Context: Captain Vancouver, Loughborough Inlet, 1792 Citation: Doe Nicholas, Perigean Spring Tide, British Columbia Historical News, 25(2), pp.28-31, Spring 1992. Copyright restrictions: For reproduction permission please e-mail: [email protected] Errors and omissions: Later references: 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 Tides 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 ocean. In some Island in Desolation Sound. 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 Vancouver Island. the probe the maze of narrow channels and ers "caught napping" as it were by the principal tidal component of the Sun 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 Moon (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 Johnstone Strait 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 Earth 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 sea-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 moons. 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.