Guest Post by Doug Hansmann

Passive solar has always topped our list of house design priorities, and here in southern Wisconsin, the trick is to capture and store as much solar energy as possible in winter while shunning summer’s overabundance.  But designing for moonlight may be a better choice.

We intend to build our new house on a south facing slope, but there’s a tree line on top of the ridge across the ravine, and if we build too far down the slope, those trees will shade out our solar gain at the winter solstice.   This past weekend, we set out to mark the ideal solar house site by measuring the altitude of that tree line to ensure we will harvest the low winter sunlight into strategic thermal mass in our living space.

...photo credit kansaspoetry, patrick, Flickr

But then it dawned on me that I had no idea how the view of the moon might be affected by our intended horizon.  Does it pass through the night sky at the same elevation as the sun?  How does it change seasonally?  For some precision answers, I went to the US Naval Observatory’s website, where you can tabulate the altitude for both sun and moon for any location, and any date from 1700 to 2100.

The sun is our source of energy, and our measure of time.  A day for each earthly revolution.  A year per orbit.  In southern Wisconsin, the sun’s fireball rises to 71° altitude every summer solstice then sinks to a mere 23° in winter, like clockwork.

But the moon moves to a more complex rhythm.

Why care about the moon?  It won’t help with the heating.  But in contrast to the baking hot sun, moonlight would fill the house with an energy that can’t be measured in degrees.  And as opposed to the sun’s blinding brightness, we can gaze wide eyed at the moon, mesmerized by that small fraction of sunlight reflecting off its surface.

It turns out that the moon tracks across the sky at wildly varying altitudes.  On late summer evenings, it travels low across the southern sky.  In January and February, it sails much higher.  During the intervening months its altitude can change so rapidly over a couple of weeks that it appears both higher and lower than the sun ever does. 

(Note:  My graph applies only to evening views of the southern sky.  The patterns differ in the wee hours.)

On top of the moon’s complex monthly movements, the 18-year Saros cycle also affects its position.  In the most extreme years of this cycle, the moon’s altitude can swing a full 20° more from high to low than it does during its sedate years.

Unable to keep pace with earth’s rapid rotation, the moon appears further to the west each day.  With this constantly changing orientation to both earth and sun, lunar phases pass from a visible crescent, to full circle and back again, every 29.5 days.   Most of us are awake long enough to see the moon in the post-twilight sky for only about two weeks out of each lunar orbit.

...Sailing above our horizon.

Back to the moonlit potential of our building site.  Our focus is on twilight-to-midnight views of the moon between new crescent to just past full, and if we don’t plan carefully, our view of the low moon in late summer could be blocked completely by leafed-out trees at the top of the ridge.

Perched on our slope at 43°N latitude, the overriding passive solar requirement is an unobstructed view of the southern sky above 23° altitude.  And if we move up the hill just a few feet more, we’ll enjoy watching the arc of the moon in the bargain — our muse and our night light.

In summary:

I really didn’t know where the moon path analysis was going to take me.  As it turns out, any good passive solar design will also provide many good moon views.

But because the schizoid moon sometimes passes through the sky even lower than the sun, I think there is a great rule of thumb to apply.  Whatever the design latitude, use the readily available passive solar rules to calculate the obstruction-free altitude to the south.  Then subtract a couple of degrees from that calculated altitude.

Here’s the beauty of designing to this lower number.  As Denise’s grandma used to say, “there’s many a slip ‘twixt cup and lip”.  In the event of some glitch that causes the final structure to deviate from the design, or if some trees grow up to obstruct the site lines (these things never happen, right?), the all-important passive solar gain will still be preserved.

So design for the moon, not the sun!  Whimsy rules!