A barely noticeable size difference
It very often happens that the Moon is closer to us because of its “elliptical trajectory”explains Éric Lagadec on the social network X. “Sometimes it’s closer to us and sometimes it happens on a full moon day.”says the astrophysicist at the Côte d’Azur Observatory, who assures us that this happens regularly. “She’s barely closerassures the scientist. Which means that the size difference (about 10%) is not noticeable.” The term “supermoon” is used when the satellite is at its shortest distance from Earth. But this expression has nothing scientific about it. It “comes from astrology”says Eric Lagadec, who rather defends the term “périgée syzygie”, less attractive certainly. We speak of apogee when the Moon is furthest from the Earth.
On the other hand, the magnifying glass effect could appear impressive at sunrise and sunset. “It’s an optical illusion because it passes in front of trees and buildings, and through the layers of the atmosphere, with a refraction effect.”explained Florent Deleflie, astronomer at the Paris Observatory, to West France last year.
A “blue moon” that is not blue
Attention, new disappointment: the “blue moon” will not be blue. Anglo-Saxon scientists use this term to designate the third full moon when four full periods occur in the same season. It is also used “when there are two full moons in the same month”, explains Eric Lagadec. This was the case in August last year with a full moon on August 1st and then a second on the 31st. This phenomenon occurs on average every two and a half years. But the definition does not correspond to the Moon that we will observe this Monday since it is August 19 and it is still happening “about 29 days between two full moons.” So there is no “nothing special from an astronomical point of view.”
So what color will the moon be this Monday night? In reality, “when the moon rises, it will be yellow/orange”, specifies Eric Lagadec. And it won’t be any brighter either: “A supermoon will not be brighter overall than a regular full moon.”according to Florent Deleflie.
Full moons less interesting for observing the satellite
For astronomy specialists, “super blue moon” episodes are rather uninteresting. “I prefer to observe the Moon when it is not full, I love to point my telescope at the terminator“, that is to say the line which separates the dark part from the light part, says Éric Lagadec. “The light is low, like when the Sun rises or sets on Earth: the shadows become larger, we can clearly see the reliefs.” “The shadows offer much more precise details […] during the first and last quarter periods”, also affirms Florent Deleflie, astronomer of the Paris Observatory. If the “super blue moon” (or rather “perigee syzygy”) remains unspectacular, it is a good “way to get people into astronomy, he concedes. Regardless, seeing a moon rise is still a magnificent event.”