Context: Captain Vancouver, Loughborough Inlet, 1792

Citation: Doe Nicholas, Perigean Spring , Historical News, 25(2), pp.28-31, Spring 1992.

Copyright restrictions: For reproduction permission please e-mail: [email protected]

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Date posted: April 2, 2011.

Author: Nick Doe, 1787 El Verano Drive, Gabriola, BC, Canada V0R 1X6 Phone: 250-247-7858, FAX: 250-247-7859 E-mail: [email protected]

Perigean Spring Tide

by Nicholas A. Doe

In late June and early July 1792. Cap­ times a year. In recent times, particular­ because of their length and shape. tend tain Vancouver's ships HMS Discovery ly large Perigean Spring have been to swap water back and forth, see-saw and HMS Chatham were anchored in accompanied by dire warnings of im­ fashion, in sympathy with the twice dai­ the Teakerne Arm near West Redonda pending earthquakes which. some seis­ ly tides of the open . In some Island in . From here mologists suggest. may be triggered by places. near the pivot point at the south­ the explorers set out in small boats to tidal forces. Not only were the explor­ eastern tip of . the probe the maze of narrow channels and ers "caught napping" as it were by the principal tidal component of the inlets that lay between them. the John­ unusual height ofthe tide, they also had (Pl:Kl) is actually greater than that of stone Strait. and the open Pacific be­ apparently not noticed that the Yaculta the (M2); and in my home town yond. The last of these expeditions was and Dent Rapids are a transition point ofWhite Rock beachgoers delight in the led by James Johnstone and Spelman between the tidal waters of the Strait of fact that the tide is always at least par­ Swain. who on Tuesday. July 3rd (by Georgia to the south, and those of the tially out at noon in the summer regard­ Captain Vancouver's reckoning). set out to the north. and that less ofthe Moon's waxings and wanings. in the Chatham's cutter and launch to there is a marked difference in the tim­ Spring Tides occur whenever there is a explore the mainland coast. They took ing of the tides on either side of the full or new moon. They are larger than with them enough supplies for a week. rapids. usual because, for a few days. the lunar The two boats made their way through Many factors go into determining the and solar tides are synchronised. Perige­ the Yaculta Rapids and along the Corde­ level of the tide - so many that each an Spring Tides occur when, simultane­ ro Channel to the entrance of Loughbo­ day's tidal cycle is almost never repeated ously, the Sun. Moon and are rough Inlet. They entered the inlet and in all its detail. My own interest in the aligned, and the Moon is at its closest camped for the night. We know that tides ofjuly 1792 stems from a kayak­ point to the Earth in its orbit around this must have been on the evening of ing trip I am planning to make some the Earth. Because the Moon is doser. July 4th as that was the day they passed day. which will involve a circumnaviga­ its contribution to the tide is larger than the entrance to the Nodales Channel, tion of Vancouver Island: it would, I usual. There is a similar effect for the and they spent the whole of the next thought. be interesting to try to time Sun. but because the Earth's orbit is day, the 5th, examining Loughborough my passage under approximately the very nearly circular, the effect is less Inlet. Vancouver records that that same tidal conditions as pertained 200 pronounced. night, i.e. the night of the 4th! morning years ago. I was also puzzled as to why Perigean Spring Tides are often asso­ of the 5th. the crew were "incommod­ such keen observers of the Moon and ciated with major flooding. particularly ed" by the flood tide which they had ex­ tides as our 18th century friends should when accompanied by strong onshore pected to be low. as the Moon was then have been so taken by surprise that winds. The rise ofthe tide is accelerated passing the meridian. Archibald Men­ night. because when the Moon is aligned with zies, the expedition's naturalist. also re­ Loughborough Inlet is deep. has steep the Sun, the Sun's gravitational field dis­ corded the event. In a diary entry for sides, and almost no islands. There are torts the Moon's orbit, making it more July 12th, the day Johnstone and Swain few campsites; there is therefore a good elliptical. so that the Moon swings by returned to the ships, he writes that: possibiliry that Johnstone and Swain the Earth closer than is normal at peri­ "in this arm they stopped the second eve­ camped near the mouth of Gray Creek gee. As it does so, its orbital velocity in­ ning and thought themselves secure .from (l25°32'W. 500 32'N)j two small is­ creases. and because the Moon's orbital any disturbance by pitching upon a small lands there are marked on both British rotation is in the same direction as the island fOr their place of rest, but in the and Spanish charts. If they found this Earth's axial rotation, the Moon appears middle of the night they were hastily site especially welcoming because of to "dwell" in the sky and the lunar tidal roused.from their repose by the flowing of mats ofsoft. green -grass. the author forces, enhanced by the close passage of the Tide, which had risen so much higher can vouch for the fact that they were the Moon. are given extra time to do than they expected 6- rushed (sic) upon not the last to make such a mistake! their work. them so suddenly, that every person got The Moon, as is well known, is the Whilst Captain Vancouver was survey­ completely drenched before they could re­ main cause of the tides; but the Sun also ing the coasts of British Columbia and move to higher ground. " makes a significant contribution. Theo­ Alaska in the 1790s, he reckoned his The tide that so "incommoded" the retically. the solar tide is only 46% the time as being 16 hours ahead of Green­ explorers was an interesting example ofa strength of the lunar tide. but in coastal wich. not as we do today eight hours be­ Perigean Spring Tide. Such tides occur areas. and in narrow passage ways. this hind. Consequently we can identify the at irregular intervals about two or three ratio is often enhanced. The Straits of night of the flood as actually being the Georgia and Juan de Fuca. for example. night of the 3rd I morning of the 4th,

B.C. Historical News· Spring 92 28 July 1792 Gulian Day 2375759.8). Sun, by the gravitational Figures 1,2, and 3 show the astronom­ anomalies of the Earth, Figure 3: Sun &Moon Declinations and by the other planets ical conditions for these two days. 30 rl)egr...=....:.88.:..:.·------.,,------,N Figure 1, records the angular distance of the solar system. The JUy 3nl PST JUy'" PST average time between 501«-""'- between Sun and Moon. An angular ZO distance of 0° corresponds to an eclipse close approaches to the of the Sun, and an angular distance of Earth, perigee, is 27.5 10 180°, to an eclipse of the Moon. The days in contrast to the Figure shows that there was a full moon 29.5 days between new o f---+-----.:\---+--Jf--~!'!L-\______..j on the night of the flood, Ouly 3rd or full . Conse­ 2300 PST), but an eclipse was missed, quently perigee seldom as it often is, by a few degrees. coincides with a new or Figure 2 plots the distance between the full moon, but as Figure ·20 s Earth and the Moon. Distance is signif­ 2 shows, on the night of -30f-LL'-'+'u...u.+'-"-.J..J..t-u...u'+'-'""'-'+.LU..y..w~J....LL-'-t-'-LL..I..jf_1_L.l_'_t_'_.LU..~ icant because the closer the Moon is to the flood it did. In fact 6/1 6/6 8m 6/16 6/Z1 6/26 711 7/6 7/1 1 7/16 7121 7126 the Earth, the stronger is the lunar tidal perigee came just 1 hour Month/day 1792 (ETaOO:OO) force, so much so that each 1% decrease before full moon, a very unusually close co- the time of month when the Moon's Figure 1: Phase of the Moon incidence. declination is at its most northerly (posi­ Figure 3 shows plots of tive) value and it is lunar mid-summer. 1eo rSU_n._Moon_Ang...... ::.U_Iar_Dis_·_tanc8----e(:...0eg..::"""'----')----e---r --, the Sun's and Moon's At other times, the Moon appears very declinations. The decli­ low on the horizon, even at midnight. nation of a heavenly This is lunar mid-winter. body is one of those in­ Figure 3 shows that at the time of the timidating terms that is flood, it was lunar mid-winter. This is actually fairly simple. It no surprise as the path of the Moon is is the latitude on the sur­ never more than five degrees from that face of the Earth at ofthe Sun (the ecliptic) and consequent­ 0\5 ...... •...... which the body appears JIAy 3rd PST JI6y 4th PST ly, the lunar season is always the oppo­ directly overhead. Thus, site ofthat of the Sun at full Moon, and if the declination of the the same as that of the Sun at new oru-u..p..LL.Lt-U-,-,+,u....u-t.J..L.J..J..t-U-U4-J...l.JllfJ...L..L1..f-I-I..l..4.J.J..l.-'-t-'-J...LL.ifJJ..U Sun is zero, it appears di­ moon. However, the high positive and IVl &'6 IVl1 8118 6/Z1 6/Z6 7/1 7/6 7/11 7/16 7121 7/26 Month/day 1792 (ET.00:00) rectly overhead at noon negative declinations of the Sun and on the Earth's equator. Moon had two effects on the tide on the in distance results in a 3% increase in This is the time of the night of the flood. Firstly, because the force. Most of the variation of distance equinoxes. In the (northern) spring­ line joining the Moon through the cen­ is a consequence ofthe Moon's approxi­ time, the declination of the Sun slowly tre ofthe Earth to the Sun, was strongly mately elliptical orbit around the Earth, increases until it reaches a positive maxi­ tilted with respect to the equatorial and I say approximately, because the mum on mid-summer's day. The Sun plane, the levels of the two daily tides smooth predictable curve beloved of is then directly overhead at noon on the were appreciably different. This may mathematicians is constantly perturbed Tropic of Cancer at latitude 23°27'N, have contributed to the element of"sur_ in a very complicated manner by the and because the northern half of the prise". The other effect was that be­ Earth is tilted towards the cause at the peaks of the declinations, Figure 2: Earth-Moon Distance Sun, it gets warmer there. the rate ofchange ofdeclination is zero, The Moon goes all of the orbital motion was directed in 1E·3 AU 1000'0 km 3,..------r------=-.:..:...:..-.:..:., 450 through exactly the same exactly the same direction as the Earth's July 31d PST July 4th PST cycle as the Sun, except axial rotation, thereby maximizing the

2.8 4Z0 that it does so once a effect of the increased velocity due to APOGEE month instead of once a the approach of perigee. The increased year, and the angles are a velocity prolonged the length of the ti­ Z.6 390 little different and not so dal day by 12 minutes at the time ofthe constant. Probably every­ flood, three minutes of which was, by . 360 one has noticed that my calculations, attributable to the fact '~330 sometimes, particularly that the Moon had reached its most Z.Z ...... ' during the winter, the southerly declination. Twelve minutes I Moon appears very high may sound insignificant, but when the Z f-LLl.-'-t'-'..L.l+J +'-'-'-'+' ", I !", I'" ! I' " 1300 in the sky, rising in the tide on a gently sloping beach is rising at 6/1 6/6 6/16 61Z1 6126 7/1 7/6 7/11 7/16 7121 7126 norch-east and setting in a rate ofseveral vertical feet per hour to­ Month/day 1792 (ET.00:00) the north-west. This is wards one's campsite, it does not seem

29 B.C.lIbtorical News ·Spring 92 Figure 4: Tides Loughborough Inlet & Desolation Sound phy, the highest tide of on Botanical Beach near Port Renfrew the day at Redonda Bay based on the timing of the tide at Am­ Feel 20 ------immediately followed the bleside Beach in West Vancouver! It Jufy 3rd July 4th 179'2 lowest, whilst at Heydon was a long way to go to see surf sweep­ R__ Bey TH FLOOD DnolotJon Sound, " Bay in the inlet, the re­ ing up to the salal at the top of the 1S ...... L.\. verse was true. beach. / For those interested in Calculating the delay between the

. 10 the relative contributions tides at different places is not quite as ofvarious components of straightforward as it may seem. Because the tide that night, I have the pattern of the rise and fall varies S plotted in Figure 5 the from day to day, and from location to semi-diurnal (Le. twice location, any comparison based on the daily) and diurnal (Le. timings of a particular point in the cy­ 9 12 1S 1S 21 0 3 6 9 12 1S 1S 21 once daily components) cle, high high water (HHW) for exam­ Pacific Standard Time of both the solar and lu- ple, is likely to give a different answer nar tides. The Moon's from a comparison based on the timings diurnal and the Sun's ofsay low low water (LLW). What we diurnal and semi-diurnal need is a comparison method that in­ Figure 5: Components of the Tide components contributed cludes all of many cycles, not just one equally to the "incom- particular point. 4 rFeet He_yd:...-O"_Ba..:.y_-Lo---=Ugh:....bor..:....:..:OU~gh_..:.lnI...:.:.;et THE FLOOD modity", while the Engineers have long since had the so­ Moon's semi-diurnal lution to problems of this sort - what component contributed they do is to look for the peak in the as much as these three cross-correlation function of the two components together. patterns. This sounds terribly technical, The next morning, all but in fact is quite simple. Imagine you four components were had two rolls of film each ofwhich had -1 close to their minima, been exposed to a light whose intensity and the tide was within varied with the height of the tide at the -3 . inches of being as low as two separate locations. The dear patch­ ""'rardln2 JIAy 4th it ever gets. es on the films would correspond to low ~r-'--L-+--L.-L-+--'---''_+-'-_'__j--U'-'-+___'--L._t_-'-'--4--'--'--l 12 16 IS 21 0 3 6 9 12 During their passage tide. The pattern of light and dark Pacific Standard Time through the rapids, John- would be different on the two films, but stone and Swain had to find a best match, you could lay the that way at all! moved from the waters of the Strait of films together, hold them up to the Figure 4 shows the tide that resulted Georgia to those more akin to the open light, and then slide one strip of ftlm from these particular alignments, and coast. They sure enough, shortly after midnight at had obviously 0054 Local Apparent Time (0121 pSn, observed the Figure 6: Tidal Stations - Vancouver Island (see Figure 7) there was a tide exceeding 15 feet in tides of the Loughborough Inlet when the Moon Strait quite Britiob Columbia CANADA was 13° past the meridian (i.e. past due closely, for it is south). The next morning at eight, the a general rule tide sank to the lower low water level for there that large tides. Spring Tides At Redonda Bay, near where the ships are low when were anchored, the evening tide on the the Sun or 3rd peaked between six and seven Moon are due south. Howev­ o'clock, which would be a good time to NORm make camp. Unfortunately, in Lough­ er, on the open PACIFIC OCEAN borough Inlet the tide at this time had coast it is vety already been ebbing for several hours different. and it began to flood again a little more The author than an hour later. The evening ebb first became may not have been obvious because the aware ofthis af­ evening low tide in the inlet was much ter planning a higher than the morning low tide. It is very unsuccess­ also interesting to observe in Figure 4 ful trip to see that, because of differences in topogra- the tidepools

B.C. HIstorical News· Spring 92 30 Figure 7: Tidal Delay - Vancouver Island the ends ofthe rapids. As shown in Figure 7, Johnstone and Minutes (late on Tofino) Hours Swain in a short journey, had moved 300IiieSOlaii"On-s,,-uiidl- 15 from a tidal region where the presence ~C' I 270 - I e, ! of the Moon due south, signalled low I tide to one where, the tides being a sub­ 240 - I 4 Dent RaPlds stantial fraction of a 13 hour semi­ 210 - I l diurnal tidal day earlier, it signified al­ Upper/Lower Rapids I' most exactly the opposite. 180 I 13 Could the flood have been foreseen? !------II:IE.YD.QlUIAY 150 loughborough'lnlet ibwen Bay i Most certainly yes. The movement of

leA) 1 the Moon was closely observed by Cap­ 120 .I 12 tain Vancouver, which he used almost 90 ­ exclusively for fIxing his longitude. The o see Figure 6 unusual alignment of Sun and Moon at 60 - perigee was not only tabulated in his 30 - G) Nautical Almanac, but exaggerated, as Totino noted in Figure 8. The series of tidal Or.l-~~,....--r'--"" -10 rapids obviously marked connecting o 10 20 30 40 50 60 70 80 90 100 % Distance trom Totino (clockwise) points between substantial bodies ofwa­ ter. Possibly everyone was too busy to notice: the expedition lacked the pres­ over the other until the maximum and fall of the open ocean; when it is ence of a professional astronomer, and amount of light could be seen through high tide at Tofino, it is within an hour as Vancouver remarks in his Journal on the two ftlms. The offset of the two and a half of being low tide in the hearing the news of the death of the as­ ftlms is then a measure of the time delay Strait. tronomer William Gooch, who was to between the two patterns. Consequently at either end ofthe Strait, have joined, the expedition in August Using a computationally equivalent water pours in and out continuously 1792: technique, I have plotted in Figure 7 the through the narrow confines ofthe Gulf relative time delay between the tides at and San Juan Islands to the south, and ". ..we had little leisure for making such miscellaneous observations as would be TOfillO and the various points around the and Desolation Vancouver Island shown in Figure 6. Sound Islands to the north. The back very acceptable to the curious, or tend to The picture that these calculations paint and forth flow along the Strait ofJuan the improvement ofastronomy" '~ is as follows. Envisage the Strait of de Fuca is fairly evenly distributed, but Perhaps we should add . .or keep the Georgia as an inland sea whose level ris­ through the narrow channels of the crew's bedrolls dry". es and falls with little variation in the north the flow becomes, almost literally, ********** timing of the tides around its shores. precipitous, with no let up in the pow­ The rise and fall of this inland sea is erful and turbulent currents that result The author is an engineer living in White Rock. His interests inclMde sea-kayaking, and J8th cen­ close to being in antiphase with the rise froth the differing heights of the tide at tury navigational techniques.

ACKNOWLEDGEMENTS The author gratefully acknowledges the help of Dr. Myles Standish of the Jet Propulsion Laboratory. Pasa­ dena who supplied an accurate Ephemeris for the Moon 1792 (DE-118 + LE-062), and also of Mike Foreman and Fred Stephenson at the Institute of Ocean Sciences, Sidney BC who supplied harmonic constants, sample predictions and other useful data for the tidal calculations.

SELECfED BIBUOGRAPHY Doodson, A.T. & Warburg, H.D., Admiralty Manual ofTides, HMSO London 1941. Forrester, W.O., Canadian Tidal Manual, Dept. of Fisheries & , Ottawa 1983. FIGURE 8: Captain Vancouver's Nautic;lI Almanac shows the Moon's parallax peaking at 61'32' on Lamb, W. Kaye Ed., A Voyage ofDiscovetyto the the night of the 3rd July 1792 (Green\\ieh timel_ Parallax is a measure of the closeness of the Moon North Pacific Ocean 1791 - 1795, , to the Earth and was an impnrlanl Iigun: in IRth century navigational calculiuinns. The tabulated Hakluyt Sociery, London 1984. parallax is the maximum value that can c\'cr ne achit:\'ed! a very rare event. The;: Moon comes this close Newcombe, C.F. Ed., Menzies' Journal ofVancouver's to us only once or twice a century, the last lime being in 1912. Howc\o·cr. on this particular occasion. Voyage April to October 1792, Victoria BC 1923. the Nautical Almanac is in error: the l:orrect figure was 61'26". The avcragc value of IUllar parall"x is 57'u3" At lhe July \7')2 perigee the Moon was 8% closer than Wood, ].F., The Saategic Role ofPerigean Spring average, and the lunar tidal forces 2Y'~ strunger than average. Tides in Nautical Historyand North American Coast­ Also tabulated in the Almanac is the apparenl size (semi·diametre) of the 1I.">on's disc, al Flooding, 1635 - 1976, US Dept. ofCommerce, 1976.

31 D.C. Illstorlcal News ,Spring 92