Astrophysics and Chronobiology: Does Moon Phase Matter? By: James Nash

The dimple you can see in the center of some of these craters is where a magma eruption occurred from the meteor penetrating the crust of the moon. Photo is taken through a Sig Sauer Oscar8 spotting scope.

In the 1930’s Dr. Frank Brown had some oysters flown from Chesapeake Bay to his laboratory in East Chicago. He kept the oysters in the dark, and for the first week, they opened during the time that would be a high tide in Chesapeake Bay, just as they would if they’d been left in their native environment. During the second week, the oysters adjusted their schedule. They opened and closed simultaneously, but they now did so when the moon was directly overhead of underfoot of Chicago, the lunar position of high tide.

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Dr. Brown concluded that the oysters were able to sense something subtle, perhaps gravitational, that made them aware of the moon’s position. He published this research, and like all scientists who discover something that confronts popular belief, he was criticized. As research continued during and after his life, more of his theories were proven as fact.

Whether and how moon phase affects animals is a polarizing and hotly debated subject to this day. But speaking of polarity, Dr. Brown also suggested some animals navigate based on magnetism, an idea which was soundly rejected then and thoroughly embraced today. He spent his life researching how animals were able to sense micro-subtleties and adapt their behaviors to them. Interesting stuff.

Today, we can use something called a Solunar table to help predict periods of activity outside of the normal crepuscular behavior of game animals. Most animals fall into one of three categories: Diurnal means they are active during the day, Nocturnal means they are most active and night, and Crepuscular meaning they are active during dawn and dusk. Deer, elk, and many other game species tend to be crepuscular. 

The lunar cycle lasts for 27 days 7 hours and 43 minutes. Moonrise over the horizon changes about 50 minutes per day.

A solunar table uses your location on the planet and predicts periods of activity outside the normal based on the timing of sunrise/sunset and moonrise/moonset with some adjustments for moon phase, meaning how much of the moon can reflect sunlight onto your part of the earth at night. A new moon starts out 233,000 miles away from the earth with 0% illumination, this is the dark side of the moon facing the earth, the side of the moon that never sees the sun. From that point, the moon “waxes” meaning that each night it shows a larger percentage of the portion that faces the sun which is then reflected through the night sky onto earth. The first day will gain about 3% illumination, just a crescent. The next day is 9%, the day after 17%, and so on. When half of the moon is lit, that is called the first quarter and you will have 50% illumination. This is the closest the moon gets to earth during its cycle at 226,000 miles away. About 15 days after the new moon, a full moon will occur with 100% illumination and then the moon will “wane” until it is a new moon again. The entire cycle takes about 28 days.

The human body is mostly water. One of the biggest effects we know of from the moon is the changing of the tides, which is because of the moon’s gravity. High tides occur every 12.5 hours, low tides the same. The largest tidal swing in the country is near Anchorage, Alaska, and shifts over 40 feet of water. How much does that gravitational pull impact us as humans? The weight of a mosquito on your arm is greater than the reduction of weight on your body by a moon directly overhead. That said, there are 438 million cubic miles of ocean on this planet that get slammed back and forth every 6 hours and 12 minutes, so the impact of that subtle gravity is actually massive.

I know you want me to get to the part where I tell you when a 187” whitetail buck with a single drop tine on his right and a scar under his left eye is going to saunter under your tree stand and pause comfortably at 23 yards. The answer to that one is always, right before you got here or right after you left. Science can make some predictions about when animals are more likely to be on the move. The algorithm to figure that out is wildly complicated because it takes into account too many factors to control and understand, but we can try.

Elk don’t have a hunting season. They are being hunted every day of their lives, and survival is their foremost priority, even more than procreation. They can live to reproduce another year. Take lessons from the predators who make their living pursuing elk and you’ll go far.

Here’s some of what we know about how moon phases affect people. During a full moon crimes increase. People sleep less during full moons, even in completely black rooms. People report being more fatigued during full moons. Bullying at schools increases during full moons. 81% of mental health professionals in one study reported an increase in mental illness during full moons. The word “lunatic” is derived from the word “lunar.” The United Kingdom even staffs more police officers on nights with full moons and some hospitals staff more emergency room nurses. What is also peculiar, is that there are small spikes of all of these incidents of behavioral change during a new moon, which indicates that illumination is not the sole cause.

Rodent activity decreases as nights become brighter, and peaks on nights with 0% illumination and cloud cover. I even conducted my own study of this when I was evaluating the Sig Echo3 Thermal reflex sight. Each night when I went into the loft of my cabin to go to bed, I opened the window and scanned a one-acre area I staked out that contained tall grass adjacent to the building. I wrote down the percentage of illumination that night, the cloud cover, and then tallied how many mice I could see in the thermal optic. After four months the results became obvious, the darker the night, the more mice were out. The maximum distance I was able to see a mouse was around 100 yards, which was outside of my one-acre square anyways but is pretty impressive for the optic.

The number of croaks a frog makes or chirps a cricket produces also increases with darkness. Predators hunt and move more on darker nights. When we begin thinking about an animal’s life from their perspective, it makes sense that their rhythms would change based on these conditions. Have you ever noticed that some animals’ eyes glow when hit by artificial light at night? This is because they have a reflective membrane in the back of their eye that looks a bit like tin foil which catches and reflects into the pupil the maximum amount of light available. It lets them see in the dark. I’ve heard many hunters say that animals don’t feed at night during a new moon because they can’t see, and this simply doesn’t hold up to the animal’s actual visual acuity. 

Taken from a deer’s perspective, some of their predators hunt primarily by sight (like felines) or by a combination of sight and scent (like canines) and all of their predators hunt by sound. In dry climates, ungulates like deer and elk, are more likely to feed at dawn and dusk when the wind is switching directions so they can be aware of predators on multiple sides of them and because relative humidity increases during those times which makes the dry plant matter more digestible. Any time that prey species are active, predators will be too. Prey animals get to choose when they are active and do so at times that give themselves the greatest advantage against predators. They also choose their resting locations in places that give them good escape routes and early warning against the approach, which is why it is full-bars not advisable to hunt bedding areas. Moon phase and illumination absolutely play into this dynamic. Even though deer can’t see as well during a new moon as they can during a full moon, it is to their advantage to use these dark times for movement which is when they are most vulnerable. 

We killed this terrific bull during a waxing quarter moon this fall. For more breakdown on hunting check out

What does this mean to me as a hunter? If there was only one day of the month I could hunt, it would be the waxing quarter. There is 50% illumination, the moon rises at around 1500 and sets around 2300. I have had lots of “luck” during this moon phase on multiple continents and for multiple species including fish, birds, and big game. For my money, the waxing quarter is a balance of the good and bad that comes with the lunar cycle. Half the night is half lit, the other half all dark. As a guide, I hunt every day of every season and don’t get to pick the weather, moon phase, or much of anything. That has given me perspective on how behavior and timing change. This evidence is anecdotal, meaning it fits firmly into the “old wive’s tale” category at best and superstition at worst. “Let me make the superstitions of a nation and I care not who makes its laws…” Mark Twain

The bottom line is that the timing of the rise and set of the moon, and its effect on our oceans, our wildlife, and ourselves is real, but hard to explain. I don’t believe any moon phase is a good reason to not go hunting. But I firmly believe that taking into account the dynamic relationship between predators and prey species and how light affects that relationship will help you make better decisions about how you hunt when you are able to go. We don’t understand all these factors yet, and we need to keep that spark of wonder and curiosity alive so that we can better understand them in the future. After all, “There are more things in heaven and Earth, Horatio, than are dreamt of in your philosophy.” Shakespeare, Hamlet

I want to hear from you. Is moon phase-based chronobiology hocus pocus or hard science? Do you have a favorite moon phase? Leave a comment below and let’s talk about it.