First came the Super moon. Then came the red moon, the rose, the blood moon, the blood moon superblue, the blue red moon Super moonthe extra Super moon and the super bright, to name a few. I am a fervent supporter of the non-proliferation of useless adjectives when we talk about astronomical events. First, because they are not necessary and, second, because they are confusing. So, from here, I make a humble appeal for calm. Let’s strip our satellite of epithets because, what at first glance may seem a mere description of qualities, actually hides a more complex phenomenon that has its roots in the fascination that popular beliefs continue to exert in our societies, and in the culture of click promoted by some media.
I am very sorry to disappoint the reader, but the full moon that will take place tonight will not have any special color associated with it. Why then do you insist on calling it a blue moon? Well, by tradition and, moreover, wrong. A seasonal Blue Moon is an old definition and refers to the third full moon in a season that has four full moons. Hence the origin of the popular expression, “once in a blue moon” as something that happens infrequently. According to this definition, the blue moon would correspond to the full moon that occurred on August 1 and not to the one on the night of August 30.
The concept arises from the old English almanacs that gave a poetic name to each full moon of the year: since there are twelve full moons in most years and four seasons, we have three full moons per season. But every ten seasons we have one with four full moons (we’ll see why later) and the third one that occurs in that season is called a “blue moon”. A misinterpretation of this definition is being applied to tonight’s full moon, August 30, and the second full moon of the same calendar month is being called a “blue moon.”
Let’s see then what is Super moon. To continue with my condition of killjoy, I must say that there is nothing special about it either. At least for a non-bionic human eye (meaning one that doesn’t have a professional telescope attached to the cornea), the Super moon it is a phenomenon indistinguishable to the naked eye.
The orbit of our satellite around the Earth is not a circle, it is an ellipse. That means there is a point in the orbit, called the perigee, at which it is a little closer to us. When the perigee occurs on a full moon they call it Super moon and that’s what happens tonight. In this case, the term was coined by an astrologer (please do not confuse with astronomer or astrophysicist) and we all know that astrologers do not live on air. And although we also know that neither are astronomers, at least the latter do not feed on the proliferation of beliefs without any solid basis that attribute influences on humans to the movement of the stars.
The perigee point (let’s say the short distance) of the lunar orbit moves due to the effect of solar gravity and the combined gravity of all the planets in the solar system. This means that if we draw an imaginary line that joins the Earth and the Moon at their closest and furthest points (known as the line of apses), we would see how that line rotates in space and makes a complete turn every 8 .85 years.
Our satellite moves around the Earth in its orbit and therefore it is illuminated differently by the Sun from our point of observation. In its full phase it is in the opposite direction to the Sun, and we see it complete. The lunar month, or synodic month, is the time it takes the Moon to complete a complete cycle of phases, and it lasts 29.53 days. That is the interval in which the full moon and the new moon occur, 29.53 days and that is why every so often two full moons coincide in a month of our calendar.
When the perigee point and the full moon coincide, they call it Super moon. And that is why headlines appear stating that tonight’s phenomenon is rare, because the line of apses and the phase of the full moon coincide. I understand that it is easier to call Super moon, but I find the knowledge behind describing and predicting its movement infinitely more fascinating, even if it is a little more complicated. Remember that the term Super moon it is not recognized by the International Astronomical Union, the body responsible for coining the names of celestial objects.
We have been attributing supernatural powers to our satellite for millennia and, like a layer of dust that has accumulated in an abandoned house, it is difficult for us to shake off these old beliefs. But with knowledge we have removed fear, we have stepped on the Moon and we will do it again soon. It is time to dust yourself off and enjoy the spectacle of the Moon tonight: a simple full moon.
Eva Villaver ands Research Professor at the Instituto de Astrofísica de Canarias and Director of the Office of Space and Society, Spanish Space Agency
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