Earthshine

The World Book defines albedo as “the ratio of light reflected to light received by a planet or other heavenly body.” Earthshine arises from sunlight reflected from the Earth to the dark of the moon and back to the nighttime Earth. The albedo is because of the Earth’s cloud cover and the diversity of landscapes. Leonardo da Vinci (1452–1519) first explained Earthshine in the 15th century. A simple technique was developed to measure the amount of sunlight that bounces off the Earth’s surface and is subsequently reflected by the moon. Earthshine is faintly visible to the naked eye on the darker side of the crescent moon as an ashen glow. For many phases of the moon, Earthshine is easily visible by the naked eye.

André-Louis Danjon (1890–1967) performed the first rigorous Earthshine measurements in 1926. He captured 200 data points over a period of five years with his cat’s eye photometer, and using a technique he pioneered called the Danjon scale. Danjon found that it was important to take the measurements well after and well before a full moon or a new moon. Having an even area of bright and dim light when observing the moon’s surface is needed to gain an accurate reading of Earthshine. After then, few measurements were taken, until NASA funded Project Earthshine in the 1990s to obtain albedo numbers. A value of 0.297 was obtained from the study. The research team used a 6 in. refractor telescope housed at California’s Big Bear Solar Observatory. Earth’s albedo decreased 2.5 percent during a period of five years. Satellite measurement of solar irradiance reported a variation of no more than 0.1 percent during an entire 11-year solar cycle. Some experts believe that this deviation is too small to change climate or leave a terrestrial footprint of the solar activity cycle. Thus, Earth’s reflectivity may be what is magnified in an indirect role of the sun in climate change. If this ability to reflect light is degraded, by even as little as 1 percent, global warming could be accelerated.

Earthshine measurements using low-power Earth-bound telescopes are practical since they are cost-effective, easy to conduct, and immediately cover a large portion of the Earth’s surface. Satellite determinations of the albedo, however, are costly and require precise calibration to obtain good results. In addition to capturing measurements when there is a clearly defined moon crescent, it is important to average a large number of data points. This is to remove the effects of a single measurement location where a nighttime measurement would see more light reflected by a large landmass on the opposite side of the planet, or where less light would be reflected by an ocean. This process of averaging provides a more accurate reading of a changing albedo. Modern techniques enable a measurement accuracy of 2 percent for each reading, which is equivalent to measuring Earth’s emission temperature to within 0.8 of a degree C.

Earth’s cloud cover dominates the “shininess” feature of the planet. Clouds reflect around 50 percent of incoming sunlight. Snow and ice reflect even more sunlight, usually from 50 and 90 percent. The melting of polar ice and disappearance of Greenland’s ice cover leaves more water, which reflects only 8 percent of sunlight, and uncovers more land, which is also a poor reflector of sunlight, usually with a reflectivity of 10–25 percent. The darker the surface, the lower the albedo, and the more solar energy absorbed. The dark side of the moon usually refers to the side of the moon that the human eye cannot see from Earth, even with the aid of a telescope. In the controversial world of climate change and the use of Earthshine as a tool to gauge global warming, the dark side of the moon now refers to the dim side of the moon in its crescent configuration. The moon’s surface provides astronomers and climatologists another means for measuring both sunshine and Earthshine, and to assess the global warming phenomenon.