In San Francisco, while Thursday, Friday and Saturday nights were disappointing for astronomers, the cloud layer beneath the big moon made for some photo fun. Friday’s full moon coincided with its periapsis, the point in the moon’s orbit when it is closest to the earth, a combination that happens about every year and a half. This perigee full moon was the biggest moon – as seen from earth anyway – that we’ll see this year. It was 14% larger in diameter and therefore about 30% greater in area and brightness than the apogee full moon, occurring when the moon is at its farthest from the earth. For the angularly inclined, the perigee full moon occupied about 33.9 minutes of arc while the apogee is about 29.9 minutes.
Apparently, native Americans – at least the Algonquins – called the January full moon a Wolf Moon. This had nothing to do with the more popular connection between full moons and the lycanthropes known as werewolves. Our eagerness to associate a full moon with madness and other weird stuff no doubt stems from folklore and ancient myths, reinforced by the statistically and mentally challenged news media. For example we all know that suicide, homicide, problems in psychiatric wards, sleepwalking, birth rates and fertility are dramatically affected by a full moon. Except none of it is true. Or at least the moon’s effect on these things cannot be detected by analysis of an embarrassing wealth of available statistical data.
A reason commonly given for why the full moon could impact behavior is that we all have mass and the moon’s gravity exerts a force on us. However the gravitational effect of the perigee moon would be the driver, not the full moon. There is a statistical correlation between extreme weather and the perigee moons, even perigee new and quarter moons. In other words, the moon’s tidal force depends on its distance from earth, not its phase. While the period between full moons (synodic period) is 29.5 days, the time between perigees is only 27.5 days. Higher tides occur during full moons, but also during new moons. This is not because of the gravitational force of the moon, but because the tidal force of the sun lines up with that of the moon at such times.
Well, how about the female menstrual cycle – that correlation with full moons is obvious right? Not really. Human cycles average 28 days, but chimpanzees average 35. The moon’s synodic period is 29.5 days.
Why, you might ask, do we modern intelligent folk seem to notice so many oddities of behavior on a full moon. Owen, my Psych friend, calls this cognitive bias with communal reinforcement. We didn’t know nuthin bout that back home, but we used to say that you always find a thing in the last place you look for it. A bit earlier, Aristotle’s take on the matter was that to give a fair decision it is necessary to be rather an arbitrator than a party to the dispute.
Full moons cause one thing for certain – photography.
