Tides, Currents, Sun and Moon – Explained

Short articles that answer questions people ask while looking at a tide table. They are written for boaters and curious people, not specialists, and they use San Francisco Bay and the Delta as the running example.

Tides and currents

• Why are there two tides per day? – the two tidal bulges, the way the Earth and Moon orbit a shared balance point, and why high tides come about 12 hours 25 minutes apart.
• Why doesn't slack equal high or low tide? – in bays and rivers the tide travels like a slow wave, so the water can keep flowing after the level has peaked.
• Why can you ride a flood but not an ebb? – why a trip up the Bay toward Stockton or Sacramento can ride the flood most of the way, and the trip back cannot ride the ebb.

Sun and moon

• What is the longest day of the year? – daylight versus the solar day measured from noon to noon, and why the two answers differ.
• How much sun will I get today? – the UV index by hour of the day, compared with the strongest day of the year.
• Can the sun rise and set to the north? – yes, every summer north of the Tropic of Cancer, and how that works.
• Why doesn't the moon follow the sun's path? – the tilt of the moon's orbit and the 18.6-year lunar standstill cycle.

Back to the numbers

When you are done reading, open today's tides and currents, or pick a spot from the chart of locations.