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Ken Tapping, October 22, 2008

In the sky this week...

> Saturn rises in the early hours.

> Jupiter is still prominent in the southern sky during the evening.

> Venus is getting more and more conspicuous in the southwest after sunset.

> The Moon will reach Last Quarter on October 21 and will be New on October 28.

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At the end of this month we will be treated to a really attractive spectacle: the Old Moon in the New Moon's arms. In the western sky we will see the crescent of the New Moon, with the rest of the Moon's disc visible, coloured bluish or greenish grey. This phenomenon is not rare, and is usually called the Old Moon in the New Moon's arms. On October 31 we will have the treat of this spectacle with Venus shining brightly nearby. What we will be seeing is part of the Moon, the crescent, lit brightly by the Sun, with the rest of the Moon's disc lit by sunlight reflected from the Earth.

In our Solar System the only object we can see by its own light is the Sun. All the other bodies, whether comets, asteroids or planets, are only visible because they are being illuminated by the Sun. Part of that light is reflected, and it is that reflected light that we see. Although the Moon dominates the night sky with its brightness, it really reflects only about 12% of the sunlight falling on it. It has an albedo of 12%, or 0.12. By contrast, Venus, which is perpetually covered in thick, white clouds, has an albedo of 0.65, or 65%. Our Earth, a largely ocean-covered world with large patches of white clouds, reflects about 40% of the sunlight, depending on the amount of cloud cover. Because the light reflected by the oceans is mainly blue, it imparts a blue tinge to the part of the Moon's disc it illuminates. Earthlight falling on the lit crescent part of the Moon is so much weaker than sunlight that it is invisible by comparison. However, when there is nothing else lighting up the Moon, we can see the lunar landscape lit up by an ashy, blue glow.

Imagine what it would look like if we were astronauts standing on the Moon's surface. If we were on the sunlit part, we would see the Sun much as it looks from Earth, but against a black sky. The landscape would be brightly lit, with inky shadows. Perhaps lit with bluish earthlight. The Earth would be hanging in the sky looking about 4 times the diameter of the lunar disc as we see it from Earth, that is, covering nearly 16 times more sky.

If you have a telescope, you can explore the earthlit part of the disc, seeing the mountains, plains and craters lit an eerie blue-green. However, if we were astronauts standing on that part of the Moon, the Earth would be huge in the sky, with blue oceans and milky clouds, so bright and colourful compared with the lunar surface. All around us the lunar landscape would be lit with a soft, blue-green light.

However, the Earth would not be contributing much heat, and with the Sun below the horizon it would be very cold, well below -100°C. When the Sun rises again, the temperature rockets to well above +100°C. These extremes are due to the Moon having no insulating atmosphere. Which is why, when we have bases on the Moon, they will be underground, protected from solar radiation and the appalling variations in temperature. It would still be a wonderful experience though, to stand on the austere, lifeless surface of the Moon, with the Earth hanging in the sky and the surrounding landscape lit in a soft blue-green.


Ken TappingKen Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca