineers.
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