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The Urban Astronomer

Sun and Moon and Ebb Tides

30 Jul 2014, 23:06 UTC
Sun and Moon and Ebb Tides
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A kayaking friend of mine referred me to a question posed by a fellow kayaker regarding tides in and around the Golden Gate: Why are Ebb Tides strongest in SF Bay at night during the summer and during the day in winter? I love this kind of question, where the celestial mechanics of the Solar System impact the daily experiences of hobbyists and average people on Earth. First of all, why are there differences in size of high & low tides throughout the month and year? Each month the highest highs and lowest lows occur when the Moon is Full or New. That’s pretty simple. But there are two important factors regarding the Moon’s proximity to Earth on any given Full or New Moon. And there is one important factor regarding the Sun’s proximity to Earth on any given Full or New moon. 1. The Sun’s Changing Proximity The Earth’s orbit around the Sun is elliptical, meaning that there is a close approach each year on January 4th (“perihelion”) and a corresponding far point in our orbit in July (“aphelion”). The New and Full Moon phases just before or after January 4th have higher high tides at the noontime tide, ...

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