Why the Moon Pulls the Oceans
🌊 Gravity & tides
Question
How can the Moon, 385,000 km away, pull the oceans up and down twice a day?
Gravity falls off with the square of the distance, so the side of the Earth facing the Moon is pulled a bit more strongly than the far side. The near-side water bulges toward the Moon; the far side is effectively 'left behind' - two bulges. The Earth rotates underneath them, so most coastlines get two high tides a day. When the Sun and Moon line up (new and full Moon) their bulges add (spring tides); at right angles they partially cancel (neap tides).
The math
Tidal force scales as M/d³. Moon vs Sun: (Moon's mass ÷ Sun's mass) × (Sun's distance ÷ Moon's distance)³ = 3.69×10⁻⁸ × 389³ = 2.2. The Moon's tidal pull is ~2.2 times the Sun's - even though the Sun's gravity on Earth is 180 times stronger. Tides depend on the difference in pull, not the pull itself.
Why it matters
Newton's gravity predicts the tides - twice a day, bigger at new/full Moon, exactly where and when - which is why tide tables are accurate to the minute. A flat Earth with a 'close Moon' cannot reproduce this pattern.