Understanding the Astronomical Tide John Brubaker Virginia Institute of Marine Science
July - Windmill Point Date Day Time Height Time Height Time Height Time Height Time Height 07/01/2009 Wed 01:21AM LDT 0.2 L 06:58AM LDT 1.0 H 01:19PM LDT 0.1 L 07:36PM LDT 1.3 H 07/02/2009 Thu 02:21AM LDT 0.2 L 07:55AM LDT 1.0 H 02:13PM LDT 0.2 L 08:31PM LDT 1.3 H 07/03/2009 Fri 03:16AM LDT 0.2 L 08:49AM LDT 1.0 H 03:05PM LDT 0.2 L 09:22PM LDT 1.3 H 07/04/2009 Sat 04:07AM LDT 0.2 L 09:39AM LDT 1.0 H 03:54PM LDT 0.2 L 10:10PM LDT 1.3 H 07/05/2009 Sun 04:53AM LDT 0.2 L 10:26AM LDT 1.0 H 04:41PM LDT 0.2 L 10:54PM LDT 1.3 H 07/06/2009 Mon 05:35AM LDT 0.2 L 11:09AM LDT 1.1 H 05:26PM LDT 0.2 L 11:35PM LDT 1.3 H 07/07/2009 Tue 06:15AM LDT 0.2 L 11:49AM LDT 1.1 H 06:08PM LDT 0.2 L 07/08/2009 Wed 12:13AM LDT 1.3 H 06:52AM LDT 0.2 L 12:28PM LDT 1.1 H 06:48PM LDT 0.2 L 07/09/2009 Thu 12:50AM LDT 1.3 H 07:28AM LDT 0.1 L 01:06PM LDT 1.1 H 07:28PM LDT 0.2 L 07/10/2009 Fri 01:25AM LDT 1.3 H 08:03AM LDT 0.1 L 01:42PM LDT 1.1 H 08:07PM LDT 0.2 L 07/11/2009 Sat 01:59AM LDT 1.2 H 08:37AM LDT 0.2 L 02:19PM LDT 1.2 H 08:48PM LDT 0.2 L 07/12/2009 Sun 02:35AM LDT 1.2 H 09:11AM LDT 0.2 L 02:58PM LDT 1.2 H 09:30PM LDT 0.3 L 07/13/2009 Mon 03:13AM LDT 1.2 H 09:48AM LDT 0.2 L 03:39PM LDT 1.2 H 10:18PM LDT 0.3 L 07/14/2009 Tue 03:55AM LDT 1.1 H 10:28AM LDT 0.2 L 04:26PM LDT 1.3 H 11:11PM LDT 0.4 L 07/15/2009 Wed 04:44AM LDT 1.1 H 11:14AM LDT 0.2 L 05:19PM LDT 1.3 H 07/16/2009 Thu 12:11AM LDT 0.4 L 05:40AM LDT 1.0 H 12:08PM LDT 0.2 L 06:18PM LDT 1.3 H 07/17/2009 Fri 01:16AM LDT 0.3 L 06:42AM LDT 1.0 H 01:07PM LDT 0.1 L 07:22PM LDT 1.4 H 07/18/2009 Sat 02:21AM LDT 0.2 L 07:48AM LDT 1.0 H 02:10PM LDT 0.1 L 08:27PM LDT 1.4 H 07/19/2009 Sun 03:24AM LDT 0.2 L 08:53AM LDT 1.1 H 03:14PM LDT 0.0 L 09:29PM LDT 1.5 H 07/20/2009 Mon 04:23AM LDT 0.1 L 09:55AM LDT 1.2 H 04:16PM LDT -0.2 L 10:28PM LDT 1.6 H 07/21/2009 Tue 05:17AM LDT -0.1 L 10:54AM LDT 1.3 H 05:16PM LDT -0.2 L 11:24PM LDT 1.6 H 07/22/2009 Wed 06:08AM LDT -0.2 L 11:49AM LDT 1.4 H 06:13PM LDT -0.2 L 07/23/2009 Thu 12:17AM LDT 1.6 H 06:58AM LDT -0.2 L 12:43PM LDT 1.4 H 07:09PM LDT -0.2 L 07/24/2009 Fri 01:08AM LDT 1.6 H 07:45AM LDT -0.2 L 01:35PM LDT 1.5 H 08:04PM LDT -0.2 L 07/25/2009 Sat 01:58AM LDT 1.5 H 08:33AM LDT -0.2 L 02:27PM LDT 1.5 H 08:59PM LDT -0.1 L 07/26/2009 Sun 02:48AM LDT 1.4 H 09:20AM LDT -0.2 L 03:20PM LDT 1.5 H 09:55PM LDT 0.0 L 07/27/2009 Mon 03:38AM LDT 1.3 H 10:08AM LDT -0.1 L 04:13PM LDT 1.4 H 10:52PM LDT 0.2 L 07/28/2009 Tue 04:30AM LDT 1.2 H 10:57AM LDT 0.1 L 05:08PM LDT 1.4 H 11:51PM LDT 0.3 L 07/29/2009 Wed 05:25AM LDT 1.1 H 11:50AM LDT 0.2 L 06:07PM LDT 1.3 H 07/30/2009 Thu 12:51AM LDT 0.4 L 06:23AM LDT 1.0 H 12:45PM LDT 0.2 L 07:06PM LDT 1.3 H 07/31/2009 Fri 01:52AM LDT 0.4 L 07:22AM LDT 1.0 H 01:42PM LDT 0.3 L 08:05PM LDT 1.3 H All times are listed in Local Standard Time(LST) or, Local Daylight Time (LDT) (when applicable). All heights are in feet referenced to Mean Lower Low Water (MLLW).
Hourly predicted tidal levels for Windmill Point
3-week calendar of predicted tides for Windmill Point Why: tides? two tides each day? are consecutive tides sometimes similar? are consecutive tides sometimes different? are times of high and low tide later each day? does tidal range vary through the month?
3-week calendar of predicted tides for Windmill Point Why do we care? Storm surge adds on to astronomical tide level. Impact depends on storm timing relative to tide timing.
Predicted astronomical tides at Windmill Point for July 2009
Rotation of the Earth-Moon system
Distribution of tide-producing forces over the earth. Moon
Horizontal ( tractive ) tide-producing forces move water effectively. Vertical tide-producing forces can t compete with earth s own gravity.
Equilibrium tide Completely water-covered Earth. Water adjusts to equilibrium with resultant tide-generating force at all locations Equilibrium shape for water surface is ellipsoid with bulges toward and away from Moon. Water maintains equilibrium shape aligned with moon, even though Earth spins. Not entirely realistic (obviously!), but helps explain some tidal phenomena.
Why tides, and why two cycles per day? Earth s daily spin takes a given location on Earth through two bulges (high tides) and two thin sections (low tides) of the water layer. Two cycles/day implies half-day period: semi-diurnal tides.
Sizes of consecutive tides is influenced by the Moon s orbit. 1 3 4
Why are consecutive tides different on some days, similar on other days? Large declination of Moon (positions 1 & 3) produces strong diurnal inequality ( Tropic tides ). Zero declination of Moon (positions 2 & 4) produces no diurnal inequality ( Equatorial tides ). 1 3 4
Predicted astronomical tides at Windmill Point for July 2009 Moon declination: -26.5º tropic 0º equatorial 26.5º tropic 0º equatorial
Why are times of highs and lows later each successive day?
Why variation of amplitude within a month? Add Sun: combined tidal effects of Moon and Sun lead to spring-neap cycle. Spring tides Neap tides (b)
Spring tides Neap tides
Timing of the spring-neap cycle.
Predicted astronomical tides at Windmill Point for July 2009 Moon phase: Full spring Last Q neap New spring First Q neap
Another cycle within the Moon s orbit: earth-moon distance. Tidal range is: greater at perigee smaller at apogee
Predicted astronomical tides at Windmill Point for July 2009 Moon declination: -26.5º tropic 0º equatorial 26.5º tropic 0º equatorial Moon phase: Full spring Last Q neap New spring First Q neap Moon distance: 406,232 km apogee 357,464 km perigee