§ 17 · Tides — differential gravity
Filed 2026.05
Why are there two bulges?
Everyone is told the Moon pulls the ocean toward it. Fine. Then why is high tide on both sides of the Earth at once? Drag time forward. Drag the Sun. The arrows are the gravitational pull on the water minus the pull on the Earth's center — the difference, not the force. Stretching, not lifting. That is the whole secret.
Time0 h
Gauge tide height+0.00
Sun · Moon angle0°
RegimeMoon only
Controls
Time
0.0 h
Speed
2.0×
Sun phase
0°
Gauge ° E
0°
Reading the picture
The yellow arrows at the surface are the
tidal force: the Moon's gravitational pull on a parcel of water minus its
pull on the Earth's center. Near the Moon they point toward it; on the far side
they point away. Either way, water gets stretched outward — two bulges, not one.
The blue outline is the resulting water
surface, exaggerated about a million-fold so you can see it.
Tide gauge · height at marked surface point window · 72 hours
What you are looking at
§ NotesA
Gravity falls off as
1/r². The Moon pulls the near ocean more strongly
than it pulls the Earth's center, and pulls the far ocean less strongly.
Relative to the Earth the near water is yanked toward the Moon and the far
water is left behind. Two bulges.
B
The Earth rotates underneath the bulges. Each spot passes through two highs and two
lows a day — the semi-diurnal tide. Watch the red dot on the rim and the
strip plot below.
C
Turn the Sun on. When Sun and Moon line up (new or full moon) the bulges add — a
spring tide. When they are 90° apart (first or last quarter) the Sun's
bulge partly fills the Moon's trough — a neap tide. Sweep the Sun-phase
slider to see it.
D
Tides are not flooding. The real bulge is roughly half a metre over 6,000 km of
ocean. Coasts amplify it through resonance and shape. The page exaggerates the
vertical to make the geometry visible.