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The diagrams below represent the tidal bulges (blue) at two extremes of the relative positions of the Sun, Moon, and Earth. Of course the diagrams are not to scale, and the actual effects of the changing declination of the Moon and Sun are more subtle than these diagrams would indicate. | |
 | In the figure to the left, the Moon and Sun are aligned with the equator, as might happen on the equinoxes. Note that the tidal bulges are also aligned with the equator. A mid latitude observer would experience 2 high tides of approximately equal height and range (symmetrical tides) - at X1 and 12h 25m later at X. "Two high tides a day" tides are called semidiurnal tides. Note also that near and at the poles (Y), tidal effects are small, if noticeable at all. |
| In the figure to the right, the Moon and Sun are both at maximum declination, as might occur on the summer solstice. Note that there is 1 tidal bulge north of the equator, and the other is south of the equator. A mid latitude observer would still experience 2 high tides (semidiurnal tides), but they would be assymetrical - the tide at X1 would be greater in both height and range than the tide 12h 25m later at X. Note also that at higher latitudes, there is only one relatively small high tide that day. "One high tide a day" tides are called diurnal tides |  |
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As noted above, these diagrams represent extremes of the Earth, Moon, and Sun positions, and they are relatively rare. Most days, the tidal bulges will form somewhere in between the positions shown above. And it is important to remember that many other factors - including Moon phase, Earth-Moon and Earth-Sun distance, local ocean depth and seafloor topography, and the shape of the coast line - all have an effect on the local time, height, and range of tide. | |
| For further exploration, check out this Tide Table for Prudhoe Bay on the arctic coast of Alaska, noting especially the small range of tide and the seasonal changes in tide heights. | |