How the Moon's Phases Form and Why They Drive Ocean Tides
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In brief
The Moon's phases change because sunlight hits different parts of its surface as it moves around Earth, not because of Earth's shadow; one full cycle of phases, called a synodic month, averages 29.5 days. The same Moon also pulls on Earth's oceans with uneven gravitational force, creating two high tides and two low tides roughly every 24 hours and 50 minutes, with the Sun strengthening or weakening that pull twice a month.
What makes the Moon appear to change shape
The Moon does not produce its own light. It reflects sunlight, and as it orbits Earth the angle between the Sun, the Moon and Earth keeps changing. That shifting geometry means a different slice of the Moon's sunlit half faces Earth from night to night, according to NASA's Jet Propulsion Laboratory. The visible change is not caused by Earth's shadow; that only happens during a lunar eclipse.
One full cycle of phases, from new moon back to new moon, is called a synodic month. The U.S. Naval Observatory's Astronomical Applications Department defines the month precisely by the angular separation between the Moon and the Sun along the ecliptic: new moon at 0 degrees, first quarter at 90 degrees, full moon at 180 degrees and last quarter at 270 degrees. The average length of this cycle is 29.5 days.
The eight phases in order
New Moon — the Moon sits between Earth and the Sun; the side facing Earth is unlit
Waxing Crescent — a thin sliver of light grows on the right edge (for Northern Hemisphere observers)
First Quarter — the right half of the Moon's disk is illuminated
Waxing Gibbous — more than half is lit, but not yet full
Full Moon — Earth lies roughly between the Sun and Moon, so the entire visible face is lit
Waning Gibbous — the illuminated portion begins shrinking
Last (Third) Quarter — the left half is illuminated
Waning Crescent — a thin crescent remains before the cycle returns to new moon
How the Moon pulls on Earth's oceans
Tides are not caused by the Moon lifting water upward in a simple pull. NASA's Science division explains that the Moon's gravity is slightly stronger on the side of Earth closest to it and weaker on the far side. This difference in pull — called a tidal force — stretches Earth's water (and, to a tiny degree, its solid body) into an elongated shape, producing a bulge of water on the side facing the Moon and a second bulge on the opposite side.
As Earth rotates underneath these two bulges, most coastlines pass through a high tide, then a low tide, twice in roughly one day. NOAA's National Environmental Satellite, Data, and Information Service (NESDIS) states that this produces two high tides and two low tides approximately every 24 hours and 50 minutes — slightly longer than a 24-hour day because the Moon itself is also moving in its orbit.
Why tides are sometimes bigger: spring and neap tides
The Sun also pulls on Earth's oceans, but because it is about 390 times farther away than the Moon, NASA notes its tide-generating force is a little less than half of the Moon's. Whether that solar pull adds to or subtracts from the Moon's pull depends on the three bodies' alignment, which repeats on a roughly two-week rhythm tied to the lunar phases.
Spring tides vs. neap tides
Feature
Spring tide
Neap tide
When it occurs
Full Moon or New Moon
First Quarter or Last Quarter
Sun-Moon-Earth alignment
In a line (syzygy)
At roughly right angles
Effect on gravitational pull
Solar and lunar pull combine
Solar pull partly cancels lunar pull
Resulting tide range
Higher highs, lower lows
Smaller difference between high and low
Frequency
About every two weeks
About every two weeks, offset from spring tides
NOAA and NASA both describe spring tides as occurring during the full or new moon phase, when Earth, Sun and Moon line up and the two bodies' gravitational effects combine. Neap tides follow about a week later, around the first or last quarter phase, when the Sun sits at roughly a right angle to the Moon as seen from Earth and partially offsets the Moon's pull, producing a smaller gap between high and low water.
The Moon's gravity tugs at Earth, it shifts Earth's mass, distorting its shape ever so slightly into that of a football — elongated at the equator and shortened at the poles.
— NASA Science (science.nasa.gov)
Questions
How long does a full cycle of moon phases take?
A complete cycle from new moon to new moon, called a synodic month, averages 29.5 days, according to the U.S. Naval Observatory.
Why are there two high tides a day instead of one?
The Moon's uneven gravitational pull creates a water bulge on the side of Earth facing the Moon and a second bulge on the opposite side; as Earth rotates through both bulges, most coasts get two high tides and two low tides roughly every 24 hours and 50 minutes, per NOAA NESDIS.
Does the Sun affect tides, or only the Moon?
Both affect tides. NASA states the Sun's tide-generating force is a little less than half the Moon's because of its much greater distance, but it still strengthens tides during full and new moons (spring tides) and weakens them during quarter moons (neap tides).
What is the difference between a spring tide and a neap tide?
Spring tides bring the highest highs and lowest lows and happen at full or new moon, when Sun, Earth and Moon align. Neap tides are more moderate and occur about a week later, near the first or last quarter moon, when the Sun's pull partly cancels the Moon's.
Does the Moon's shadow on Earth cause its phases?
No. Moon phases are caused by changing sunlight angles on the Moon's surface as it orbits Earth, not by Earth's shadow; Earth's shadow only affects the Moon during a lunar eclipse, a separate and much rarer event.
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