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<\/a><\/figure>\n<\/div>\n\n\nTidal heights are measured relative to a refence sea level or tidal \u201cdatum,\u201d which is mean lower low water. This datum is determined by empirical observation of the tides over a period of nineteen years at, for example, Tide Station Golden Gate, the oldest continually operating tide station in the western hemisphere. This is called a tidal epoch<\/em>.<\/p>\n\n\n Golden Gate tidal gauge<\/p>\n\n\n\n But why nineteen years? This video<\/a> shows some of the complications of the Moon\u2019s orbit. It\u2019s an ellipse and is tilted approximately 5\u00b0 to the plane of the ecliptic, while the Earth itself is tilted 23.5\u00b0. In addition, the line of the apsides, which is the line between the Moon\u2019s perigee (closest point) and apogee (farthest point), rotates around the Earth in a period of 8.85 years, which is called the \u201cprecession of the apsides.\u201d The plane of the orbit also wobbles, like a plate settling on a table, in a retrograde direction every 18.6 years. This is called the \u201cregression of the nodes.\u201d These variations in the Moon\u2019s orbit change the Moon\u2019s effect on tides, which is why NOAA uses a \u201cmean\u201d or average height of tide over nineteen years to enable a steady reference. (The Greeks noticed a similar but not directly related period of the Moon\u2019s orbit, discovered by Athenian Meton in the 5th<\/sup> century BC, and so called the Metonic cycle. Babylonians, Polynesians, and native Americans observed this, too.)<\/p>\n\n\n\n The Earth\u2019s seasons occur because although the Earth\u2019s tilt doesn\u2019t change as it orbits (or only over a period of 26,000 years, which we can ignore), it either tilts at 23.5\u00b0 towards the Sun in northern hemisphere summer, or away from it in northern hemisphere winter, and this affects the amount of gravity the Sun directly exerts in different parts of the globe. The Earth\u2019s orbit around the Sun is also an ellipse and has similar precessions as the Moon, but in addition is affected by the gravity of the planets. The speed of both the Moon and Earth vary throughout their orbits, as was discovered by Kepler.<\/p>\n\n\n\n There are even more variables to the relationship between Earth, Moon, and Sun, and tides, and if this is starting to get a bit dizzying, don\u2019t feel bad. No less an authority than Sir Isaac Newton said that contemplating the Moon\u2019s orbit gave him a headache. Writing a century before Newton, Galileo did not believe the Moon could affect the tides at all, despite centuries of mariners noticing the conjunction of tides and phases of the Moon. This was because, like Einstein with regard to quantum entanglement, the idea that a distant object could affect the Earth struck him as something akin to witchcraft. To be fair to Galileo, even though we understand what gravity does, we still don\u2019t understand what it is. But tell you what, let’s leave that one alone.<\/p>\n\n\n\n Here is a fun fact that is the result of relationships between orbit of the Earth and Moon: When you see a waning half-Moon, the Earth will be where the Moon is in about three and a half hours\u00ad\u00ad\u2014but it will be gone, so don\u2019t worry. Here\u2019s why: The orbit of the Earth is an ellipse, but let\u2019s not be fancy and call the distance to the Sun from Earth about 93 million miles. The diameter of the Earth\u2019s orbit is double that, or 186 million miles, and multiplying by pi<\/em> we get the circumference, which is 584,040,000 miles. The year takes 365 days, so dividing by 365 we get 1,600,109 miles per day, and dividing that by 24 we see that the speed of the Earth around the Sun is 66,671 miles per hour. Average Earth-Moon distance is 238,900 miles. The Moon orbits the Earth counterclockwise, in the same direction that the Earth orbits the Sun, and when it is ahead<\/em> of the Earth in the earth\u2019s orbital path, you\u2019ll see the left half illuminated. It will be visible from about midnight to noon. We divide the Moon\u2019s distance by the speed of the Earth, and get approximately three and a half hours to get to the moon\u2019s location at the earth\u2019s speed.<\/p>\n\n\n\n We’ve just scratched the surface of all the things affecting tides. For example, on a new Moon when both Sun and Moon are on the same side of the Earth, why do we still have two tides, instead of one big one? Why, under the same Moon and Sun, does the Bay of Fundy have tides of over fifty feet while Tahiti has about one? What is a seiche? The boffins at NOAA\u2019s National Ocean Service earn their pay by figuring this stuff out in intricate detail. If questions like this intrigue you but you aren\u2019t fond of knotty math, check out Beyond the Moon.<\/em><\/a><\/p>\n","protected":false},"excerpt":{"rendered":" In the left column of the daily pages in your tide book, you\u2019ll see the phases of the Moon, and in the columns to the right of a full and new Moon, you\u2019ll see that some of the heights are … Continue reading <\/a><\/figure>\n<\/div>\n\n\n