The Moon's gravity pulls Earth's oceans into two tidal bulges. Earth spins through both every day. Watch it happen, compare spring and neap tides, then predict the tide from the moon phase.
⏱️ About 10–15 minutes · Do the four steps in order, finish with the Challenge
Try this: Press play. The two water bulges stay lined up with the Moon while Earth spins underneath them, like a merry-go-round turning under two hills of water. Watch the red figure ride around and pass through each bulge. Every pass is a high tide, so count how many it gets in one full spin (24 hours). Then let it keep running to watch the Moon make its slow, month-long trip around Earth and change phase as it goes.
High Tides Today
2
every ~12 hours
Low Tides Today
2
between the highs
Observer's Water
High tide
facing the Moon
Moon Phase
New Moon
changes as the Moon orbits
Controls
Speed1.0×
How fast time runs. Press Pause to stop the simulation.
Observer's Tide Gauge
Water level
High tide
Sea level
Low tide
Time of day (hours)
The blue line traces the observer's water height over one day. Two peaks = two high tides. Watch the range grow at spring tides and shrink at neap tides.
Why two bulges, not one? The Moon pulls the near side of Earth more strongly than the center, and the center more strongly than the far side. The near-side water gets pulled toward the Moon. The far-side water gets "left behind" as the rest of Earth gets pulled away from it. The result is two bulges on opposite sides. Earth spins through them, so most coasts get two high tides a day. (A few coasts, including parts of the Texas Gulf, get only one a day because of basin shape, but the cause is still the same two bulges.)
Why doesn't the Moon zip around the screen? Earth spins all the way around once a day, but the Moon takes about a month to orbit Earth. So Earth turns about 28 times for every one trip the Moon makes. The Moon barely moves while the observer whips around, which is why you get about two high tides every day. (The Sun pulls on the oceans too. That's Step 2.)
Try this: drag the Moon around Earth. Watch the tide range bar swing from huge (spring) to tiny (neap). Where is the Moon when the range is biggest?
Earth, the Moon, and the Sun
Tide Type
Spring
Sun + Moon aligned
Tide Range
100%
high tide minus low tide
How big is today's range?
Neap (small) → Spring (big)
Moon position0°
0° = New Moon (Sun side) · 90° = First Quarter · 180° = Full Moon · 270° = Third Quarter
Jump to a moon position
How the Sun fits in. The Sun's gravity pulls Earth too, just less than the Moon's. When the Sun, Earth, and Moon line up, the Sun's pull adds to the Moon's and the bulges get taller. When the Sun is at right angles to the Moon, the Sun's pull fights the Moon's bulges and flattens them.
Try this: tap each moon phase. Notice which phases give the biggest tide range and which give the smallest. Watch the tide curve switch between tall (spring) and short (neap).
Sun, Earth, and Moon
Predicted Tide for Today
Spring Tide
Sun, Earth, and Moon are in a line. Solar bulges add to lunar bulges. Tide range is biggest.
Pick a moon phase
Phase
New Moon
Range Today
Big
How to predict a tide. Spot the moon phase, picture the Sun-Earth-Moon line, and ask: are they lined up or at right angles? Lined up means spring. At a right angle means neap. A new moon and a full moon both give spring tides because the Sun and Moon are on the same line either way.