I think that a full Moon is one of nature’s most beautiful events, and yet it is one of those naked-eye phenomena that seems to escape the notice of most of the population, probably because it happens so regularly.
Last night, if you were lucky enough to have clear skies, you would have seen the full Moon at its biggest and brightest. It appeared up to 14% bigger and 30% brighter because it is at the point in its orbit when it is closest to the Earth. Usually, the Moon is approximately 384,000km away from us, but last night it was only 356,400km away.
This phenomenon is known as a perigee full Moon, and it happens only about once every two or three years. Perigee is derived from two Greek words; the prefix “peri” meaning close, and the suffix “gee” is derived from Gaea, meaning Earth. Hence the perigee full Moon is closest to Earth. It is more traditionally known as the Flower Moon, because of the seasonal connections.
The reason for this variation in distance is because the Moon has an elliptical orbit around the Earth.
Elliptical orbit of the Moon (not to scale).
An elliptical orbit is different from a circular one in that it follows the shape of an ellipse. This means that it is longer than it is wide, i.e. a stretched out circle. Yesterday, the moon was at a point in its orbit where it was at its closest to the Earth. This point is called the perigee. When the moon is at its furthest away from the Earth, it is at the apogee (see my pretty picture above).
Astronomers use the term eccentricity to describe how elliptical an orbit is, or how stretched out the circle is. The Moon’s orbit has an eccentricity of 0.0546. This, compared to Pluto’s eccentricity of 0.2488 is fairly small, but it is still substantial. The eccentricity of the Moon’s orbit can be further increased by the tidal effects of the Sun’s gravitational pull when the Moon is either full or new. At this point, the major axis of the Moon’s orbit is aligned with the Sun and Earth. The major axis of an ellipse is the distance between the furthest points on the ellipse (see the picture above).
It is the combined effect of the Sun’s gravitational pull and the eccentricity of the orbit that causes the apparent difference in size and brightness of the Moon.
The Moon doesn’t create its own light; it only reflects light from the Sun. The Moon may seem bright in our night skies, but it actually only reflects about 7% of the sunlight that strikes it! The brightness of the Moon is determined by two things: how intense the light from the Sun is, and how far the reflected light has to travel between the Moon and the Earth. So when the Moon is closest to our planet, it is also closer to the Sun. This means that the reflected light has a higher intensity and a shorter distance to travel, making it appear brighter than when it is further away.
As our closest celestial body, the Moon has a significant effect on our tides. We are all aware of the gravitational force that keeps our feet firmly on the ground and the planets orbiting around the Sun. When the Sun and Moon combine their gravitational pull on Earth, the water in our oceans is “bulged” outwards, making it appear slightly elliptical. This bulge appears at high tide. Tides are at their strongest when the Earth, Sun and Moon are all lined up. These are called Spring Tides. Although the Sun is much larger than our Moon, its effects on our tides are much smaller as it is so much further away. These high tides are not the same every month because the Moon moves closer and further away from the Earth. At perigee, when the full Moon is closest, the gravitational pull will be greatest, creating higher tides in our oceans.
Spring tides, when the Sun, Moon and Earth are all lined up (not to scale).
So, if you are fortunate enough to have had clear skies last night, and you live by the beach, you will have the best of both night-time and day-time wonders of the Supermoon: a beautiful night-light and fantastic waves to surf on! I’m not jealous in the slightest.