The final full moon of 2017 as well as the first two of 2018 are called “perigeean,” the full moon taking place within 24 hours of the transit of the Moon to the perigee of its orbit, meaning its closest point to the Earth. The expression “supermoon” is a creation of astronomers – and popularized by the media – to mark this feature involving a phase of the Moon and the distance of the Moon from the Earth.
A bigger lunar disc?
The expression “supermoon” implies that the lunar disc on this occasion appears to be larger than usual. CAREFUL! The situation is more complex. The Moon’s orbit is eccentric, not circular, and as a result its distance from the Earth changes constantly. That distance ranges from 356,700 km, at the Moon’s perigee, to 406,300 km at its apogee, with an average distance of 384,400 km.
For an observer on Earth, that translates to an apparent lunar-disc size of 31 minutes of arc on average and to an apparent size of 33 arc minutes when the full moon occurs at the perigee of its orbit. This difference of roughly six to seven percent cannot be observed with the naked eye.
Even bigger in January
The Moon is full on December 3 at 10:47 a.m. EST and reaches its perigee (357,486 km) at 3:52 a.m. on December 4, in other words seventeen hours and five minutes later. The biggest full moon of 2017 will be December’s. Nevertheless, the full moon on January 1, 2018 will be even more “important,” the Moon reaching its perigee less than five hours before becoming a full moon.
Enjoy the show
The full moon on December 3 rises at 5:44 p.m. in Montréal. A reddened disc deformed by the Earth’s atmosphere, it will be impossible not to notice the excessive size of that disc when it rises on the horizon. But it’s an illusion, a misinterpretation by the brain.
To see for yourself, observe the Moon again in evening when it’s higher in the sky. It will seem smaller, and yet it will have moved closer to the Earth since rising – don’t forget that it will reach its perigee around four in the morning on December 4!
A good reason all the same to observe the Moon and the starry sky.