Tides

The lunar day

Goal: explain why tide times march forward by about 50 minutes a day.

Predict

If Earth rotates once in 24 hours, and the tidal bulge points at the Moon, shouldn't high tide be at the same time every day?

Learn

While Earth turns once, the Moon moves along its orbit, about 13.2° per day. So after 24 hours, the Moon is not where it was. Earth must turn about 13.2° further to bring it back overhead.

13.2° ÷ 15°/hour ≈ 53 minutes

The lunar day is 24 h 50.5 min. That is the tidal clock, and it is why tide tables march forward through the week instead of repeating.

This is the same effect as the synodic/sidereal month gap from module 02, one moving thing measured against another moving reference.

Run

moonfield tide rough

Look at the predicted events: consecutive highs are about 12 h 25 min apart, half a lunar day.

Checkpoint

Next: Spring and neap.