Star Spangled Banter


Chances are you have never heard the word albedo. It refers to the percentage of light reflected from any given body. A high albedo appears bright, and a low albedo means it appears dark. Simple enough? Then answer this. What is the albedo of a bright star?

Wrong! Stars have no albedo, because they give off their own light, and albedo is a measure of reflected light. On the other hand, the moon and planets, and everything in the world that we see by the light of something else is visible because it reflects some of that light. You are reading this article now because the newsprint reflects more of the sun's light (or your reading lamp's light) than the ink which forms the letters. Paper has a higher albedo than ink. The opposite is true for those greeting cards printed in white ink on dark paper, but if there were no difference the ink would be invisible. Perhaps an exception should be made for those viewing this article on a glowing monitor screen.

Albedo can be an important factor in the study of celestial bodies. For example, it has been widely accepted that the nucleus of a comet is not much more than icy slush, sometimes with traces of frozen ammonia, or methane, but mostly frozen water, mixed with a lot of dirt it picked up in roaming outer space for the past few billion years. It should come as no surprise that the dirty skin is not uniformly dark, partly because, as the comet approaches the sun, pockets of gas form and bubbles blast off pieces of the icy crust (this forms the coma and the tail of the comet). As the comet rotates, its brightness appears to vary, as we see parts with a higher albedo alternating with parts having a lower albedo. The darker parts absorb more sunlight and heat, and thus expand, throwing off the center of gravity somewhat, and altering its orbit a trifle in the process.

Venus (covered with clouds) has a higher albedo than Mercury, which is mostly dark rock. The interior of the Hubble Space Telescope is coated with a substance known as "Martin Black," probably the darkest substance known. Its low albedo was deliberately used to prevent internal reflections that might lower the contrast of the telescope's image.

The "man in the moon," is really formed by huge areas called maria (Latin, for seas, because their discoverer, Galileo, thought they were oceans). They are actually nothing more than lava flows, filling large basins on the moon. Lava has a lower albedo than the surrounding mountains, so it appears darker. The "white" portions of the moon have a somewhat higher albedo because its rocks have been shattered, resulting in billions of tiny surfaces that reflect more light in all directions.

Astronauts landing on the moon brought along color film, but they might as well not have bothered, because there is almost no color on the moon. Technically, however, each color has its own albedo. Red objects have a low albedo in green, and blue objects have a low yellow albedo. The albedo of each individual wavelength of light is what gives an object its characteristic color. Overall, the color of the moon is not that of green cheese, nor blue cheese (except possibly once in a blue moon!), but gray. Everything there seems to be either gray or black or white.

What may come as a real surprise is that even the brightest parts of the moon have a very low albedo -- about the same as an asphalt road. Why then does the moon shine so brightly?

The answer is that even with such a low albedo, the moon's rocks still reflect a lot more sunlight than does the adjacent empty space. No one who has seen full moonlight on the White Sands in New Mexico could ever forget its unearthly beauty (no pun intended). "Bright as day," everyone seems to comment. But no. Full daylight (10,000 lumens per square meter) is still 100,000 times brighter than full moonlight (0.1 lumen per square meter). Obviously, the moon is brightest at that time because more of the lit portion is showing, but also because the moon, Earth, and sun are more or less in a line. When the sun is at our backs we gaze upon the full moon. Each rock and grain of sand on that body hides its own shadow. At other times the sun, moon and earth are not in line; parts of the shadow of every mountain, rock, and other object are visible then, lowering the overall albedo of the moon.

There is another factor I have not mentioned, and that is the micro-geometry of rough particles. Magnify rough rock, sand, or a handful of table salt or sugar. You will see they are made of innumerable different surfaces, oriented in random directions, overall. Every surface will be oriented in its particular direction, but surfaces whose planes are in certain favored orientations will reflect more light back to us; these tend to be the surfaces at right angles to the line of sight (which at full moon are seen to greater advantage, when multiplied by the uncounted billions of surfaces involved). Just by virtue of their geometric shape, rough particles of any solid surface will appear brighter (i.e.,have a higher albedo) when the light source (e.g., the sun) and the viewer (e.g., the earth) are in line with the viewed object (e.g., the moon). This occurs at or near the full moon.

The new moon lies between the earth and the sun, so we see the shadowed side of our satellite. On the other hand, astronauts or little green men on the moon would be seeing "full earth," and in turn, we can see "earthshine" on the moon. The so-called "Old Moon in the New Moon's arms," was once highly feared by superstitious people who believed it foretold bad happenings. (Sir Patrick Spens was warned, "Late late yestreen I saw the new moone, wi' the old moone in hir arme, and I feir I feir, my Master Deir, that we will cum to harme.")

Earthshine is much brighter than that which an astronaut on the moon would see of the full moonlight reflected from the earth, partly because the earth would appear much larger in the lunar sky, but also because the earth's clouds and continents have a high albedo, i.e., they reflect a lot of light, which shows on the new moon's surface as earthshine.

* Appeared in The Kenwood Press June 15, 2002
© May 28, 2002, by Nathan B. Miron, Ph.D.