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Sunday, May 20, 2012

Moon Phases

In Canada we still celebrate Queen Victoria’s birthday on the 24th of May and, since it is adjusted to fall on the closest Monday to give us a long weekend, it is a public holiday tomorrow. Traditionally, this  is the day we plant our seedlings in the garden because it is so rare that we get a frost this late in May.  However, I checked using Google just the same to see the phase of the moon because I, like most people, believed that the nights with full moon are coldest and most likely to be frosty. To my surprise, I found that my firmly held conviction has no basis in fact and that the moon actually has no power to effect the weather at all!  It turns out to be one of those human quirks we have, where we clearly remember the times something that’s predicted does happen and easily forget the times it doesn’t.  It’s called ‘conformational bias’.

Like most people, until recently I couldn’t have explained why the moon waxes and wanes through monthly cycles until I found a video of a classroom demonstration and a diagram like the one on the right. To make sense of the diagram, you have to take into consideration that: 
1.We are looking down on one of the poles of the Earth in the centre as it is turning on its axis so in one 24 hour period we experience both day and night.  
2. The sunshine, coming in from the right hand side in the picture, shines only on one side of the moon all the time because the moon doesn't turn on it’s axis and 
3. the moon circles around the earth in roughly a month [29.53 days]. 
The diagram shows 8 positions of the moon represented by the circle of smaller moons all with just one face lit by sun.   
To understand why the moon is full once a month, start on the left hand side of the diagram and you see the larger outside picture of the moon is totally lit by the sun coming in from the right. The dotted line from Earth pointing to the full moon shows its position when it is directly overhead once a month. Now, going counter clockwise around the circle, the next dotted line with the small bracket at the end shows our view of the moon in that new position [the picture of the larger moon outside the circle is what we see].  Following the dotted lines around, when we come to the one pointing directly to the right, we can’t see any moon at all in that position because the sunlit side is away from us. The Earth is also shown sunlit on the right but remember it is turning on its axis and night will soon come – but still with no moon in sight. Three or 4 days later the dotted line gives our view where we see only a partly lit crescent moon and so on.
If you’re still having trouble visualize all the motions of the sun shining on the moon,  the moon moving around the earth and the earth rotating, it should be easier to envision if you click on this video.
As astronomers say when they sign off - Clear Skies!    Rie

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