Feb 192015
 

Have you noticed how fast the earth has been moving lately?  Probably not, but in fact the earth moves faster in the winter than in the summer.  The197px-Ecliptic_plane_3d_view reason is because the earth moves around the sun in an elliptical orbit, not circular, so as the earth gets closer to the sun it speeds up and as it flies away from the sun it slows down.  In North America, the winter half of the year is approximately eight days shorter than the summer half.

Now, if you are like me, you are thinking, if we are closer to the sun now, why isn’t it warmer?  The answer is that because of the tilt of the earth, the sun’s rays are slanted and therefore don’t warm us like they do when hitting us more directly.

800px-Precision_sundial_in_Bütgenbach-BelgiumNow, if you are really perceptive, you may be wondering how that affects our clocks.  As you may guess, our clocks do not speed up or slow down.  They are based on the mean speed of the earth.  But, the ancients knew better and their clocks were based on the actual speed of the earth, not some arbitrary average.  That is why a sun dial is more accurate than a watch throughout the year.   I think it is part of the arrogance of man that at some point we stopped living by the natural clock and started managing time to suit our convenience.   A good example is Daylight Savings Time which has completely fouled up sundials.

So, what does that mean for the poor moon trying to hang on as we speed up and slow down each year?  TheMoon_phases_en moon orbits the earth in an elliptical orbit, also.  So, it speeds up as it gets closer and slows down as it gets further away.  It also appears larger and smaller as it gets closer or moves further away.  Now, think about the fact that the earth is moving through space as it circles the sun!  So, when the moon is moving the same direction as the earth, it has to speed up even more to pass.  Then, when it heads back the other way, it has to slow down to let the earth pass.  What a ride!

Well, to complicate things even more, the earth is tilted in relation to the sun which causes the seasons.  And it also causes the phenomenon of the sun appearing to move across the horizon each day north or south.  When the sun sets due west, that is the equinox, and it happens twice a year.  When it sets at its northernmost point, that is the Summer Solstice.  When it sets at its southernmost point, that is the Winter Solstice.  The solstices are also called “Solar Standstill”, because for a couple of days, the sun appears to set in the same spot before moving back the other way.

Lunar_standstill diagramThe moon has a similar apparent motion across the horizon.  But, the moon’s cycle is not yearly, it happens every four weeks or so.  For two weeks, the moon sets north of due west and for two weeks, it sets south of due west.  And, like the sun, when it reaches its northernmost or southernmost point, that is called a Lunar Standstill.  What is interesting about the moon, though, is that unlike the sun, the northernmost point this month is different from the northernmost point next month.  In fact, the moon will not have its Lunar Standstill in the same spot for another 18.6 years!

Why is that?  Because the attitude of the moon’s orbit changes.  As the attitude gets steeper, the gap between the southern standstill and northern standstill gets wider and wider for 9.3 years, then it gets narrower and narrower for the next 9.3 years.  We call that “Minimum Standstill” and Maximum Standstill”.

Phew!  That’s complicated!  The last Maximum Standstill was in 2006.  That means that this year we will be able to witness the Minimum Standstill.  Since the standstills happen twice a month, I like to observe the half moons rising.  For fun, you can mark the horizon for each half moon and you should see the gap between the first half moon and half quarter moon narrowing (very subtle, have to watch over time).  You will also observer that the moonrises will bounce around.

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Maximum Lunar Standstill, Chimney Rock Colorado

According to “EarthSky.org”, “In September 2014, … the moon swings farthest south on September 3 (18.6o south of the equator) and again on September 30 (18.5o south of the equator). About midway between these two dates, the moon climbs farthest north for the month on September 16 (18.6o north of the equator).”

 Stay tuned, I will be writing more articles on this phenomenon.  Next, we will take a look at one of the Anasazi’s most impressive Lunar Standstill viewing sites–Chimney Rock, Colorado.

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  2 Responses to “Native American Skies: Lunar Standstill”

  1. […] During the month, the moon rises at different points across the eastern horizon.  When it reaches the farthest point north it pauses, or rises in the same spot for a couple of days, and then reverses course.  This pause is called a “Lunar Standstill”.  The same thing happens two weeks later at its farthest point south.   You may have noticed that the sun does the same thing, but it takes the sun a year to move from its farthest point north (Summer Solstice) to its farthest point south (Winter Solstice) and back again.  At each solstice, the sun pauses before reversing course and this is called a Solar Standstill.  [refer to last week’s article: Native American Skies: Lunar Standstill] […]

  2. […] In any given month, the rising moon swings between two extremes on the eastern horizon, similar to the oscillation of the rising sun during the year.  When the moon reaches its maximum northern or southern declination, it has a “standstill” similar to the sun at summer and winter solstices.  The standstills could be said to be the moon’s equivalence to the Solar Solstices.  [for details on lunar standstills, refer to Native American Skies: Lunar Standstills] […]

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