Old moon, new moon

Ken Tapping, February 8, 2012

In the sky this week…

  • Jupiter and Venus continue to dominate the southwestern sky in the evenings.
  • Mars rises around 8 p.m. and Saturn at midnight.
  • The Moon will be Full on the February 7 and Last Quarter on February 13.

One of the first astronomy books I ever read was “The Sky and Heavens.” In that book, in addition to star maps and information about objects in the sky, there was a painting depicting the western sky soon after sunset. There was a glow on the horizon showing where the Sun had disappeared, and the sky was that amazing shade of blue that seems to be duplicated nowhere else in nature. In the sky was the Moon, with a thin crescent lit brightly by the Sun, but with the rest of the disc faintly visible, glowing blue-green-grey. Nearby was the planet Venus, glowing like an aircraft landing light. 

Although our observatory specializes in observing radio emissions from the cosmos, it is an excellent site to see the sky optically. The need to protect the observatory from radio interference, by controlling housing development close to the site, produces the added benefit of a dark site for enjoying the sky. 

A couple of weeks ago I was leaving the observatory to drive home. It was dark and clear, and, as usual, I had a look around the sky. There, in the western sky was exactly what was depicted in that painting, except even better. There was a thin crescent Moon, brightly lit by the Sun, with the rest of the Moon's disc glowing grey-blue. Venus was shining brightly nearby and, as an additional benefit, higher up in the sky shone Jupiter; it was almost as bright as Venus, but more yellowish. 

The phenomenon of seeing a bright crescent embracing the much more faintly-lit rest of the lunar disc, is known as the Old Moon in the New Moon's arms. Of course, there are more prosaic names for it — ashen light, or more generally, planetshine. What we are seeing is part of the Moon lit by sunlight shining directly on it, and the rest of the Moon's disc lit by sunlight reflected from the Earth. This explanation is another thing we can attribute to Leonardo da Vinci, back in the 16th Century. 

Although we now have satellites to make such measurements, we still find that determining the brightness of the ashen light is a good way to estimate the degree of cloud cover on the side of the Earth facing the Moon. Images of the Earth taken from space show land to be dull brown or grey, the oceans to be brilliant blue, and the clouds shining snowy-white, because they are very effective at reflecting and scattering sunlight. 

If you have ever flown in an airplane on a cloudy or rainy day you will have noticed this. Below the clouds all is grey and dim. Then you are in the cloud and, as you climb through it, the cloud glows brighter and brighter. When you emerge on top the light is blinding. You are experiencing the fraction of the sunlight the cloud is sending straight back into space, perhaps in the direction of the Moon, so that someone else on Earth, where the Sun has just set, can see it lighting up the part of the Moon's disc not lit by the Sun, as the Old Moon in the New Moon's arms. Measuring cloud cover using the Moon's ashen light remains an important step in understanding climate change on Earth. Even though the Old Moon in the New Moon's arms is something we have known for centuries, it remains scientifically useful, as well as being beautiful.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton,  BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355

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