The Moon’s Light
The brightness of the moon, as seen from Earth, varies drastically throughout its different phases. The light we see is reflected from the sun as it lights up different portions of the moon. However, there is a common misconception that the moon does not emit any light itself. There is a thermal emission coming from deep within the lunar soil, which we cannot see. This is because the moon is emitting light within the radio wavelength, invisible to the human eye. This can be proved using a radio telescope and measuring the radio brightness of the moon over a period of time.
To collect our data, we used the 20-meter radio telescope at Greenbank Observatory in West Virginia. Radio telescopes are giant pieces of machinery, but this one is considered small, only four times larger than the world’s largest optical telescope. That being said, they are also quite complex and delicate; things don’t always work to plan. I submitted an observation of the moon but it never came back, so I had to analyze my groupmate’s data.
The complexity of the telescope also makes the analysis portion more difficult because the measurements will always come back with different values, even if you use the same telescope looking at the same object. To account for this, we first had to take a radio image of an object with a known radio brightness, and use that to calibrate our observations. We chose to use galaxy Virgo A, with a known brightness of 229.2 Janskys. Comparing that to our measured brightness, we were able to subtract any technological inconsistencies from the data.
Once the moon observation was finished, we used a program called Radio Cartographer to process the image. The program specifically removes any radio frequency interference, and removes any noise from the background. That way, we could be sure that any data we took from the image was only coming from the moon. Then, we uploaded the image into Afterglow, and performed aperture photometry. This uses software to sum up the pixel values, which gives the total brightness of the object. We then calibrated the brightness with Virgo A, and found the moon to have a brightness of 855 Janskys when it was 60% full. We then repeated this process, imaging the moon and Virgo A again, four days later.
We found that when the moon was only 20% full, it had a brightness of 950 Janskys. This is a small variation, which can be accounted for due to the moon’s elliptical orbit, causing it to be slightly closer to the Earth during the second observation. From this data, we can conclude that the radio emissions from the moon must be separate from the light that is reflected from the sun, because it does not vary with phase the same way. Finally, we were able to calculate the temperature of the moon below it’s surface using the blackbody equation, based on the brightness, the radius of the moon, and its distance from Earth. The temperature was between 220K and 230K for both observations, which matches the nominal value. This indicates that the thermal emission is absorbed sunlight that is remitted in the radio frequency.