Causes of Tides – Gravity of Moon and Sun
Spring Tide
Neap Tide
Moon has main
gravity effect
(2.25 x Sun)
New Moon
1st
Quarter
2 tides per day
@ 6.25 hrs from
HW to LW
2nd
Quarter
Full Moon
Hour
1 2 3 4 5 6
1/12
6
Rule of
Twelfths
Spring
range
2/12
5
Neap
range
4
3/12
3
3/12
2
2/12
1
1/12
6.25 hrs approx
Tide definitions
Charted
Elevation
Charted Vertical
Clearance
Highest
Astronomical Tide
MHWS
Spring
range
Mean Sea Level
MHWN
Neap
range
Surface of Sea at present
Observed
depth
Height of
Tide
MLWN
Drying
Height
Chart Datum
Lowest Astronomical Tide
Charted depth
MLWS