What causes the phases of the Moon, eclipses and the tides?
Describe the phases of the Moon, solar and lunar eclipses, and the tides as consequences of the motions and gravitational interactions of the Earth, Moon and Sun.
A Regents answer on the Earth-Moon-Sun system: the cause of the Moon's phases, why solar and lunar eclipses are rare, the roughly two-week phase cycle, and how the Moon's and Sun's gravity produce spring and neap tides.
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What this topic is asking
This topic ties the Moon's phases, eclipses and the tides to the geometry and gravity of the Earth-Moon-Sun system. The Regents tests the difference between phases (caused by positions) and eclipses (caused by shadows), and links the tides to the gravity of the Moon and Sun.
The phases of the Moon
The cycle runs new moon (between Earth and Sun, dark side toward us), waxing crescent, first quarter, waxing gibbous, full moon (Earth between Moon and Sun, fully lit), waning gibbous, last quarter, waning crescent, and back to new. Each major phase is about a week apart, so a quarter-to-quarter or new-to-full change is about two weeks.
The Moon also rotates on its axis in the same time it takes to revolve (synchronous rotation), which is why it always keeps the same face toward Earth.
Eclipses
This 5-degree tilt is the key Regents point: it is why we do not get an eclipse every new and full moon.
The tides
The tides are the regular rise and fall of sea level, caused mainly by the gravity of the Moon (and partly the Sun) pulling on Earth's oceans. There are usually two high tides and two low tides each day as Earth rotates beneath the tidal bulges.
The alignment of the Sun and Moon controls the size of the tides:
- Spring tides (largest): at new and full moon the Sun, Earth and Moon are in a straight line, so the Sun's and Moon's pulls add together, giving the highest high tides and lowest low tides.
- Neap tides (smallest): at the quarter moons the Sun and Moon are at right angles to Earth, so their pulls partly cancel, giving a small tidal range.
Try this
Q1. State the cause of the Moon's phases. [1 point]
- Cue. The changing relative positions of the Moon, Earth and Sun as the Moon orbits Earth.
Q2. Explain why a solar eclipse can only occur at new moon. [2 points]
- Cue. Only at new moon is the Moon between the Sun and Earth, so its shadow can fall on Earth.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (style)1 marksPart A. The phases of the Moon as seen from Earth are caused by the (1) shadow of Earth falling on the Moon (2) changing distance between Earth and the Moon (3) changing relative positions of the Moon, Earth and Sun (4) rotation of the Moon on its axis. Justify your choice.Show worked answer →
A 1-point multiple-choice question. The answer is (3).
Phases are caused by the changing positions of the Moon, Earth and Sun as the Moon revolves around Earth, which changes how much of the Moon's sunlit half we can see. Earth's shadow (1) causes a lunar eclipse, not the regular monthly phases. Distance (2) changes slightly but does not cause phases. The Moon does rotate (4), but synchronously, which keeps the same face toward Earth and does not cause phases. The trap is confusing phases (positions) with eclipses (shadows).
Regents (style)2 marksPart B-2. (a) Explain why solar and lunar eclipses do not happen every month. (b) State the alignment of the Sun, Earth and Moon needed for the highest (spring) tides.Show worked answer →
A 2-point constructed-response question.
(a) 1 point: the Moon's orbit is tilted about 5 degrees to Earth's orbit around the Sun, so most months the Moon passes above or below the line joining Earth and the Sun, and no shadow falls. Eclipses occur only when the three bodies line up closely (at new moon for a solar eclipse, full moon for a lunar eclipse).
(b) 1 point: the Sun, Earth and Moon must be in a straight line (at new moon or full moon) so the Sun's and Moon's tidal pulls add together, producing the highest high tides and lowest low tides (spring tides).
Markers reward the tilted orbit for (a) and the straight-line alignment for (b).
Related dot points
- Explain Earth's rotation and revolution, the evidence for each, and how they produce the apparent daily motion of celestial objects at 15 degrees per hour, including the use of Polaris to find latitude.
A Regents answer on Earth's rotation and revolution: the evidence for each, the apparent daily motion of the Sun, Moon and stars at 15 degrees per hour, Foucault's pendulum and the Coriolis effect, and how the altitude of Polaris gives an observer's latitude in the Northern Hemisphere.
- Calculate the eccentricity of an elliptical orbit using the Reference Tables equation (distance between foci divided by length of the major axis) and relate eccentricity to orbital shape and orbital velocity.
A Regents answer on orbital eccentricity: ellipses and foci, the Reference Tables formula (distance between foci over the length of the major axis), worked calculations rounded to the nearest thousandth, and how eccentricity and the Sun's off-center position affect orbital velocity and apparent solar diameter.
- Explain how the tilt of Earth's axis and its revolution change the angle and duration of insolation through the year, producing the seasons, the solstices and the equinoxes.
A Regents answer on insolation and the seasons: why the 23.5 degree axial tilt and Earth's revolution change the angle and duration of insolation, the solstices and equinoxes, the Sun's path across the sky at New York latitudes, and why summer is warm even though Earth is near aphelion.
- Describe the structure of the solar system and use the Selected Properties of the Planets table and Kepler's laws to relate a planet's distance from the Sun to its period and orbital velocity.
A Regents answer on the solar system: terrestrial versus Jovian planets, gravity as the controlling force, and Kepler's laws used with the Reference Tables Selected Properties of the Planets so that planets farther from the Sun have longer periods and slower orbital velocities.
- Explain how ocean surface currents form (winds, the Coriolis effect) and how they redistribute heat, moderate coastal climates and connect to density-driven deep circulation.
A Regents answer on the oceans: how prevailing winds and the Coriolis effect drive surface currents into gyres, how warm and cold currents redistribute heat and moderate coastal climates (for example the Gulf Stream), the difference between surface and density-driven deep circulation, and the link to the water specific heat on the Reference Tables, with worked exam questions.
Sources & how we know this
- Reference Tables for Physical Setting/Earth Science (2011 edition) — New York State Education Department (2011)
- Regents Examination in Physical Setting/Earth Science — New York State Education Department (2026)