| MadSci Network: Astronomy |
The question:
Does a stick's shadow in moonlight point east if it is six inches or longer? I am the coordinator for our homeschool group's Magic Tree House book club. On pages 42-43 of Night of the Ninjas, the kids are lost in the woods at night and need to go east. They pushed a stick into the ground in a clearing where there was moonlight and said that if the stick's shadow is at least six inches long that the shadow is pointing east. Is this true? If so, why and how does it work? I have Googled extensively and have not found any mention of this method of telling direction.
I have never seen the method you describe, and I also did an extensive search of the Internet without finding any reference to it. There are interesting methods to find compass directions using shadows available from sunlight and moonlight, some of the better sites listed here:
The method you describe is not totally without merit, although it is not foolproof.
Concerning the length of the shadow, it of course depends on the length of the stick, and since the length of the stick isn't mentioned in your description I can only guess that it must be a short stick to cast a shadow only on the order of six inches. The point, I think, is that the shadow is long compared to the length of the stick. In that case the Moon is "close" to the horizon, but just exactly how close depends on things like your latitude. If, then, the Moon is close to the horizon it is near the East, or the West, horizon. That, in fact, is the ambiguous part of the method: the shadow can point east or west. However, if you have any idea of the time of day the ambiguity can be removed: if the Moon is near the horizon in the evening (the Sun has already set) and it is not too far from being a full Moon then it is in the East, while if it is near the horizon in the morning (the Sun has not yet risen) and it is not too far from being a full moon then it is in the West. The shadow, of course, points in the opposite direction. The caveat "not too far from being a full Moon" is probably not even required to be mentioned because if the Moon's phase is closer to new than full then there probably isn't enough light to produce a decent shadow!
But the direction found by this method will by only approximately East or West. Consider, for example, the fact that the perceived paths of the objects in or near the ecliptic generally do not move perpendicular to the horizon. So if the Moon is "near the horizon" but still has two hours to go before it sets then it could be as much as 30 degrees North or South from the setting point, and the direction determined by its shadow could be that far away from true East or West.
So, if one only needs to know roughly where East is then the described method could work. On the other hand, some of the methods described in the URL references above are more accurate.
John Link, MadSci Physicist
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