Science 7–10 · Year 7

Tides at Fort Denison: reading the Moon in the tide table

Science understanding: Earth and space sciences (NSW Stage 4 focus areas Observing the Universe and Data science 1)

Practical, model not builtLow risk

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The idea

The Moon's and Sun's gravity raise two tidal bulges, so a coast sees two highs and two lows in a lunar day of 24 h 50 min, and the highs are largest when Sun and Moon line up at new and full Moon.

What you need

1 copy of the Bureau of Meteorology tidal predictions for Sydney (Fort Denison) for one month, as printed in the Transport for NSW tide booklet with Moon phase symbols
graph paper or a spreadsheet
1 ruler and 1 calculator

How to do it

For each day of the month copy the times and heights of the high and low waters.
Compute the daily tidal range (highest high minus lowest low) and plot it against date; mark the new, first quarter, full and third quarter dates from the table symbols. Count the tides listed against each date first and set aside any date with only three, because a fourth tide that falls after midnight makes that date's range too small to compare.
For three consecutive days compute the interval between successive high waters and take the mean.
Plot water height against time for one week as a curve through the highs and lows.
Compare the range at new and full Moon (spring tides) with the range at the quarters (neap tides) and report the ratio.

What you should see

From the July 2025 Fort Denison predictions the daily range (highest high minus lowest low) is 1.70 m on the new Moon day, 25 July (low 0.28 m at 02:46, high 1.98 m at 20:46), and 1.46 m on the full Moon day, 11 July (low 0.43 m at 02:47, high 1.89 m at 20:41), against 0.81 m on the first quarter day, 3 July (low 0.63 m at 08:10, high 1.44 m at 14:43), and 1.10 m on the last quarter day, 18 July (low 0.49 m at 07:40, high 1.59 m at 14:10). Most days have two highs and two lows, and successive highs average about 12 h 25 min apart: the six intervals from 00:05 on 1 July to 02:34 on 4 July total 74 h 29 min (NOAA: the lunar day is 24 h 50 min, high tides 12 h 25 min apart). The range plot shows two peaks and two troughs across the month: peaks of 1.51 m on 12 and 13 July and 1.70 m on 25 July, and troughs of 0.81 m on 3 July, the first quarter day, and 1.10 m on 18 July, the last quarter day. Four dates list only three tides because the fourth falls after midnight (6, 16, 21 and 30 July), and their apparent ranges of 0.92, 1.09, 1.24 and 0.92 m are short by a missing tide rather than small by the spring-neap cycle: 16 July sits three days after the 12 and 13 July peak, far short of the 7.4 days of a half spring-neap cycle, so it cannot be a neap minimum. The largest July range falls on the new Moon day, and the full Moon peak comes a day or two after full Moon. The mean spring-neap period is 14.77 days, half the 29.53 day synodic month, but in this month the two peaks sit 12 to 13 days apart, so the mean shows across several months rather than in one. The learner knows it worked when the plotted peaks land on the new and full Moon symbols within a day or two.

What changes

What you change: date (Moon phase)
What you measure: daily tidal range and time of high water
What you keep the same: single station
predicted heights, not observed

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

There is one high tide a day, when the Moon is overhead; there are two, the second under the bulge on the far side.
The Sun has no effect on tides; it sets the spring-neap difference.
High tide is at the same clock time each day; it drifts about 50 minutes later.

Safety card

Low riskLearners carry it out

Hazards

No hazard is listed.

Controls

No control is listed.

Note

No hazardous chemical and no flame or heating apparatus: a generic classroom risk assessment (CSIS 1.7 or RiskAssess) covers trips, spills, warm lamps and sharp edges.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC4-OTU-01 SC4-DA1-01 SC4-WS-05 SC4-WS-06
Australian Curriculum v9AC9S7U03AC9S7I04AC9S7I05

Sources

The pages the author read to write this activity.

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