Thursday, December 26, 2013

La Luna: Moonrise

Note: This program first aired December 14, 2013.

A few years ago, I started to make my own little Stonehenge. From a single spot on my land, I watched the sun as it changed its position on the horizon, with the seasons. I wanted to mark the spots on the horizon where the sun rose on the summer and winter solistices, the days that are the end points of the sun’s journey back and forth across the sky. I think I understand this pattern, and by marking it on the landscape, I could check my reckoning on a yearly basis, and test my hypothesis that this pattern is unchanging, at least on the time scale of my life. I never did move any large rocks around , but the idea still stands.

A celestial pattern I find more difficult to follow is that of the location of moonrise. One might think it would be easier to follow, being a shorter cycle and changing every night. But when I looked into it I realized just why I’ve never been able to “get” the pattern of the location on the horizon of moonrise. Its because its fantastically complicated. This may be one of those examples where a picture really is worth more than a thousand words, but I will attempt to explain it anyway.

The first thing to know is that the plane formed by the orbit of the Earth around the sun is called the ecliptic. Star gazers know this term because it is the path that the sun takes through the sky. At night it is the line across the sky where you will find the planets, as all of the planets orbit the sun on this same plane. If the Earth were spinning on an axis that was perfectly perpendicular to the plane of the ecliptic, our observations would be oh so much easier to explain. The path of the sun would be in the same place in the sky year round, rising and setting at the same spots on the horizon each day. The height of the sun in the sky would correspond with your latitude, at the equator the sun would always be directly overhead at noon, at the poles it would be on the horizon. If the Earth were rotating perpendicular to the plane of the ecliptic, it would be an equinox every day. And there is something appealing and egalitarian about that, but its not how it works. The Earth’s axis of rotation is inclined 23.5 degrees off the plane of the ecliptic, which is why we have seasons and different day lengths and all that good stuff. Its also why the sun’s point of emergence on the horizon migrates north and south over the course of a year. For example, in the summer, the northern hemisphere is inclined towards the sun, leaning into the plane of the ecliptic. That means the arc of the sun across the sky is higher in the sky than it would be if the Earth were straight up and down. The arc of the sun is higher, and the points of the sun’s emergence and stetteing on the horizon are further north (going along with that higher arc). I go through all of this for the sun and the Earth because it’s the easy example. In this case, the slowness of the year long cycle makes it easier to keep track of.

For the moon it all happens much faster. Just like the Earth orbits the sun, the moon orbits the Earth. The plane of the moon’s orbit around the Earth is not the same as the ecliptic. The moon’s orbit is inclined off the plane of the ecliptic by about 5 degrees. So over the course of a month, the moon travels around the Earth, and is at times as much as 5 degrees above the ecliptic, and at times 5 degrees below it. Put another way, half of the month the moon is above the plane of the ecliptic, and the other half it is below. This also means that there are times when it is exactly on the plane of the ecliptic, these are called the nodes, and incidentally, eclipses can only occur when the moon is at the nodes of its orbit.

Image of moon’s orbital plane vs. Earth/sun (from the Hyperphysics instructional website):

 So we know that the latitude of sun rise changes as the Earth’s angle relative to the sun changes over the course of a year. The same holds true for the moon. The apparent latitude of the moon rise on any given night is determined by the combination of the Earth sun relative position and the position of the moon in its orbit. It’s a cycle within a cycle within a cycle, which is why it is so wildly difficult to follow. It has a regularity to be sure, but one that changes constantly. I think that only the most dedicated sky watchers, and mathematicians have sorted it out.

Perhaps that is the basis of the idea that the moon makes us crazy. Not only does it change shape every night, but it moves about the sky in a different pattern each month, in sharp contrast to the plodding regularity of the sun and its seasons. At this point, I think I will stick to my solar Stonehenge, and simply keep watching the moon, delighting and finding humility in the surprise of where I find it on the horizon and in the sky each night. Maybe some day I will have watched long enough to unlock the pattern, but I am many nights way from that as of yet.


Find the time of moon rise, and set and lots more data here:

Neat stuff from Cornell University, complete with podcasts:

Most of the stuff out there about the moon’s orbital plane stems from questions about eclipses. This website is an example, it has a nice visual graphic that shows the moon’s offset orbital plane, in the context of explaining why and when eclipses happen