Below is a short summary and detailed review of this video written by FutureFactual:
Why Eclipses Don’t Happen Every Month: Ferguson’s 18th-Century Explanation of Moon-Earth-Sun Alignment
This video explains why solar eclipses don’t occur every month despite the Moon’s monthly orbit around Earth. It summarizes James Ferguson’s 1757 explanation that the Moon’s orbit is tilted about five degrees to the ecliptic, so most new Moons do not align with the Sun and Earth. Eclipses only happen when the Sun, Moon, and Earth line up near the Moon’s two nodes—points where the Moon’s orbit crosses the ecliptic. The video highlights a 17-degree allowance around the nodes, and notes that the Sun only passes the nodes twice a year, limiting eclipse frequency. It also acknowledges NASA’s educational support for eclipses and the corona visible during total eclipses.
Overview
In this concise explainer, the presenter revisits a classic astronomy question: why don’t we experience a solar eclipse with every new Moon? The answer lies in orbital geometry. The Moon’s orbit is inclined about five degrees to the ecliptic, so most new Moons miss the Sun-Earth line. The video traces this idea to a 1757 treatment by James Ferguson, whose illustrations help convey how alignment matters for eclipses to occur.
"If the moon be then new, her shadow falls upon the earth."
Moon-Earth-Sun Geometry and the Ecliptic
The Moon’s tilt means its shadow rarely reaches Earth. Only when the Sun, Moon, and Earth are in a straight line near specific points can an eclipse happen. The video uses Ferguson’s explanation to show that eclipse conditions require precise geometry rather than a monthly inevitability.
Nodes and Eclipse Conditions
A central concept is the Moon’s two nodes, where its orbit crosses the ecliptic. The transcript states the key fact clearly: "The moon's orbit intersects the ecliptic in two opposite points called the moon's nodes." When these nodes align with the Sun at new or full Moon, shadows can fall upon Earth or be cast into space, producing an eclipse. The video additionally notes the 17-degree window around the nodes that governs whether an eclipse is possible.
"When the moon is less than 17 degrees from either node at the time of conjunction, her shadow falls upon the earth."
Eclipse Frequency and Learning Resources
Because the Sun only crosses the nodes about twice a year, many new Moons do not produce eclipses. The Moon-Earth-Sun geometry thus explains why eclipses are relatively rare. The video also highlights NASA HEAT and its mission to engage learners with solar eclipses, emphasizing the corona visible during total eclipses as a striking example of the Sun's outer layers.
"Solar eclipses are a great way to experience a different side of the sun than we normally do, like literally the corona extending out from its sides."