Tide Prediction: People going into or out of a harbor, or anchoring near a shore
ID: 289799 • Letter: T
Question
Tide Prediction:
People going into or out of a harbor, or anchoring near a shore, need to know in advance about the behavior of the tide. The tide is caused by the gravitational pull of the moon and the sun on the oceans, and the rotation of the earth, but its exact pattern at any particular spot on the coast depends very strongly on the shape of the coastline and on the profile of the sea floor nearby. So even though the forces that move the tide are well understood, the tides at any one spot are essentially impossible to calculate theoretically. What we can do is to record the height of the tide at that spot over a long period of time, and use these measurements to predict the tides in the future.
A Typical Tidal Record:
The graph below shows the tidal record for two weeks (January 1-14, 1884) at Bombay, India. The tide was recorded on a cylindrical sheet that turned once every 24 hours. Each daily curve is labeled with its date. Some obvious features: Note that on the graph, time increases to the LEFT.
there are usually two high tides and two low tides each day;
the tides come about 50 minutes later each day;
during the two-week period there is considerable variation in the daily pattern of highs and lows;
there is usually a difference in height between two consecutive high tides (the diurnal inequality).
Procedure:
1. Learn how to follow each date, starting with January 14, by outlining the date curve with a colored pencil. Now use this website to find the phases of the moon for these dates, Jan 1 – 14, 1884. http://www.timeanddate.com/calendar/?year=1884&country=35.
2. Match the appropriate moon phases from the website with spring and neap tide dates and times from the graph, for the entire month of January. You have 2 weeks of data on the graph, and must extrapolate backwards in time to find the remaining possible tide heights. Type your answers into the chart below:
Moon Phase Tide Height Date Time
Spring tide:
Neap tide:
Spring tide:
Neap tide:
From George H. Darwin's article "Tides" in the 9th edition of the Encyclopaedia Britannica. Note that in this graph, time increases to the left.
Please type the answer.
Midnight P M A M 4 rt. ort. ft. Fio. 3.-Tide-ourve for Bombay from tho beginning of tho civil ycar 18S4 to the midnlght cading Jan. 14, 1884, or from 12b Dee. 31, 1383, to 1 Jan. 14, 1884, astronomical tiu Zere of ungeExplanation / Answer
We know that the spring tide is in the Full moon or the New moon and the neap tide is at the first and third quarter of the moon cycle. From the website we know the 6th of Jan was the first quarter, 12th Jan was Full moon 20 Han was the Third quarter and 28th Jan was the New moon. Considering a quarter of moon cycle as an average 7 days 10 hrs, let us complete the table:
Moon Phase Tide Height Date Time Spring tide: Full Moon 18.5 ft 13th Jan 12:30 AM Neap tide: First Quarter 13 ft 5th Jan 3:30 PM Spring tide: New Moon 18.5 ft 27th Jan 8:30 PM Neap tide: Third Quarter 13 ft 20th Jan 10:30 AM