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"Super-Lune of the century": what to know by this evening
& Quot; Super-moon of the & quot; : What to know by this eveningThe full moon 
seen from the International Space Station (NASA)
The full moon of Monday 14 November will be the biggest since 1948. Explanations 
to understand the phenomenon.
Jean-Paul FritzJean-Paul FritzPublished on November 13, 2016 at 17h01
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Why is not the full moon always the same size? Depending on months and years, the 
apparent diameter of the Moon varies, and those who like to raise their eyes 
to the sky have noticed it.
The difference is important: the biggest full moon has an apparent diameter 14% 
larger and a brightness 30% more intense than the smallest full moon.
Compared to a full "average" moon, the difference will be 7% in diameter, and 16% in 
brightness. Enough to change a landscape for a photographer, or illuminate a 
beautiful starry night. Or, in a tiny way, increase the height of the tides, 
perhaps one or two centimeters ...
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Apparent size difference between the moon at the apogee (farthest, right) and perigee 
(nearest, left). (Galileo Project, Nasa)
So what's going on? First, the well-known phenomenon of the phases of the Moon 
depends on an alignment between the Sun, the Earth and the Moon. When the 
latter is on the same axis as the Sun, one is entitled to a full moon or a 
new moon. When it is perpendicular, it is the first or last quarter. Between 
two full moons, there is a lunar "month" of 29 days and a half on average.
Phases of the Moon. Sizes and distances are not to scale. (Editing from Nasa Pictures)
But the Moon revolves around the Earth, and its orbit is not a circle but an 
ellipse. So that the moon returns to the same point of this ellipse, it is 
necessary to wait a little more than 27 days. The difference with the phase 
cycle? The Earth also moves around the Sun and takes the Moon with it. The 
alignments that are the phases are therefore not repeated at the same place 
of the lunar orbit.
The lunar orbit is not a circle
An ellipse also means that the Moon will sometimes pass closer to the Earth, 
sometimes even further. The nearest point, the perigee, is about 360,000 
kilometers. The farthest point, the apogee, is at 405,000 kilometers. It is 
this difference that explains the variation in apparent size of the Moon seen 
from the Earth. When a full moon occurs in the vicinity of the perigee, it is 
called a "super-moon." And the biggest possible moon is when the moon passes 
through the perigee when it gets full.
The orbit of the Moon is elliptical, and its distance to the Earth varies. 
Distances and sizes are not to scale. (Editing from Nasa images)
But there are other factors to be taken into account. The axis of the Moon's orbit 
also moves, and performs a complete rotation in 3,233 days (just over eight 
years and ten months). This is called precession.
The axis of rotation of the Moon is also moving (Rfassbind via Wikimedia Commons)
It is also necessary to know that the Moon does not move at the same speed 
according to its position on its orbit, 
It is also necessary to know that the Moon does not move at the same speed 
according to its position on its orbit, as defined Kepler. The closer it is 
to Earth, the faster it goes.
Rendezvous Sunday and Monday
Depending on all these elements, it becomes rare for the Moon to be full when it passes 
exactly through its closest point to the Earth. The phenomenon is not very 
regular, and that is why the Moon has not been so close to its perigee since 
1948, and that it will be necessary until November 25, 2034 to have a passage 
so close.
This "super-moon of the century", which will occur just two hours from the passage 
to the perigee, takes place at 14:52 Monday ... What prepares us at least two 
beautiful nights full moon.
The best time to watch the moon is at sunrise or sunset. Because of an optical 
illusion whose cause is not completely elucidated, the Moon appears bigger, 
all the more so if you observe it above trees or constructions.
In this case, this interesting moment will be the setting of the moon, which will 
occur in the early morning before sunrise (around 7:20 in Paris on the 14th, 
for example, but the show is also on Tuesday before dawn ).
Hopefully there will be no clouds from Sunday evening to Tuesday morning, to enjoy the 
view. To console himself, we shall have another super-moon, less fat, on the 
14th of December. But there, instead of offering us a beautiful spectacle, it 
will spoil it: the lunar luminosity will make it much more difficult to 
observe the swarm of shooting stars of the Geminids.
Jean-Paul Fritz

   

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