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Skywatching

Taking a closer look at the Moon

Lunar odds and ends

We have all seen how much bigger the Moon looks when it is close to the horizon compared with how big it looks when it is higher in the sky.

If you have ever tried to photograph a moonlit landscape, the image of the Moon is much smaller than the object we remember seeing in the sky. It is just a little dot, not the romantic disc we wanted to record.

These illusions are the result of our brains playing tricks on us.

Try this simple experiment. Take an ordinary drinking straw, like the ones we get with soft drinks. The one I have sitting on the ridge at the top of my computer keyboard is 20 cm long and six mm in diameter.

When I look through it, the field of view is just under two degrees in diameter. You might think that such a small field of view couldn't capture the complete lunar disc. However, when you try it, you'll see that the Moon covers only about a quarter of that field of view. It has an angular diameter of about half a degree.

Things seem bigger when they are close to something, and they seem bigger when we concentrate on them. That's where in movies and pictures, the Moon has to be drawn too large, or it just does not look right. Don't try this looking-through-the-straw experiment with the Sun.

The Moon, like the Earth, is always half-lit by the Sun. If we are between the Sun and Moon, we see the lit half effect - the full Moon. If the Sun, Earth and Moon are not in line, we see part of the lit half and part of the dark half, so we see a half disc or a crescent. The only time half the Moon is not lit by the Sun is when, on rare occasions, it passes right through the Earth's shadow, giving us an eclipse of the Moon.

When the Moon is a crescent—so we see just one edge of the lit half face—we can determine the direction of the Sun easily.

Imagine the Moon is a bow. Now imagine there is an arrow on that bow. The arrow points towards the Sun. Quite often in drawings and cartoons set during the night, the orientation of the Moon shows the Sun would be still in the sky. One of the most surprising cartoons showed a star within the crescent. For this to happen there would have to be a hole right through the unlit half of the Moon.

The Moon rotates on its axis once for each time it orbits the Earth, so we only get to see one side of it. It's like dancing. When you whirl with your partner, you see his or her face the whole time. We often refer to that unseen side of the Moon as the “dark side.” That is definitely not the case.

That unseen side gets as much sunlight as the side we see. When the Moon is “new,” it is between us and the Sun. The familiar side of the Moon is dark, so we don't see it. and the unseen side of the Moon is basking in sunlight.

Back in the early days of astronomy, the Moon's surface was depicted as being one of jagged mountains and craters. The logic was that here on Earth, mountains start off jagged, and then they get rounded off and ground down by glacial ice, frost, rain and wind blown sand.

On the Moon there are no glaciers and no weather, so the mountains stay jagged. However, one important element was left out of this way of thinking—plate tectonics.

Little of the Earth's surface dates back more than a billion years or so. It has constantly been recycled by subduction and the emergence of new crust. The Moon has no plate tectonics and its surface is billions of years old. It might have had jagged mountains billions of years ago but ages of big daily temperature changes and impacts of micrometeorites have produced the smoothly rounded, dusty surface the astronauts have explored, photographed and sampled.

•••

• Mars lies low in the dawn twilight.

• After sunset, Venus lies close to the southwest horizon, with Saturn to its left and then Jupiter. Venus shines brightest and Saturn the faintest.

• The Moon will be full on Dec. 18.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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