High tide in Sydney Harbour: the secret astronomical facts

Tide gauge at Fort Denison, photo by Nick Lomb

Tide gauge at Fort Denison, photo Nick Lomb

Tide gauges have been measuring tides at Fort Denison ever since George Robarts Smalley, Government Astronomer at Sydney Observatory set one up in 1865. Tomorrow (Wednesday 4 June 2008 at 8:38 pm the high tide is predicted to peak at 2.08 metres. This is the highest predicted tide for several decades though another high tide is expected next year on 25 June. Today’s Sydney Morning Herald reports on the high tide, but the astronomical alignments that cause the high tide are not mentioned as they were deemed too difficult. Here they are:

Each month there are two lots of high tides, one at new Moon and one at full Moon. At those times both the Sun and the Moon are pulling on the oceans in the same line though not necessarily in the same direction. If at one of those times the Moon happens to be at its closest to Earth for the month (perigee) then it is an exceptionally high tide.

Thus the high tide is due to a coincidence betwen the synodic month (new Moon to new Moon) and the anomalistic month (perigee to perigee). The perigee occurs 11 pm Tuesday (3 June 2008) while new Moon is at 5:23 am Wednesday (4 June 2008).

What raises the tide even higher is that the Moon is almost as far north as it can go, mainly because it is winter and it is roughly in the same direction as the Sun, but also because of the Draconic month or cycle. This is node to node and called Draconic as it is related to eclipses and according to some cultures eclipses happen when a dragon eats the Moon. The draconic cycle can increase or decrease how far the Moon is north or south. On Wednesday it is making it 4 degrees further north than otherwise.

Late addendum Thursday 5 June 2008: Sydney Ports Corporation has informed me that the high tide on 4 June 2008 did not quite reach the predicted level at Fort Denison and was 2 cm less, possibly due to the poor weather. However, the predicted level was reached at Botany Bay as recorded at the Bulk Liquids Berth.

6 responses to “High tide in Sydney Harbour: the secret astronomical facts

  • Mr Jon Bell. Our pastor at C3 church has a waterfront home (tidal waterfront) and always knows when to lift his boxes of bibles and other holy literature out of the lowest level (twin garage up to the next level 1st level of storage), I recommend you come along to a C3 church and receive the holy spirit and then you will always know when your prosperity is going to be threatened, as God wants you to be rich and pay your tithe (10% before tax) so the good word can be spread

  • Hi, On Sat 30th March this year, the forecast Ft Denison tide was around 1.82m at 10pm The actual tide, by a rough measure of how nearly the water came through my front door at Woronora, was 2.02m. The creek was not flowing faster than usual, so it wasn’t catchment water, so I assume it was storm surge out at sea. Is there any other evidence of higher than usual tides along the coast? Is there a prediction system available to the public that reports such things? Living close to the water as we do, it would be nice to know a few days before a scheduled high tide if it is going to be 200mm higher than the printed data says.
    Jon Bell

  • How do I find out what the highest annually recorded tides have been since 1865.
    For example, highest recorded tide for 1865, highest recorded tide for 1866 and so on until 2009.
    Your help is appreciated.
    Brian Sampson

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