Friday, January 13, 2012

Orange Moon Rising

Looking at the rising moon is a great experience! The rising moon has a different appearance than a regular "high in the sky" moon. There are two noticeable differences:
  1. The moon looks much larger, even as 2-3 times as it seems later at night.
  2. The moon color is not white, but orange or even red.
Rising Orange moon
Orange moon about 12 minutes after it rises.

How to observe the rising moon
No special equipment is required besides your eyes. However, in order to see the large and red moon it is best to view it as soon after it rises as possible. There are many places where you can check when the moon rises in your area (it varies for different latitudes) for example use the heavens above site. Also make sure that no buildings or mountains block the view in the required direction.  After you find a nice location from which to observe, just wait for the moon to rise and observe its color and size.

Why does the moon look so large?
This is a very old question but even today there is not a full explanation for this. The moon of course in not really larger when it rises, quite the opposite. The moon's distance from the observer is larger when the moon rises than during the night (add the radius of the earth), and the moon is actually about 1.5% smaller! You can test this with your camera. Take a picture of the moon when it rises and when it is high in the sky and count the number of pixels (make sure that the same focal length or zoom is used in both pictures). There will be a slight difference, but even if you don't see the difference you will see that the size is more or less the same and that the large moon illusion is indeed an illusion.

Another known method to measure the moon size is by taking a small coin (a dime usually) and holding it in front of your eye with a straight arm. The coin will completely cover the moon. Try different coins with different sizes. Try it when the moon is low and when the moon is high and you will see that the coin covers the moon at the same distance from your eyes which means, again, that there is no size difference.

The "Large moon illusion" has been known for hundreds of years and even Ptolemy mentions it and tries to provide an explanation. The explanation he gave is partially correct. The moon seems to be large because our brain compares it to other low objects such as trees or mountains. Since we know the moon is much larger than the tree the brain interprets it as being large. However, even looking through a tube at the moon, it will still look large even when there is no other object to compare to. Also the direction of your gaze influences this phenomenon. Try either one of the following: try laying down on your stomach and then raising your head, or try to bend down and look upside-down at the moon and see for yourself what happens.

Why is the orange moon or red moon?
The moon is red because of the earth's atmosphere. It is the same reason why the sun looks red when it rises or sets. The atmosphere scatters most of the moon's light (which is the reflected sunlight) but not the red light which has the longest wavelength. The distance that the moon's light travels through the atmosphere is  greater when the moon is low in the sky. When the moon is higher the light is not scattered so much and the moon seems to be white.

Note: The moon will look red and large also when it sets. So make sure to check the moon set hour and location as well if it is more convenient to observe.

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