What causes solar and lunar eclipses, and why do they not happen every month?
The causes and appearance of solar eclipses (partial, total, annular) and lunar eclipses (partial, total), including the terms first, second, third and fourth umbral contact.
A focused answer to Edexcel GCSE Astronomy statements 3.8 to 3.10, covering the causes of solar and lunar eclipses, the appearance of the Sun during partial, total and annular solar eclipses and of the Moon during partial and total lunar eclipses, the umbral contact terms, and why eclipses do not occur every month.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Edexcel statements 3.8 to 3.10 want you to understand the causes of solar and lunar eclipses, and the appearance of the Sun during partial, total and annular solar eclipses and of the Moon during partial and total lunar eclipses, including the terms first, second, third and fourth umbral contact.
What causes eclipses
The two are easy to keep straight by which body is in the middle: Moon in the middle blocks the Sun (solar eclipse); Earth in the middle blocks the sunlight reaching the Moon (lunar eclipse). The shadow of each body has a dark central umbra (full shadow) and a lighter outer penumbra (partial shadow), which determines whether the eclipse seen is total or partial.
Why eclipses are not monthly
This 5 degree tilt is the whole reason we do not get two eclipses every month. Only when the alignment falls near a node do the bodies line up well enough for a shadow to be cast and received. This explains why eclipses come in seasons a few times a year rather than at every new and full Moon, and is a favourite "explain why not" question.
The appearance of eclipses
Annular eclipses arise because the Moon's orbit is elliptical: when it is far from Earth its angular size is slightly smaller than the Sun's, so it cannot quite cover it. The red colour of a totally eclipsed Moon comes from red sunlight bent through the Earth's atmosphere into the shadow. Note that a total solar eclipse is visible only from the narrow path of the umbra, whereas a lunar eclipse is visible from the entire night side of the Earth.
Umbral contacts
These four terms apply to both kinds of eclipse and let observers describe and time the stages precisely. Second to third contact marks the period of totality. Learning the order (touch, fully in, start to leave, fully out) is exactly what statement 3.8 and 3.9 require.
How Edexcel examines this
This is naked-eye Paper 1 content and a frequent extended-answer topic. The core compare-and-contrast question rewards the Moon-between geometry at new Moon for a solar eclipse (the Moon covers the Sun, corona visible at totality) and the Earth-between geometry at full Moon for a lunar eclipse (the Moon enters the Earth's shadow and reddens), plus the point that a total solar eclipse is seen only along the umbra's narrow path while a lunar eclipse is seen across the whole night side. The "why not every month" question rewards the 5 degree orbital tilt and the need for alignment near a node. Annular versus total tests whether you know the elliptical orbit changes the Moon's angular size. The umbral contact terms are tested by ordering or labelling the four stages. The biggest error is swapping the solar and lunar geometries, so anchor each to which body sits in the middle.
Try this
Q1. State the positions of the Sun, Earth and Moon during a total lunar eclipse. [1 mark]
- Cue. The Earth is between the Sun and the Moon, and the Moon is in the Earth's shadow (full Moon).
Q2. State what is meant by second umbral contact. [1 mark]
- Cue. When the Moon is fully inside the umbra, marking the start of totality.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 1AS0 20214 marksExplain the difference between a total solar eclipse and a total lunar eclipse, describing the positions of the Sun, Earth and Moon and what is seen in each case.Show worked answer →
In a total solar eclipse the Moon passes directly between the Sun and the Earth (at new Moon), and the Moon's shadow (umbra) falls on the Earth, so observers in the umbra see the Moon completely cover the Sun's disc, revealing the Sun's faint corona (2 marks). In a total lunar eclipse the Earth passes directly between the Sun and the Moon (at full Moon), so the Moon moves into the Earth's shadow and the whole Moon dims and often turns a dull red (2 marks). Markers reward the Moon-between geometry for the solar eclipse (Moon covers the Sun) and the Earth-between geometry for the lunar eclipse (Moon enters the Earth's shadow). A lunar eclipse is seen by everyone on the night side, but a total solar eclipse only from the narrow path of the umbra.
Edexcel 1AS0 20223 marksExplain why a solar eclipse does not happen at every new Moon, even though the Moon passes between the Earth and the Sun once a month.Show worked answer →
The Moon's orbit around the Earth is tilted by about 5 degrees to the plane of the Earth's orbit around the Sun (the ecliptic) (1 mark). So at most new Moons the Moon passes a little above or below the line between the Earth and the Sun, and its shadow misses the Earth, giving no eclipse (1 mark). An eclipse only happens when a new Moon occurs near a node, where the two orbital planes cross, so the three bodies line up exactly (1 mark). Markers reward the 5 degree tilt of the Moon's orbit and the need for the new Moon to occur near a node for the shadow to line up.
Related dot points
- The relative sizes and distances of the Earth, Moon and Sun, the Sun's mean diameter, and how Eratosthenes and Aristarchus determined the sizes and distances from observations.
A focused answer to Edexcel GCSE Astronomy statements 3.1 to 3.4, covering the relative sizes and distances of the Earth, Moon and Sun, the Sun's mean diameter of 1.4 x 10^6 km, and how Eratosthenes measured the size of the Earth and Aristarchus estimated the sizes of and distances to the Moon and Sun.
- The relative effects of the Sun and Moon in producing spring and neap tides, and the precession of the Earth's axis, its effects on the sky and its use in archaeoastronomy.
A focused answer to Edexcel GCSE Astronomy statements 3.5 to 3.7, covering how the gravitational pull of the Moon and Sun produces high and low tides and spring and neap tides, and how the slow precession of the Earth's axis changes the position of the celestial pole and the equinoxes, with its use in archaeoastronomy.
- The difference between sidereal and synodic (solar) days and months, the lunar phase cycle, and the astronomical significance of equinoxes and solstices.
A focused answer to Edexcel GCSE Astronomy statements 4.1, 4.9 to 4.14, covering the difference between sidereal and synodic (solar) days and months, the cause of the lunar phase cycle, and the astronomical significance of the equinoxes and solstices and the Sun's changing apparent path.
- Safe solar observation by pinhole projection, the ecliptic and Zodiacal Band, retrograde motion of the planets, and the configuration terms conjunction, opposition, elongation, transit and occultation.
A focused answer to Edexcel GCSE Astronomy statements 5.1 to 5.6 and 5.8, covering safe solar observation by pinhole projection, the ecliptic and the Zodiacal Band, the cause of retrograde motion of the planets, the First Points of Aries and Libra, and the configuration terms conjunction, opposition, elongation, transit and occultation.
Sources & how we know this
- Pearson Edexcel Level 1/Level 2 GCSE (9-1) in Astronomy (1AS0) specification — Pearson (2017)