How do the motions of the Earth, Moon and Sun cause day and night, Moon phases and eclipses?
Explain how Earth's rotation causes day and night, how the Moon's orbit causes its phases, and how the alignment of the Sun, Earth and Moon causes solar and lunar eclipses (Virginia 2018 Earth Science SOL ES.11).
A SOL-level answer on the Earth-Moon-Sun system for the Virginia Earth Science EOC: how rotation causes day and night, the cause and sequence of Moon phases, why eclipses happen only at certain alignments, the difference between a solar and a lunar eclipse, and why eclipses are not monthly, with worked exam questions.
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What this topic is asking
Virginia Earth Science SOL standard ES.11 asks you to explain the motions and alignments in the Earth-Moon-Sun system: day and night, Moon phases, and eclipses. The EOC tests this with rotation-versus-revolution items, Moon-phase diagrams (match the phase to the position), and eclipse items (which alignment, and why eclipses are not monthly). These ideas also connect to the tides in the oceanography module.
Rotation: day and night
The phases of the Moon
The cycle, in order, is: new moon (Moon between Earth and Sun, its dark side toward us, so it appears unlit) → waxing crescent → first quarter (half lit) → waxing gibbous → full moon (Earth between the Sun and Moon, the lit side fully toward us) → waning gibbous → third (last) quarter → waning crescent → back to new. "Waxing" means the lit part is growing; "waning" means it is shrinking.
Eclipses
Why eclipses are not monthly
If the Moon orbited in exactly the same plane as Earth orbits the Sun, there would be a solar eclipse every new moon and a lunar eclipse every full moon. But the Moon's orbit is tilted about 5 degrees, so most months the Moon passes a little above or below the direct Sun-Earth line, and no eclipse occurs. Eclipses happen only when the Moon is new or full and is crossing the plane of Earth's orbit, so the three bodies line up exactly. This tilt is the key reason eclipses are occasional rather than monthly.
Try this
Q1. Explain what causes day and night, and contrast it with what causes a year. [2]
- Cue. Day and night are caused by Earth's rotation (spin) once about every 24 hours; a year is caused by Earth's revolution (orbit) around the Sun.
Q2. State the alignment of the Sun, Earth and Moon during a lunar eclipse. [1]
- Cue. Earth passes directly between the Sun and the Moon (at full moon), so Earth's shadow falls on the Moon.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Earth Science SOL 2023 (style)1 marksWhat causes day and night on Earth? (A) Earth's revolution around the Sun. (B) Earth's rotation on its axis. (C) the phases of the Moon. (D) the tilt of Earth's axis.Show worked answer →
A 1-point multiple-choice item on Earth's motions.
The correct answer is B. Day and night are caused by Earth's rotation on its axis once every 24 hours: the side facing the Sun has day, the side facing away has night. Revolution (A) takes a year and (with the tilt) causes seasons, the Moon's phases (C) are separate, and the tilt (D) causes seasons, not the daily cycle.
The test rewards distinguishing rotation (spin, day and night) from revolution (orbit, a year).
VA Earth Science SOL 2024 (style)2 marksA solar eclipse happens only at a new moon, but there is not a solar eclipse every new moon. (a) Explain the alignment of the Sun, Earth and Moon during a solar eclipse. (b) Explain why eclipses do not happen every month.Show worked answer →
A 2-point item on eclipses.
(a) 1 point: in a solar eclipse the Moon passes directly between the Sun and Earth (at new moon), so the Moon's shadow falls on Earth and blocks the Sun.
(b) 1 point: the Moon's orbit is tilted (about 5 degrees) relative to Earth's orbit around the Sun, so usually the Moon passes a little above or below the direct line; eclipses occur only when the Moon is new or full and crosses the orbital plane, so the three line up exactly.
Markers reward the Moon-between-Sun-and-Earth alignment in (a) and the tilted orbit explanation in (b).
Related dot points
- Explain that Earth's seasons are caused by the tilt of its axis (about 23.5 degrees) as it revolves around the Sun, which changes the directness of sunlight and the length of daylight (Virginia 2018 Earth Science SOL ES.11).
A SOL-level answer on the seasons for the Virginia Earth Science EOC: why the tilt of Earth's axis, not its distance from the Sun, causes the seasons, how the directness of sunlight (insolation) and daylight length change, the solstices and equinoxes, and why the hemispheres have opposite seasons, with worked exam questions.
- Describe the structure of the solar system (the Sun, terrestrial and gas-giant planets, and small bodies), explain that orbits are elliptical (Kepler), and explain how gravity and inertia keep planets in orbit (Virginia 2018 Earth Science SOL ES.2 and ES.11).
A SOL-level answer on the solar system for the Virginia Earth Science EOC: the Sun and the inner terrestrial versus outer gas-giant planets, asteroids, comets and other small bodies, the nebular hypothesis, Kepler's elliptical orbits, and how gravity and inertia combine to keep planets orbiting, with worked exam questions.
- Describe how wind generates ocean waves and the parts of a wave, and explain that tides are caused by the gravitational pull of the Moon and Sun, including spring and neap tides (Virginia 2018 Earth Science SOL ES.10 and ES.11).
A SOL-level answer on waves and tides for the Virginia Earth Science EOC: how wind makes waves, the parts of a wave (crest, trough, wavelength, height) and what fetch controls, why tides are caused by the gravity of the Moon (and Sun), the daily pattern of two high and two low tides, and spring versus neap tides, with worked exam questions.
- Explain that stars produce energy by nuclear fusion, describe the Hertzsprung-Russell diagram and the relationship between a star's color, temperature and luminosity, and outline the life cycle of a star (Virginia 2018 Earth Science SOL ES.12).
A SOL-level answer on stars for the Virginia Earth Science EOC: how stars produce energy by nuclear fusion, what the Hertzsprung-Russell diagram shows about color, temperature and luminosity, the main sequence, and how a star's life cycle depends on its mass, with worked exam questions.
- Describe galaxies and the scale of the universe, explain the Big Bang theory and its evidence (redshift and the cosmic microwave background), and outline how the electromagnetic spectrum and telescopes are used to study space (Virginia 2018 Earth Science SOL ES.12).
A SOL-level answer on the universe for the Virginia Earth Science EOC: galaxies and their types, the vast scale measured in light-years, the Big Bang theory and its evidence (the redshift of distant galaxies and the cosmic microwave background), and how the electromagnetic spectrum and telescopes let us study space, with worked exam questions.
Sources & how we know this
- 2018 Science Standards of Learning (Earth Science) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)