The moon, our closest neighbor, circles the earth roughly every 27 plus days and, from our perspective, we see the moon wax and wane during that time from new to full and then back to a new moon based on the geometry between the moon’s position and the way the sun illuminates its surface.
At the time of the new and full moons, the sun, the earth and the moon are more or less aligned which has the effect of increasing the gravitational pull of the moon and sun on the earth. It is at these times that we experience spring tides – tides which are higher then the mean because of the increased gravitational pull of the sun in alignment with the moon. When we see a half moon, the moon and the sun are more or less at right angles to each other. The gravitational pull of the two is at a minimum and this results in neap tides which are typically smaller then the mean. Thus over 27 days, we se two neap tides and two spring tides with the intervening periods being between the two.
In addition to the above, the moon’s orbit is slightly elliptical which means that it has both an apogee (time when it is furthest from the earth) and a perigee (a time when it is closet to the earth). The technical term is perigee-syzygy (courtesy of Wikipedia) – you learn something new everyday. At any rate, at perigee-syzygy, the moon appears 14% larger and 30% brighter. Getting back to tides, the spring tides at perigee-syzygy are higher then normal spring tides because of the increased gravitation pull of the moon during its closest approach to earth. Gravity is not linear and varies as the square of the distance so these effects are larger then one might guess. Tidal ranges during the supermoon were upwards of 19 feet. Whoa to the sailor who anchored in 17 feet of water at high water – the thing that went bump in the night was his boat hitting the rocks on the bottom.
While in BC, we had a supermoon on August 10th when we were at Nimmo off McKenzie Sound.
The entrance thru McKenzie Sound is thru Kenneth Narrows and, in other posts, I have talked about the tidal rapids which are part of cruising life in BC. The larger then normal tides cause more extreme tidal flows as the water rushes thru the narrow, shallow passes that separate larger bodies of water on either side.
Kenneth Narrows is generally pretty benign with tidal flows at mid tide (the time of the strongest flow) in the 2 knot range or so. Not so on the supermoon. While certainly not dangerous, it was exciting as we negotiated whirlpools and eddies that were racing at speeds approaching 6 knots thru the narrows. I can’t imagine what some of the more infamous rapids like Devil’s Hole, Whirlpool and Seymour would have looked like with speeds approaching 19 knots.
The following are from Kenneth Narrows.
And finally, this pretty but somewhat eerie shot with a low lying bank of fog. There was nothing else like this any where around the area.