For those of us who tend to be fascinated watching the tide flow in and then flow out again, Monday, September 28 holds a treat for us. This September 28, one day after a full lunar eclipse, the Earth will experience the lowest and highest tides in 18.6 years. We will not experience tides this extreme again until 2034.
The astronomical events that cause the tides work in various periodic cycles. The Moon has the greatest influence on the tides, but it is not the only one. The Sun also plays a role and when the two bodies work together it is called a spring tide (nothing to do with seasons) and when they work at 90 degrees to each other it is called a neap tide. Let’s examine these cycles.
- Spring tides are linked to the full and new Moons when the solar and lunar tides are aligned, on average every 14.77 days or half the synodic cycle.
- The Sun’s influence on the Earth-Moon system results in an especially small lunar perigee when the major axis of the lunar orbit aligns with the Sun. This happens every 206 days.
- Perigean Spring tides occur near the time of an equinox at 4.43 year intervals, half of the 8.85 year period of the turning of the major axis of the lunar orbit.
- The plane of the lunar orbit is tilted 5 degrees to the ecliptic plane. The Sun’s gravity acting on the Moon causes the lunar orbit to wobble like a child’s top. The period of the resulting retrograde precession of the lunar nodes relative to the equinox is 18.61 years.
All of these above mentioned mechanisms come to be on September 28 which will result in the extreme tide I mentioned at the beginning. The best place to observe this is the Bay of Fundy where the tide will rise over 55 feet or 16.8 metres. Our local tides will be noticeably higher than usual for observers like myself who like to watch the water come in and go out. Others who might notice are dock workers. Some of the older wharfs in the area might actually be submerged at the peak of the tide.
The tide charts for Vancouver predict the lowest tide is at aproximately 12:30 pm and the highest tide of the day will be at aproximately 6:40 pm. Please check tide charts on the day as well as tide charts for your local spot as the geography of our coastline influences the timing of the tides. For those of you stuck in office buildings on the 28th, take your binoculars and have a peek at the water front from time to time to watch the tide.
For lots more detailed information about the tides please refer to the Royal Astronomical Society of Canada’s Observer’s Handbook 2015. Almost all the information above was taken from this book.
Leigh Cummings