Why Does the Full Moon Seem Brighter in Winter?

I was sitting outside on my patio, enjoying a particularly clear (and cold!) almost-winter night right before Thanksgiving. Stars that I normally don’t see were clear as day; the wind was calm for once – a rarity where I live; the just-past-full moon was big and bright.

In fact, the Moon was so bright that I got to thinking about it…and then I realized that full moons always seem to be brighter in the winter! Why?, I wondered, and so I did some more thinking, and then a little research. Now here I am sharing my thoughts with you on the subject.

After brushing up on my astronomy online, I came to the conclusion that winter full moons seem to be brighter than summer full moons for two reasons: one, the Moon is closer to the Earth at night in the winter than in the summer; and two, the cooler temperatures mean that less water vapor is trapped in the air.

Wet air

Let me start with the water vapor part, as this concept may be slightly easier for most people to grasp.

When the air is cooler, water vapor is more likely to condense. Higher condensation of water vapor = less water staying in the air (this is why we get dew on the grass in the fall and spring, and frost in the winter, but don’t seem to get much – if any – of either in the summer). In addition, what little water vapor does manage to stay airborne is made of smaller, denser particles that are spaced farther apart due to contraction, not to mention aren’t moving around quite as much. (I know, I know, I’m asking you to remember all the way back to Chemistry 101 here. On a side note I’m starting to appreciate that class. However, I digress.)

Fewer, smaller, less active particles of water vapor in the air quite simply mean one thing: less atmospheric interference. Atmospheric interference refers to the particles in the atmosphere – water vapor, dust, pollution, volcanic ash, etc. – bouncing light around, not only distorting images but also physically blocking our view of the objects beyond, however slightly. This is the reason why the sky appears pinkish or orangish in and around a city or other developed area, but not out in the middle of nowhere: the light from the city is literally reflecting off the various particles in the air. It’s also why you can see more stars on a dry night than on a humid night, even if there are no clouds.

All this atmospheric interference affects the way the Moon appears to us down here on Earth. The dustier and more humid the air, the duller the Moon seems. In the summer, the air is generally more humid than in the winter (due to the Sun’s heat warming up the Earth, in turn causing more water to turn to vapor, in turn “clogging” up the air), so, dust and volcanic eruptions and other such occurances notwithstanding, that alone is enough to affect the relative brightness of the Moon.

I’ll get back to “wet air” in a few moments. But first, a bit of (VERY) basic astrophysics.

Earth farther from Sun = Earth closer to Moon

This second part may take a little bit more explaining, but I will do my best to make it as clear and easy to understand as possible.

The Moon orbits around the Earth, and the Earth orbits around the Sun. Both of these objects sit in a relatively flat plane around the Sun’s equator. However, the Earth does not sit straight up and down like a freshly-released spinning top; rather, it sits on a considerable bit of a tilt to the Sun. So even though the Earth orbits the Sun in a pretty flat (rough) circle, its angle relative to the Sun is not perpendicular. This “leaning” of the Earth is what gives us seasons; the hemisphere closer to the Sun is in summer while the other one is in winter, and vice versa.

In the image above (please pardon my Windows Paint skills), the yellow circle represents the Sun (no, really?), while the blue circles represent the Earth at opposite points of its orbit. The white line through the Earth is an exaggerated representation of the Earth’s axis (the line from pole to pole around which the Earth rotates, giving us day and night) for purposes of demonstration, though it isn’t quite that severe in actuality.

Assuming that you, like me, reside in the Northern Hemisphere, the Earth at the left is in winter, while the Earth on the right is in summer. To give a little bit of clarification to this, draw an imaginary line across the Earth, perpendicular to the axis – this separates the Northern and Southern Hemispheres. Once you’ve done that it become more obvious that the Northern Hemisphere is closer to the Sun in the summer.

Now look at my amateur orbit graph again, and this time look at the small white circle next to each Earth. These white circles represent the (rough) position of the Moon at night. Notice how the Moon is closer to the hemisphere that is in winter? This is also due to the tilt of the Earth along its axis. The Moon orbits the Earth in a pretty darned regular pattern (though it is ever-so-slowly moving farther away from the Earth – but that’s another article), which like the Earth’s orbit is relatively flat to the Sun’s equator. Because the Moon’s orbit is flat to the Sun’s equator and not the Earth’s, it’s position relative to the Earth’s Northern and Southern Hemispheres changes accordingly.

In short: the farther away a point on the Earth is from the Sun, the closer that point is to the Moon, and vice versa.

This “movement” of the Moon in our sky does two things: one, it puts us closer to the Moon in the winter than in the summer, thereby decreasing the distance that the light reflected off the Moon has to travel to us, as well as making the Moon appear ever-so-slightly larger (think of a flashlight; when you look at it from across a room it seems much bigger and brighter than from across a football field); and two, it puts the Moon higher in the sky relative to our vantage point (similar to the Sun at high noon in the summer), limiting the chance for atmospheric interference to distort or dim the image we see.

If you don’t believe me about atmospheric interference and moon brightness, I have an example for you that demonstrates exactly what I’m talking about, regardless of other factors such as humidity, pollution, etc. The next time you have a full moon in your neighborhood, go out early enough to watch the moonrise. Notice how the Moon is not only much larger in appearance than when it’s high in the sky, but also much darker? Perhaps it’s a yellowish color, or even orange or brown, or red. Whatever the case, it’s certainly not white (or particularly bright) when it’s right near the horizon like that.

That change in size and color is due to atmospheric interference. When looking out across the surface of the Earth, much more of your sightline is through the thickest part of the atmosphere, where most of the airborne dust, water vapor, etc. resides – in fact about 80% of the entire atmosphere’s mass is in this thin layer (called the troposphere), making it pretty darned dense, relatively speaking. This causes the Moon to appear darker and distorted in color. In addition, the water vapor in the air magnifies the Moon to make it appear larger (though some experts argue this is really just a psychological phenomenon due to relative comparison to nearby objects).

When you look straight up, however, instead of looking through hundreds and hundreds of miles of dense air, you’re only looking through roughly 4-10 miles or so – and it gets exponentially thinner from there. (This is why jets flying long distances usually fly so high; the air is thinner so resistance is lower, thus costing less fuel to travel the same speed.) This straight-up view means that there are considerably fewer particles in the air to interfere with your lunar viewing, meaning that the Moon appears brighter and whiter.

Summary

In conclusion, it is my steadfast and humble non-professional theory that full moons really do appear brighter in the winter than in the summer, and that it’s due to a combination of atmospheric interference and the position of the Moon relative to the Earth. Winter = drier air = less interference, and winter = hemisphere farther from Sun = same hemisphere closer to Moon = less distance for reflected light to travel + less chance for atmospheric interference to begin with.

I only call this a “theory” as I have not, as of yet, been able to locate a single reputable source of astronomical information that addresses this particular topic. I have been able to locate some smaller, relatively unknown and rather unprofessional-looking websites that give information on the ideas behind my theory – to a small extent anyway – but nothing that directly tackles the topic head-on, and especially not reputable sources. (I consider “reputable” sources to be NASA, National Geographic, major universities and astronomical groups, etc.)

Hey – think I can get a multi-billion dollar government grant to study this? 🙂

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