Tides

Spring and neap tides

Goal: predict the tidal range for the week from the Moon's phase alone.

Learn

The Sun raises its own tide, about 46% the size of the Moon's. What matters is whether the two line up.

• New and full Moon: Sun and Moon in line, bulges add → spring tides,

largest range

• First and last quarter: at right angles, partly cancel → neap tides,

smallest range

"Spring" has nothing to do with the season. It means to spring up.

Because springs happen at both new and full Moon, the cycle repeats twice per month, roughly every 14.8 days, with about 7 days between a spring and the next neap.

The combined range:

range ∝ √(1 + r² + 2r·cos(2 × elongation)),   r ≈ 0.46

Note the factor of 2. A bulge pointing at the Moon and one pointing away are equally good at aligning with the Sun.

Run

moonfield phase --no-art
moonfield tide rough

The range factor line: about 1.46 at springs, 0.54 at neaps.

Try it yourself

1. Find the next spring tide from the phase alone, then verify with a tide table
2. Check whether the real spring/neap ratio at your port matches ~1.46/0.54
3. Springs often lag the full Moon by a day or two, find out why (friction)

Checkpoint

• I can predict spring or neap from the Moon's phase
• I know why the cycle repeats twice a month
• I know "spring" is not about the season

Next: Predict → Check → Explain.