There has been much discussion in the popular press about the so-called supermoon this past weekend. While the phenomenon itself has been around for a very long time – billions of years, in fact – the term supermoon has only been around for a few decades, and the popular attention currently being paid to the phenomenon is all quite recent.
The entire concept of a supermoon arises from the fact that the moon's orbit around the Earth is not a perfect circle, but rather is an ellipse. Every month, the moon passes through both its closest point to the Earth, called perigee, and its farthest point from the Earth, called apogee. When the moon's phases are aligned such that a full moon occurs near the time of perigee, it appears larger than other full moons might appear, and thus such events can be called a supermoon. Strictly speaking, the term supermoon can also apply to the occasions when a new moon occurs near perigee, but since the moon is not visible at those times these events don't get the publicity that the full moon or supermoons get.
This past weekend, the moon was at perigee shortly before noon MDT Aug. 10, and the precise instant of full moon was only half an hour later, thus it would qualify as a supermoon. Last month's full moon on July 12 and next month's full moon Sept. 9 also occur pretty close to perigee, and thus they are supermoons as well. Interestingly, next month's supermoon also coincides with the annual Harvest Moon phenomenon, which, due to the orientation of the moon's orbit with respect to the Earth's orbit, the near-full moon rises only a few minutes later each evening around the time of the autumn equinox.
From an astronomical perspective, however, the entire concept of a supermoon is rather trivial. While the moon's orbit around the Earth is indeed elliptical, it is only slightly elliptical; if it were to be drawn accurately to scale on a sheet of paper, it would be indistinguishable from a circle. The moon's average distance from Earth is 239,070 miles; at perigee it is roughly 225,300 miles — a difference of less than 6 percent — and at apogee it is roughly 251,900 miles — again, a distance of less than 6 percent — indeed, the difference between perigee and apogee is less than 12 percent. Last weekend's supermoon appeared only 8 percent larger than the average full moon, and only 15 percent larger than a full moon at apogee. While the difference might be detectable if the respective full moons could be viewed side-by-side, in reality the unaided eye cannot detect any real difference between a supermoon and any other full moon.
This entire discussion recalls the fact that the orbits of all objects in our solar system — and elsewhere in the galaxy and the universe, for that matter — are ellipses. The sole exceptions are some long-period comets that travel in open-ended orbits called parabolas or hyperbolas, but since these are in fact open-ended the comets in question will never return to the inner solar system. Earth's orbit around the sun in indeed elliptical, but is even less elliptical than the moon's orbit: the difference in distance between Earth's closest point to the sun's perihelion, and its farthest point from the sun's aphelion, is only a little over 3 percent. Although this seems somewhat counter-intuitive for those of us in the northern hemisphere, Earth is actually at perihelion in early January each year, and at aphelion in early July.
The idea that the planets' — and other bodies' — orbits are ellipses and not circles is a rather recent development. Ever since the first views of the nature of the solar system were being developed a couple of thousand or more years ago — which primarily held that the Earth was the center of the solar system, and indeed of the universe as it was perceived at that time — the various planets' orbits were all considered to be perfect circles. The rationale was that, since the heavens are perfect, the planets' orbits must likewise be perfect. The idea that the planets' orbits might be anything other than circles was never even considered. Even when the Polish astronomer Nicolaus Copernicus introduced his sun-centered view of the solar system in the mid-16th Century, he still considered the planets' orbits around the sun to be circles.
In the early 17th Century the German mathematician Johannes Kepler was trying to calculate the orbits of the various planets from the positional measurements that his mentor, the Dutch astronomer Tycho Brahe, had made of the planets. Brahe, curiously, still believed in the Earth-centered view of the solar system when he was making these measurements. Kepler found that Brahe's measurements, especially those of Mars, could not be reconciled with the assumption of a circular orbit, and made the outside-the-box leap that the orbits are in fact ellipses — which did fit the data. Mars' orbit is indeed moderately elliptical, the difference in distance between its perihelion and aphelion points being over 20 percent.
This determination that the planets' orbits around the sun are ellipses became the first of what are now known as "Kepler's Laws of Planetary Motion." There are two additional such laws, these being more mathematical descriptions of various parameters of the orbits, and as with the first law Kepler determined these empirically from Brahe's positional measurements. Kepler's Laws can be derived mathematically from the "Law of Universal Gravitation" that the British physicist Isaac Newton developed near the end of the 16th Century.
The supermoons that are getting all the publicity — and which occur at least once per year — are accordingly just the result of naturally occurring phenomena that take place all over the universe and that have been going on since the universe's beginning. Nothing, really, to get excited about .
Alan Hale is a professional astronomer who resides in Cloudcroft. He is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His web site is http://earthriseinstitute.org.