Dodge tides

 

The Australian term Dodge Tide refers to a time when at least one tide is skipped, during a neap tide.   

 

A popular description of Dodge Tides at Gulf St. Vincent, Adelaide Region, South Australia: 

https://www.abc.net.au/news/2017-02-17/the-story-behind-adelaides-dodge-tides/8279870

  

Another view showing a Dodge Tide at Adelaide:

 

Another site, like Adelaide in Gulf St. Vincent, is Edithburgh, which has a more striking Dodge Tide is exhibited.  

 

 Tide tables are not useful, at such locations. 

It's the Earth, on Earth Day

 A link to a National Geographic webpage: Milestones in Space Photography.

 

Can we agree that our home is a very, very fine home, and that we must do everything we can to preserve and conserve it?   

Several images of the Earth from space are nicely presented here.

Earth Rise

This image is of Earth Rise from the Moon is widely touted as a spark that started the Environmental Movement.  True or Not, such photographs as these have inspired many of us to reconsider the fate of our trash.

Astronauts have commented that our atmosphere appears from space as a thin layer, approximately at the same scale as a layer of paint on a basketball.  

 

Juxtaposed photographs, another approach to visualization of high and low tides.

An alternative approach to visualization of tides has been provided by Ron Hipschman, of San Francisco's Exploratorium.  A friend had asked, wasn't there a more direct way to portray the high and low tides, than a graph, or just numbers?  

These two panoramas were compiled from series of photographstaken at high and low tide.  The subject is Pier 15 (site of the Exploratorium), on the left, and Pier 17 on the right, at San Francisco.   I don't know the dates and times.  These were obviously taken from out in the Bay, probably from a boat.  How else?

Satellite Altimetry: Tide Gauge Network on Steroids // Sea Level Rise

 This article caught my eye.  

 

 https://earthobservatory.nasa.gov/images/147435/taking-a-measure-of-sea-level-rise-ocean-altimetry

 

Here's a link to the whole Earth Observatory Sea Level Rise collection 

 

I am remembering that in 1996, when I started teaching Environmental Science on the island of Saipan, it was difficult to find news articles online to print for student use.  There were very few.  Even before this, I had taught Environmental Science for a number of years in Chuuk Lagoon, predating, I think, the Internet.  Needless to say, there was Zero presence of news articles about Sea Level Rise.

Sadly, there are places on the globe, even today, where sea level rise is an actual fact of every day life, but where no Internet resources are available to educate or prepare the islanders or their decision making leaders.  

It is a profoundly obscene actuality: publishers of scientific journals want to charge even those islanders---assuming any ability to access them through the Internet---up to or more than 52.00 for a single article.  These islanders do not use or carry money.  

Such arrogance.  How arrogant we are.

Declination of the Sun

Over the course of a year, to an observer on Earth, the Sun travels from extreme North in June to South in December, and back again.  The Seasons are determined by these changes in the Declination of the Sun---its angle North or South of the Equator.  Tides respond to the Declination of the Sun as well.   The following animation was constructed from photographs of the Earth over time from the  perspective of an observer beyond the Solar System..  . 

 

https://twitter.com/weatherdak/status/1274383843215110145

 

In reality, it is the Earth that is moving, relative to the Sun. 

 

 

Space Age Understandings of Tides

Tide science long suffered from a lack of measurements of tides in mid-oceans.  Am impressive number of tide gauges have dotted the coasts.  In the central Pacific, for example, tide records have been available only for isolated, scattered islands.  In recent years, satellites have been able to measure sea level with an accuracy of a few centimeters at thousands of points all over the globe.

William Whewell led one of the first science projects: he organized volunteers all over the globe to measure the tide.  In her blog, Caren Cooper describes his "great tide experiment":

With the consent of the British Admiralty, Whewell coordinated thousands of people in nine nations and colonies on both sides of the Atlantic in the synchronized measurement of tides. At over 650 tidal stations, volunteers followed Whewell’s instructions for measuring tides every 15 minutes, around the clock, during the same two week period in June 1835. Volunteers in the “great tide experiment” included dockyard officials, sailors, harbormasters, local tide table markers, coastal surveyors, professional military men, and amateur observers.

Drawing on this data, Whewell compiled a tide map:

Whewell brought it all together into maps illustrating how the tides progressed across the Atlantic Ocean and onto shores, inlets, ports, and into rivers and estuaries.

With no measurements mid-Ocean, he had to fill in the gaps with guess work and deduction.   Later, in 1909, Rollin Harris had more data available when he produced the following map in Popular Science Monthly of the tides of the Atlantic.  The lines represent the progress of the tide over time, each line representing one hour. Roman numerals represent the hours.

Until quite recently, knowledge of the tides of the central Pacific Ocean were limited to records of tide gauges on islands.  Whewell deduced certain features from known features of the tides around the coasts of the Ocean.   The following shows a global map, somewhat more recent.

 

 The following figure is a much more modern iteration of such a map based on satellite altimetry, from NASA.  

 A particularly effective illustration of Amphidromes is found on the University of Pennsylvania website.

 The tides of Tahiti are famed for their minimal amplitude range, and have drawn the attention of a number of tide scientists in the 19th and Early 20th Centuries, due to this and other peculiar characteristics.  Captain James Cook remarked, in Tahiti, that "The tides are perhaps as inconsiderable in these Seas as in any part of the world."  In attempting to explain seeming anomalous times of high and low tides in  Cook's  measurements, Henry Maskelyn, a Royal Astronomer, suggested that measuring the tides at certain times was problematical:

The times of high and low water seem to be subject to great irregularity on particular days; no doubt owing to the small rise of the water ... which renders it more liable to be disturbed by the action of the winds and other causes: part of the irregularity may be attributed to the difficulty of observing the time of flood or ebb, with any degree of certainty.

The following  two animations are models of global tides for two days, the first  near the time of 1st Quarter Moon of early March, 2020, during a period of Neap Tides.  The second represents a period of Spring Tides, when the moon is Full in April 2020, during Spring Tides.  I have taken the liberty of modifying  Svetlana Erofeeva's animation by position a red star near the position of Tahiti, to illustrate the relationship between the "inconsiderable" tides of Tahiti and the nearness to the Amphidrome. 

Neap Tide

Spring Tide

Spring Tide.  A red star is placed near Tahiti.

This animation shows water sloshing around the oceans, under the influence of the Moon and Sun.  Because of the shape, size, and depth of the Pacific Basin, the movement at Tahiti, near the Star, is indeed inconsiderable. T  hese animated models amount to  Space Age tide tables.  Availability of satellite sea level data has made it possible to predict tides even in mid-Ocean, where, for Centuries, no tide data, or very little, could possibly exist. 

  These two animations are modified versions of the TPXO9 model products of Oregon State University. These animations By Svetlana Erofeeva and Gary Egbert cannot be overestimated in their contribution to modern tide science.  My  general understanding of the workings of the tides has benefited immensely from their study.  Their work is at the forefront of the long history of the science of tides.

Dark background: Tritanopia and normal