Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
The brightest object in the night sky, our Moon is an unmistakable sight.
The brightest object in the night sky, our Moon is an unmistakable sight.
The Moon, in a crescent phase, can be seen setting with the Statue of Liberty in the foreground. The Moon’s phases change from new to crescent to half-full to gibbous to full to gibbous to half-full to crescent to new again periodically every 29.53 days. (Tayfun Coskun/Anadolu Agency via Getty Images)
Beyond its skyward motion and changing phases, naked-eye lunar observations yield tremendous scientific knowledge.
As seen from the northern hemisphere versus the southern hemisphere throughout its various phases, the Moon will appear to be oriented differently owing to the Earth’s spherical shape. The perspectives of observers at different latitudes ensure that the Moon will be tilted relative to the horizon and the sky, as dictated by a person’s orientation on Earth. (OPEN UNIVERSITY / CREATIVE COMMONS)
1.) The Earth is round.
Depending on an observer’s latitude and perspective, they will see the Moon oriented differently, as their horizon and orientation is determined by their position on a round Earth. Someone at the north pole would see the Moon flipped by 180 degrees from someone at the south pole. The diagram is not to scale; the Moon is much farther away than illustrated. (KELVIN CASE / PUBLIC DOMAIN)
Arctic versus Antarctic perspectives are completely flipped, with latitudinal variations indicating Earth’s shape.
By looking at the curvature of the Earth’s shadow that falls on the Moon, we can reconstruct the relative size of the Moon versus the Earth’s shadow-cone, allowing us to geometrically reconstruct the Earth-Moon distance. The Earth’s shadow falling on the Moon teaches us that our planet is more than 3 but less than 4 times the diameter of the Moon, and spheroidal in shape. (FRED ESPENAK / MRECLIPSE.COM)
Additionally, the Earth’s shadow during lunar eclipses reveals our planet’s spheroidal nature.
Compared to an apogee (most distant) Moon, a perigee (closest) full Moon can be approximately 14% larger and 30% brighter. The Moon’s apparent size not only changes from full Moon to full Moon, but throughout the lunar month as it travels in an elliptical orbit around Earth. (TOMRUEN / WIKIMEDIA COMMONS)
2.) The Moon’s orbit is elliptical, not circular.
Before we understood how the law of gravity worked, we were able to establish than any object in orbit around another obeyed Kepler’s second law: it traced out equal areas in equal amounts of time, indicating that it must move more slowly when it’s farther away and more quickly when it’s closer. This effect was clearly visible for the Moon since antiquity, as the smaller angular size and slower speed near apogee and the larger angular size and faster speed near perigee is evident. (RJHALL / PAINT SHOP PRO)
Changes in the Moon’s apparent size indicate large variations in its distance from Earth.
The cycle from new Moon to full Moon to new Moon again coincides with increases and decreases in apparent size as the Moon moves along its elliptical orbit. Because it moves faster at perigee and slower at apogee, but has a constant rate of rotation, we see slightly more than 50% of the Moon over the course of a lunar month: this is the phenomenon of lunar libration. (WIKIMEDIA COMMONS USER TOMRUEN.)
As seen from Earth, a less-than-full Moon will have a portion of its face illuminated by reflected sunshine, but the remainder of the Moon isn’t fully dark. Instead, it’s lit up by Earthshine: the reflected sunlight from Earth that falls on the Moon. By observing the Earth-illuminated portion of the Moon, we can determine the reflectivity of the Earth: known as the Earth’s albedo. (Alan Dyer/VWPics/Universal Images Group via Getty Images)
3.) How reflective is the Earth?
The brightness of the portion of the Moon not directly lit by the Sun, but instead illuminated by Earthshine, will change over time, dependent on how reflective the Earth is, which is dependent on a number of factors, including cloud cover, ice cover, the time of day and the Earth’s rotation, and even the seasons. (Frederic Larson/San Francisco Chronicle via Getty Images)
The unlit part of the Moon is brightened by Earthshine: sunlight reflected from Earth.
This amateur photograph shows a crescent Moon in detail: where a portion of the Moon is illuminated by the Sun, where craters are particularly visible along the terminator (the line between night-and-day), and the remainder of the Moon is dimly illuminated by reflected sunlight from Earth: Earthshine. (ROB PETTENGILL / FLICKR)
The Moon as seen from a view above the majority of Earth’s atmosphere. Earthshine illuminates the majority of the Moon, which has only a tiny sliver lit up by the Sun. The light from the Moon passes through the Earth’s atmosphere a little bit, with the atmospheric color slightly affecting the camera’s view. (NASA)
4.) Earth’s atmosphere bends red light more than blue.
When observed very close to the horizon, light from the Moon must pass through the maximum amount of Earth’s atmosphere. The atmosphere preferentially scatters blue light away while allowing red light to pass through more easily, resulting in a redder appearance near the horizon. (Gary Hershorn/Getty Images)
During moonset/moonrise, the Moon appears redder, as blue light is scattered away.
As the Moon appears farther away from the horizon, its light passes through less of Earth’s atmosphere before reaching our eyes, causing its color to appear truer to its original color: white, that of reflected sunlight. The closer the Moon is to your apparent horizon, the redder it will appear. (STEVEN SCHIMMRICH / HUDSON VALLEY GEOLOGIST)
That red light is bent, meanwhile, preferentially illuminating the Moon during lunar eclipses.
During most total lunar eclipses, a partial eclipse is followed by a dark red taking over the Moon from one side, with one limb always remaining brighter and whiter than the other. If the Moon passes through the direct center of the Earth’s shadow, it may appear to be uniformly red and dim, but more blue light will fall on the Moon the closer the side of the Moon is to the end of Earth’s shadow cone. (KAZUHIRO NOGI/AFP/GETTY IMAGES)
5.) The Moon has mountains, valleys, and high crater walls.
As the Moon blocks out nearly all of the Sun, the deepest craters continue to let sunlight through, giving the effect known as Baily’s Beads. The amount of time that these beads are visible, along with their intensity, enables us to infer the heights and depths of crater walls and valleys during solar eclipses. (PHIL HART / HTTP://PHILHART.COM/CONTENT/SOLAR-ECLIPSE-QUEENSLAND-14TH-NOVEMBER-2012)
When the Moon’s shadow falls on the Earth, as it did during this 1999 total solar eclipse, its entire shadow can be seen from the right perspective. Contrary to the expectations of many, the Moon’s shadow won’t be perfectly spherical, but will be elongated and irregular due to geometric alignments and the cratered and mountainous terrain found on the Moon. (MIR / ROSCOSMOS)
The lunar shadow’s irregular shape on Earth, during total eclipses, reveals the heights of crater walls.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
