Author: Jihun Oh
Subtitle: Why Moon phases, Earth’s seasons, and eclipses aren’t quite what they seem
Image credit: “Solar Corona of the 2024 Total Solar Eclipse” by Brucewaters (8 April 2024, Sherbrooke, Québec). CC BY 4.0; unmodified. Source: Wikimedia Commons. Licence: Creative Commons Attribution 4.0.
A first-quarter Moon looks half illuminated. A drawing of Earth orbiting the Sun can suggest that summer arrives when our planet is closest to it. A simple sketch of the Sun, Earth, and Moon in a straight line can seem to suggest an eclipse would happen every month. Each picture captures something real, but leaves out a detail needed to understand the system fully.
A simplified diagram is not necessarily wrong. It selects which relationships to show, but it does not explain them by itself. Readers interpret the image and draw conclusions; if they mistake a partial view for the whole system, they may reach the wrong conclusion. Three familiar astronomical diagrams show how overlooking an observer’s viewpoint, Earth’s axial tilt, or the Moon’s inclined orbit can change how we interpret what we are seeing.
1. A “quarter Moon” that looks half illuminated
Sunlight illuminates roughly half of the Moon’s surface at any given time, but from Earth we see only the portion of that sunlit half facing us. At first quarter, half of the Moon’s apparent disc is illuminated. The word “quarter” refers to the Moon’s progress through its phase cycle: first quarter occurs about a quarter of the way through the cycle, not when only one-quarter of the visible disc is illuminated.
As the Moon orbits Earth, our view of its illuminated half changes, thus producing the familiar phases. A phase chart can show the sequence of lunar phases, but without making the viewer’s perspective clear, it may not explain why the Moon appears differently from Earth as it moves around our planet.
An adapted version of NASA/JPL Education’s lamp-and-ball activity makes the geometry easier to see. Place a lamp on one side of a room to represent the Sun. One person stands in the middle of the room to represent an observer on Earth, while a partner slowly carries a white ball around them to represent the Moon as it orbits Earth. As the ball moves, the observer turns to keep looking at it. The lamp illuminates roughly half the ball, but the observer sees different portions of its illuminated surface as the ball moves around them. Hold the ball high enough that the observer’s body does not block the lamp’s light; otherwise, the demonstration may accidentally resemble an eclipse.
This demonstration illustrates the geometry of lunar phases, but it is not a scale model of the Earth–Moon system. In the demonstration, the ball moves close to the observer, whereas the real Moon is, on average, about 30 Earth diameters away. The model also does not represent the Moon’s actual size or orbital period—the time it takes to complete one orbit around Earth.
Nor does it account for Earth’s rotation or the Moon’s position in the sky at any time and place, so it cannot tell an observer where or when to look for the Moon. Its purpose is narrower: to demonstrate how the Moon’s position relative to Earth and the Sun determines the phase we see from our planet.
Moon phases depend on how the Moon’s illuminated half is oriented towards us. Earth’s seasons also depend on orientation, but this time, the important factor is the direction of our planet’s own tilted axis as it orbits the Sun.
2. Two hemispheres, one Earth–Sun distance
A simplified drawing of Earth’s orbit may suggest that summer occurs when Earth is closest to the Sun and winter when it is farthest away. In fact, Earth is closest to the Sun in early January and farthest away in early July. Because Earth is closer to the Sun in January, NASA estimates that Earth receives about 6.8% more incoming solar radiation than it does in July.
Although this difference in distance affects how much solar radiation Earth receives, it is only a secondary effect and cannot explain the seasons. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. Both hemispheres are therefore at essentially the same distance from the Sun at any given time.
The main cause of the seasons is Earth’s axial tilt.
Earth’s rotational axis—an imaginary line running through the North and South Poles—is tilted by about 23.4 degrees from a line perpendicular to its orbital plane (an imaginary flat surface that contains the path an object follows as it orbits another body). This angle is known as Earth’s axial tilt. As Earth travels around the Sun, its axis maintains approximately the same orientation in space. When the Northern Hemisphere (the half of Earth north of the equator) is tilted towards the Sun, sunlight strikes Earth’s surface more directly, concentrating the same amount of solar energy over a smaller area. The Sun also stays above the horizon longer, meaning the days are longer and the surface receives sunlight for more hours. Together, these effects contribute to warmer summer conditions. About six months later, the Southern Hemisphere is tilted towards the Sun and experiences summer instead.
A useful diagram of the seasons shows Earth at two opposite points in its orbit, with its rotational axis pointing in approximately the same direction in space. If the axis is instead drawn pointing towards the Sun at both positions, the diagram may suggest that Earth swivels to face the Sun as it travels around its orbit. In reality, Earth’s axis maintains nearly the same orientation, so each hemisphere takes its turn being tilted towards the Sun. The important clue is the direction of Earth’s tilted axis, not a seasonal change in Earth–Sun distance.
In the first two examples, understanding the diagrams depends on recognising the observer’s viewpoint and the direction of Earth’s tilted axis. Eclipses introduce another geometric detail: the Moon’s orbit is tilted relative to Earth’s orbital plane.
3. The five-degree tilt behind eclipses
A simplified overhead diagram can make the Sun, Earth, and Moon appear to line up at every new and full Moon. A solar eclipse can occur at new Moon, when the Moon passes between the Sun and Earth and its shadow falls on Earth’s surface. A lunar eclipse can occur at full Moon, when Earth lies between the Sun and Moon and the Moon passes through Earth’s shadow. If the three bodies line up, why do we not see an eclipse every month?
The missing clue is that the Moon’s orbital plane is tilted.
The Moon’s orbital plane is tilted, or, more precisely, inclined, by about five degrees relative to Earth’s orbital plane around the Sun (the ecliptic). This angle is called the Moon’s orbital inclination. Because the Moon’s orbital plane is tilted, the Moon usually passes slightly above or below the Sun–Earth line at new Moon, causing its shadow to miss Earth. At full Moon, the Moon usually passes above or below Earth’s shadow rather than through it.
The two orbital planes cross along a line. The two points where the Moon’s orbit intersects the plane of Earth’s orbit are called orbital nodes.
An eclipse becomes possible when a new or full Moon occurs sufficiently close to one of these nodes. A new Moon can produce a solar eclipse, while a full Moon can produce a lunar eclipse. Because the Sun appears to cross the line of these orbital nodes twice a year, eclipse seasons occur roughly six months apart. During an eclipse season, a new or full Moon may occur close enough to a node for an eclipse to take place. Outside these periods, the Moon usually passes too far above or below the alignment needed for an eclipse.
A useful eclipse diagram combines two views. An overhead view shows the order of the Sun, Earth, and Moon. A side view reveals the Moon’s inclined orbital plane and shows how the Moon can pass above or below Earth’s shadow. Marking the two orbital nodes shows where an eclipse may become possible. Without this information, a flat drawing can make it appear as though eclipses should happen far more often than they actually do.
Figure 1. Three clues that simplified astronomy diagrams can leave out: the observer’s viewpoint, Earth’s fixed axial direction, and the Moon’s inclined orbital plane and orbital nodes. Original illustration by Jihun Oh. Schematic only; not to scale. The Moon’s approximately five-degree orbital inclination is exaggerated for clarity.
Detailed description: Three panels illustrate Moon phases, Earth’s seasons and eclipse geometry. In the first, sunlight illuminates half of the Moon, while an eye marks the observer’s position on Earth. In the second, Earth appears at two positions on opposite sides of the Sun, with its rotational axis pointing in approximately the same direction during the Northern Hemisphere summer and winter. The third panel illustrates lunar and solar eclipse geometry through three views. View A shows Earth’s near-circular orbit around the Sun. View B is an enlarged extension of the Earth–Moon system shown in View A, showing the Moon’s inclined orbital path (shown edge-on) relative to the ecliptic plane (Earth’s orbital plane). The two orbital nodes mark the intersections between the Moon’s orbital plane and the ecliptic. Earth’s shadow extends directly away from the Sun. A full Moon positioned away from either node passes above or below Earth’s shadow, illustrating why a lunar eclipse does not occur every month. Similarly, a new Moon near a node can align between the Sun and Earth, allowing its shadow to fall on Earth and produce a solar eclipse. The vertical displacement of the Moon above Earth’s shadow is exaggerated for clarity and is not drawn to scale. View C shows a cross-section of the two orbital planes, illustrating their approximately 5° inclination, exaggerated for clarity, and explaining why eclipses only occur when the Moon is sufficiently close to the nodes. Distances, sizes and orbital shapes are schematic, not to scale.
4. What a diagram can—and cannot—show
A scientific model is more than a picture. It is a simplified representation built to answer a particular question. The lamp-and-ball demonstration shows how our viewpoint changes the Moon’s apparent phase. A seasons diagram can illustrate how Earth’s tilted axis maintains approximately the same direction as the planet travels around the Sun. An eclipse diagram can show how the Moon’s inclined orbital plane affects the alignment of the Sun, Earth and Moon. None needs to show every distance or movement at once, but each should make it clear which details have been simplified or left out.
Before interpreting an astronomy diagram, ask three questions: Where is the observer? Which direction stays fixed? What happens outside the plane of the drawing?
Asking these questions helps us recognise the assumptions behind a diagram and understand what it can–and cannot–tell us. A useful diagram does not show everything. It shows the relationships needed to answer a particular question—and tells us what it leaves out.
Article written by Jihun Oh, independent developer of Space Atlas Student, South Korea.
Article edited by Priscilla Wong, a recent BSc Biological Sciences (Immunology) (Hons) graduate from the University of Edinburgh and the Head Online News Editor for EUSci.
References:
NASA Science, “Moon Phases” — why the first-quarter Moon appears half illuminated and how the phase cycle works. https://science.nasa.gov/moon/moon-phases/
NASA/JPL Education, “Moon Phases” — a lamp-and-ball activity for modelling lunar phases and observer perspective. https://www.jpl.nasa.gov/edu/resources/lesson-plan/moon-phases/
NASA Space Place, “How Far Away Is the Moon?” — average Earth–Moon distance, about 30 Earth diameters. https://spaceplace.nasa.gov/moon-distance/en/
NASA Space Place, “What Causes the Seasons?” — axial tilt and opposite seasons in the two hemispheres. https://spaceplace.nasa.gov/seasons/en/
NASA Science, “Milankovitch (Orbital) Cycles and Their Role in Earth’s Climate” — Earth’s axial tilt, perihelion, aphelion and incoming solar radiation. https://science.nasa.gov/science-research/earth-science/milankovitch-orbital-cycles-and-their-role-in-earths-climate/
NASA Science, “Eclipses and the Moon” — solar and lunar eclipse geometry, the Moon’s orbital inclination and nodes. https://science.nasa.gov/moon/eclipses/
NASA Goddard Space Flight Center, “Periodicity of Lunar Eclipses” — lunar orbital nodes, the approximately 5° orbital inclination and eclipse seasons. https://eclipse.gsfc.nasa.gov/LEsaros/LEperiodicity.html
Editorial disclosure: AI assistance was used in drafting and editing this article. The scientific explanations were checked against the linked NASA and US Naval Observatory sources. This article reports no original research, classroom trial, measured learning outcome, institutional endorsement or curriculum approval.
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