Learning to track the moon

A few days after seeing the halo, we were out around the same time of evening, and yet the moon was nowhere to be seen. It was a clear sky, so this seemed a bit odd. Shouldn’t the moon be in same place at the same time every evening? Each subsequent night, we went out around the same time and with increasing confusion were always disappointed by an empty sky. After about 3 weeks of this, we finally noticed a sliver of moon during the day and some vague sense of understanding calmed our nerves. But really we had no real deeper understanding of how the lunar cycle operated. It felt disorienting to be so utterly miffed by something so familiar.

We both knew that the moon would wax and wane over a month, but we’d never really paid attention to this process unfolding in real time, it was just information that had been handed to us from books. It wasn’t until about a decade later (back in 2013) that I spent any real amount of time focused on figuring out what was going on. Over the course of a month I tracked the moon across the night sky (link), and came to a better understanding of the moon’s patterns. There’s still plenty I don’t understand about the moon, (as I wrote about in the last newsletter), but the more I understand the workings of the world around me, the more connected I feel to it.

Mini Quiz: Is this moon waxing or waning? How can you tell?

Waxing or Waning: How to tell if the moon is getting bigger or smaller

In the last post, I asked if you could tell if the moon in the image above was waxing or waning, and how could you tell. Take a look again before reading on.

While the moon certainly appears full in the image above, you might notice upon closer inspection that the right side of the moon (which faces west) has a slight roughness to its surface, while the left side appears mostly smooth. That smooth surface indicates a thin sliver of shadow barely obscuring the east-facing edge of the moon. When the shadow is on the leading edge of the moon’s path (the west-facing surface) then the moon is waning, or getting smaller.

ANSWER: In the photo above, where the shadow is on the trailing, or eastern edge of the moon (left in the photo), the moon is waxing (approaching full).

As the earth rotates around its own axis, we view the sky as though it is rotating around us. The sun seems to move across the sky at roughly the same pace as the moon as we make each 24 hour rotation. But the moon is also slowly rotating around the earth, completing a cycle once roughly ever 28 days. The movement of the moon relative to the earth (see video below) means that the moon slowly falls and rises about 57 minutes later every day (~28 days in a lunar cycle / ~24 hours in a day = 57 minutes).

Simple animation of the earth and moon moving about the sun (YouTube)

Some patterns about the moon’s movement

As you begin to observe the moon’s appearance over the course of a full lunar cycle, you’ll start to notice some patterns:

  1. On a full moon, the moon rises roughly when the sun is setting
  2. On a new moon, the moon rises roughly when the sun rises
  3. Every day the moon rises about an hour later than it did the day before
  4. The moon rotates around the earth, but it does not rotate around it’s own axis
    • So we only ever see the same surface of the moon
  5. The moon takes about 28 days to rotate around the earth
  6. As the moon slowly rotates around the earth, the surface of the moon facing the sun slowly changes
    • So a different part of the moon’s surface is illuminated

There’s a lot going on in the image above, but it’s worth studying. I diagrammed this out back in 2013 (you can find some of my writing on that here). Because the moon is waning right now, we don’t always get good views of it (waning moons rise after sunset when we’re sleeping). One of the other things I noticed was that the moon’s color changes as it arcs over the sky, this is particularly noticeable around the full moon. Closer to the horizon, where there’s more atmosphere between you and the moon, it takes on a yellowish glaze, but when it’s high over head it’s a brilliant white (the image immediately below was taken just after the moon rose).

Quiz Time

See if you can tell whether the images above and below are of waxing or waning moons. I find the moon to be a constant reminder to pay attention to the world around me and to try and figure out its workings on my own rather than rely on information gleaned from books (and amazing newsletters). So get outside and explore your world!

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