The Moon’s gravity raises a pair of ocean bulges. Earth turns under them, so a coast on this cartoon meets a high about twice a spin. The Sun raises a smaller tide. When the Sun and Moon line up — new or full — the bulges add: a spring tide. At a right angle — the quarters — they partly cancel: a neap tide. Pair with the Moon phase dial, with eclipses (the same lineup, plus a node), with gravity (the pull itself), and with seasons, which is Earth’s axis, not this angle.
Educational schematic — not a tide prediction, not navigation, and not guidance for Campbell harbor or any other coast. Bulges are exaggerated, the Sun is drawn in close, and distance is held fixed. NOAA notes the Moon’s distance varies by about 31,000 miles in a month, which changes the range; this slider does not. Real coasts have basins, resonance, Coriolis deflection, and a seafloor. This is geometry literacy.
Interactive · Top-down
North-pole view · Moon moves counterclockwise · bulges stay put
Ocean bulges · exaggerated
Moon · sets the strong axis
Sun · smaller tide
0° · New
0°
Literacy · words for the bulges
Tidal bulges
Two ocean bulges, opposite each other. NOAA: on the near side the Moon’s gravity pulls the water toward it; on the far side, inertia dominates and a second bulge forms. NASA Science has Earth rotating through that pair, so most shorelines see two highs and two lows a day. One high-to-high cycle takes a little over 12 hours.
Lunar day
The time to come back under the Moon: 24 hours and 50 minutes, not a 24-hour solar day. The Moon orbits the same way Earth spins, so Earth has to catch up. NOAA puts high tides in this simple picture 12 hours and 25 minutes apart.
Spring tide
New or full Moon. Sun, Earth, and Moon nearly in a line, so the Sun’s tide adds to the Moon’s. Highs a little higher, lows a little lower. Twice each lunar month, in every season. NASA and NOAA use the same lineup.
Neap tide
First or last quarter. The Sun and Moon are at about a right angle, so the solar bulge partly cancels the lunar one. Moderate tides: highs a little lower, lows a little higher than at spring. About a week after a spring tide.
Syzygy
Sun, Earth, and Moon in a line — new or full. That alignment is a spring tide. An eclipse needs the same lineup and a node, because the Moon’s orbit is tilted about 5°. This page does not draw that tilt. Most new and full Moons are spring tides and not eclipses.
Not a tide table
NOAA: if Earth were a smooth ball with no continents, every shore would see two equal highs and lows each lunar day. Continents block the bulges. Each ocean basin sets its own pattern — semidiurnal, mixed, or diurnal (one high and one low, as in much of the Gulf of Mexico). Shoreline shape and funnel bays magnify the range; NOAA’s classroom example is the Bay of Fundy, over 15 meters. Wind and air pressure move water too. None of that is in the slider.
What this cartoon leaves out
Coriolis deflection, resonance in a basin, and bathymetry — the shape and depth of the seafloor. Also the lag NASA describes: real high-tide bulges sit a little ahead of the Moon, because Earth spins faster than the Moon orbits. The rock of Earth flexes too. NASA Space Place puts those solid-Earth tides at less than 20 centimeters (about 8 inches). The oval on this page is the ocean lesson, drawn huge so a phone can see it.
Sources · public NASA / NOAA education
Cited, not invented
NASA Science — Tides — two bulges; most shorelines two highs and two lows a day; high to high a little over 12 hours; the bulges lead the Moon slightly; spring when Sun, Earth, and Moon line up (new or full); neap about a week later at a right angle (first and last quarter). Graphics not to scale.
NASA Space Place — What is gravity? — the Moon’s pull draws the seas toward it and raises ocean tides. The pair of bulges, including the far-side one, is the longer account on the NASA Science and NOAA pages.
NASA Space Place — High tide on Io — the Moon pulls a little harder on the near side than the far side. Earth’s solid ground tides are less than 20 centimeters (about 8 inches). Io’s flexing is a different world; only the Earth comparison is used here.
NOAA — What are spring and neap tides? — spring from “springing forth,” not the season; twice each lunar month at new and full, when the Sun’s pull is added; neap seven days later at the first and third quarter, when the bulges partly cancel.
NOAA education — Tidal variations — solar tides about half as large as lunar tides, a variation on the lunar pattern; spring adds, neap partly cancels; two of each per lunar month. The Moon’s distance varies by about 31,000 miles, so perigee and apogee change the range. This slider holds distance fixed.
NOAA education — The lunar day — a lunar day is 24 hours and 50 minutes; two highs and two lows in that span; highs 12 hours and 25 minutes apart in the bulge picture.
NOAA education — Types of tidal cycles — continents block a free pair of bulges; semidiurnal, mixed semidiurnal, and diurnal patterns; the U.S. West Coast tends toward mixed semidiurnal tides.
NOAA education — What else affects tides? — near-side gravity and far-side inertia; shoreline shape; funnel bays, with the Bay of Fundy cited over 15 meters; wind and air pressure. A schematic of the Sun and Moon does not include those.
Classroom picture of two bulges and a Sun–Moon angle. Not a tide table, not a navigation aid, and not a prediction for Campbell, the South Bay, or anywhere else. Kilometer, mile, hour, and Fundy figures are the ones NASA and NOAA publish on the pages above. Earthrise preview: NASA / Bill Anders (public domain).