The term “Super Moon” appears in the popular press to describe the full moon when it appears the largest. This occurs when the moon is at its closest to the earth (Perigee). The term gives the impression this is an extraordinary or even catastrophic event. Exaggerated media reports often predict flooding, earth quakes and even volcanic eruptions. Most of which of course are false. It is true that that when the moon is at its closest to the earth the tides can be somewhat higher and an alignment of the sun, earth and moon does have a measurable affect – just not extreme. And if this occurs in the spring, during a spring tide (not the same thing), then the tides can actually be a concern.
The opposite of the “Super Moon” is the “Micro Moon” which describes the smallest full moon. This occurs when the moon is at its furthest from the earth (Apogee).
The difference between the super moon and micro moon is not likely to be noticeable when viewing the moon in the evening sky. But the side by side comparison below of the super moon taken 2014-09-08 and the micro moon taken on 2015-03-05 shows just how different they are!
The Perigee Full Moon can appear as large as 34.1 arc-minutes (apparent angular size). While the Apogee Full Moon can appear as small as 29.3 arc-minutes (Wikipedia). That’s about a 15% difference. The difference in apparent size is a result of the moon being different distances from the earth.
The moon orbits the earth in an elliptical path (an oval) at an average distance of 385,000km. At its closed point – perigee – the moon is 362,600km from the earth. At its furthest point – apogee – the moon is 405,400 km away from the earth. That’s an 11% difference! The shape of the orbit is constant, so once a lunar sidereal month (360° rotation) of approximately 27.3days, the moon is at its closest to the earth, and about 2 weeks later, it is at its furthest. (It takes 29.5 days for the moon to complete a synodic orbit which returns the moon to the same orientation relative to the sun and earth – e.g. new moon or full moon.)
The phase of the moon – the part we see illuminated – depends on the alignment of the sun, earth and moon. As the moon orbits the earth, we see a different portion of the sunlight part and the part in shadow (although we always see the same side of the moon). For example, when we see the waxing crescent moon, the moon is between us and the sun and a little to the left of the sun from the perspective of earth’s northern hemisphere. So the part in sunlight is facing mostly away from us and we see only a small sliver of that. The rest of the moon’s face we see is mostly in shadow.
As the moon orbits the earth, we see a portion of the sunlit part from a different angle. So we see different phases at different points in the lunar orbit.
A full moon occurs when the sun, earth and moon are aligned (in that order) – referred to as “opposition”. A new moon on the other hand occurs when the moon is between the sun and the earth – referred to as “conjunction”. Although aligned when viewed from above, when viewed from the side, the full moon or new moon is usually above or below the earth-sun plane (the ecliptic). This is because the moon’s orbit is tilted compared to the earth-sun orbit by about 5.1°. When the side view also lines up, we get a solar or lunar eclipse.
An “ordinary” full moon can happen at any point around the moon’s elliptical orbit, so the earth-moon distance can be anything between the closest and furthest distance. In the diagram above, the full moon is a little past perigee and so not at its closest. Occasionally the full moon occurs when the moon is at perigee (closest). Because it’s at its closest, it appears larger than other full moons that occur at other points in the lunar orbit. This is what’s called the “Super Moon”!
But the moon is going to be at perigee at some point in every lunar month. For example, from the “Lunar Phases” diagram above we see the waxing gibbous moon is at perigee and therefore would appear larger than other gibbous moon views. What makes the Perigee Full Moon a “Super Moon” is that the sun-earth and moon are aligned so the combined effect of their gravity is also at a maximum. The affect is measurable and at some times of the year can result in significant, but still modest increase in tides. Hence the term “Super Moon” which is chosen to evoke a sense of awe and unfortunately panic.
The compliment to a Perigee Full Moon is the Perigee New Moon. Like the full moon, occasionally the new moon occurs when the moon is at perigee (closest).
Also like the Perigee Full Moon, the sun-earth and moon are aligned for a Perigee New Moon, so the combined effects of their gravity is also at a maximum. Since the moon and sun are on the same side during a new moon, their gravitational affects on the earth add together. I would expect any noticeable affects to be larger than the equivalent Perigee Full Moon. The term “Super Moon” then also applies to the Perigee New Moon, but because the new moon is directly in line with the sun, we can’t see it, and hence goes unnoticed and generally unreported.
The alignment of a full moon at perigee occurs about every 14 full moons. Relaxing the definition of “Super Moon” a little to occurring close to perigee, we can get a perigee new moon before and after a perigee full moon. (Or conversely a perigee full moon before and after a perigee new moon.) This means it’s possible to get 3 “Super Moons” in a calendar year; most times 2 (a full moon and a new moon) and rarely no “Super Moon” in a calendar year. (There are 12 and sometimes 13 full moons in a calendar year. )
On the other end of the scale, the new moon and full moon can occur when the moon is at apogee – furthest away. When this happens the moon will appear its smallest and the combined gravitational affects of the moon and sun will be at a minimum when compared to other new and full moon events. The complimentary term is sometimes “Micro Moon”.
When the new moon occurs at apogee, nothing really happens and since we can’t see it in the glare of the sun, it’s pretty much a non-event.
When the full moon occurs at apogee, again nothing happens, but we do get to see it. While it is 15% smaller than the perigee full moon, it’s not something that anyone would notice.
Our perception of the size of the full moon is influenced to a much greater extent by the foreground. For instance, people generally report that the full moon on the horizon is significantly larger than the same moon high overhead. Although in reality, they are the same angular size. There is no mysterious atmospheric affect at play, only our own perceptions.
The precise prediction of when a perigee or apogee moon will occur is a little complicated.
- it takes the moon about 27.3days to make a 360° trip around the earth
- but the earth has moved around the sun in that time so it takes about 29.5 days between full moons (or new moons)
- the earth takes 365-1/4 days to move around the sun changing the earth-sun alignment with the major axis of the moons elliptical orbit as it goes
- the lunar orbit precesses (the axis of the orbit rotates) once every 3232.6 days or about 8.85 Earth years which also changes the earth-sun alignment with the major axis of moons elliptical orbit
- so working out all these moving parts to find an alignment is complicated
- then there is the affects of the other planets on the orbits (perturbations) and tides which changes the shape of all the orbits and speed of rotation