Celestial Clocks: The Big Dipper and the Moon

Monica Telescope.png

One of my favorite things about general astronomical geekiness is that in addition to talking about black holes and dark matter and so on, there are some totally practical ways to use our knowledge of the objects visible in our sky. One of these is telling time!

…all right, “practical” is relative, since the chances are pretty low that you’re going to have to rely on this. But I still think it’s really cool to know how it works.

I’m going to talk about basic ways to use two bright, easily identifiable celestial objects (that are not the Sun) to tell time on one scale or another. The first is the Big Dipper (or Plough, or Great Wain, or Seven Great Sages) and the second is the Moon.

Part 1: The Big Dipper/Plough

I mentioned in another post The Big Dipper and You, which provided me with my earliest exposure to several awesome concepts, including the idea of telling time by the Big Dipper’s orientation. Basically, the Earth’s rotation means that we see the stars of the Big Dipper revolve in a circle around the North Pole not every 24 hours, but every 23 hours and 56 minutes. (This happens with any other constellation too, but the Big Dipper is one of the few that’s close enough to the pole to be visible most or all of the time from most northern latitudes.)

This 4-minute difference from the Earth’s rotation relative to the Sun means that the location and orientation of the Big Dipper change steadily throughout the year, allowing you to use evening observations to identify what approximate time of year it is. (You might think this is a totally unnecessary skill, but you’ll thank me someday when you’ve just emerged from a coma to find yourself with no idea how much time has passed, in a robot body that has no means of sensing temperature or other seasonal cues.)

For observing at 9 PM in northern latitudes, here are general positions that you can associate with each season:

Spring: Dipper is above the North Star, upside down as if emptying rain onto the Earth

Summer: Dipper is left of the North Star, bowl hanging down like a dipper (see what they did there?) of cool water

Fall: Dipper is below the North Star, sitting right side up like a pumpkin

Winter: Dipper is right of the North Star, handle hanging down like an icicle

Big Dipper Seasons.png

This involves some assumptions about seasons that may not be relevant to everyone, but I remembered it just fine as a little kid despite north Texas being pretty stingy with rain in spring, pumpkins in fall, or icicles in winter.

To tell time more precisely from here, you’d have to either have one timed observation of the Dipper go to from or already know the general date. Really, it’s mostly a cool thing to mention while you’re stargazing with others, which is as good a reason as any to know something.

Part 2: The Moon

This allows more precise time-telling than part 1, which used the Big Dipper to tell the approximate time of year based on evening stargazing. Here we use the fact that the moon goes through a complete cycle of waxing and waning phases as seen from Earth every 29.5 days (hence the month), rising and setting a little less than an hour later every day. This appears as different phases to us because of the way the moon and sun are positioned relative to each other and us throughout the month.

Regardless of the phase we’re seeing (full, new, crescent, gibbous, etc.), the half of the moon that faces toward the sun is always illuminated, so how much of the moon we see depends on how much of that illuminated half is currently facing toward us. If the whole lit half is facing us, we see a full moon; if the whole lit half is facing away, we see a new moon.

Another way to think of this: Is the moon on the same side of Earth as the sun, or on the opposite side?

For this second way of putting it, think of the eastern and western horizons as a line of first sight, moving east, and a line of last sight following on the opposite horizon. If the sun and moon hit these lines at about the same time as each other, they rise and set almost together. In this case the moon is between us and the sun, so we’re seeing mostly the unlit side (new moon). If they’re opposite each other, the only time they’re both visible in the sky is when they’re on opposite horizons, one rising and the other setting. In this case the moon is on the other side of us from the sun, and we’re seeing mostly its lit side (full moon). For any situation in between, the sun and moon spend some amount of time in the sky together, and we can compare them based on relative rise and set times.

Rotation Illustration

Here the sun and moon are on opposite sides of Earth, so we see them about 12 hours apart in our sky. Note that we see the whole lit half of the moon.

To think about comparing times, take a look at this second diagram, which shows the moon’s orbit as viewed from above the North Pole. We’re looking down on Earth so that its rotational axis points roughly toward us and we can see the plane of the moon’s orbit, and the parts bracketed in red are what we would see from Earth’s surface. The red insets illustrate the view from Earth at these points, so we can see it side by side with the orientation of the sunlit half.

Moon Clock

For all of these, remember the idea of that hemisphere of view between the line of first sight and the line of last sight, and it makes sense that the full moon rises about when the sun sets and vice versa, while the new moon is more or less in time with the sun. All the in-between phases are somewhere…well, in between, with the first and third quarters providing halfway points.

How do you tell time with this? Look for the moon! Note what phase it is and where in the sky you see it. If it’s full and directly overhead, remember it’s opposite the sun so it’s probably about midnight. (These are all pretty rough times, if you couldn’t tell.) If that same full moon is on the western horizon, it’s about 12 hours before/after the setting sun was in the same spot, so it’s probably about 6 AM. If it’s about halfway between the eastern horizon and overhead, it’s about 9 PM, 12 hours off from mid-morning.

Here’s an illustration of a different phase, the first quarter moon (halfway from new to full):

Moon Clock First Quarter.png

Shown here is the waxing moon at first quarter, halfway between new and full.

Using the intermediate waxing and waning phases is a little trickier, but remember the four major points shown here and estimate between them. (Useful tip: the moon always waxes and wanes from its west side to its east side, so right to left unless you’re pretty far south.)

You can easily make your own moon clock using this template. The directions tell you how to find the rise and set times for the moon at any phase, but you can also use it to find the current time by moving the dial so the current phase matches up with its position in the sky, and then reading the time above the south arrow. You’ll be all set for moon-gazing!

I will say that once you’re aware of the way moon phases work, you may find yourself noticing inaccurate moon depictions in various media if you think about it. For example, a small crescent should never be anywhere near high overhead late at night, since it’ll be only a few hours behind or ahead of the sun. I don’t usually care about pointing out scientific errors in non-scientific things, but for some reason I feel kind of smug whenever I notice this one. I guess we all have our things.

[A note on capitalization: In English, grammatical convention recommends capitalizing Sun and Moon when they’re referred to as astronomical bodies, but using lowercase when referring to them primarily as objects in our sky. Accordingly, I’ve used lowercase sun and moon in most of this post. Besides, “full Moon” just looks weird to me.]


One thought on “Celestial Clocks: The Big Dipper and the Moon

  1. Pingback: GPS Alternatives: The South-Pointing Chariot | Something of the Marvelous

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