Report 1 of 1 · 8/4/2026
· 8/4/2026
This report finished before verdicts existed. Its conclusions are in the full report.
The most defensible undergraduate-report diagram is a temporal radial chart referenced to Mumbai Chart Datum, not a geographic compass plot. It uses the Survey of India’s official May 2025 tide-table sheet for MUMBAI (APOLLO BANDAR) and overlays, as separate explanatory annotations, the four primary lunar phases calculated by the U.S. Naval Observatory in IST. The plotted tide heights are official published predictions; the lunar symbols and phase guide-lines are annotations and are not measurements.
The official Mumbai Port Authority reference levels provide the datum framework and an independent magnitude check: mean high water spring is +4.42 m, mean low water spring is +0.76 m, mean high water neap is +3.30 m, mean low water neap is +1.86 m, and mean sea level is +2.51 m, all relative to local Chart Datum. Therefore, the spring-tide range is 3.66 m and the neap-tide range is 1.44 m.
The May 2025 predictions show the expected lunar modulation, but not as a perfectly symmetric sinusoid. The four representative windows range from 1.90 m near First Quarter, to 3.50 m at Full Moon, 2.05 m at Last Quarter, and 4.84 m at New Moon, when range is calculated as the maximum minus minimum of the turning points printed for that date. These values are not interpolated. The First Quarter window uses 5 May, the nearest complete four-event day after the First Quarter phase on 4 May, because the official 4 May row prints only three turning points.
The diagram is labelled Apollo Bunder because that is the named Survey of India station: “MUMBAI (APOLLO BANDAR) - INDIA, WEST COAST.” The May 2025 tide-table sheet gives the station position as 18°55' N, 72°50' E, or approximately 18.9167° N, 72.8333° E. The Permanent Service for Mean Sea Level independently identifies the long-term station as MUMBAI / BOMBAY (APOLLO BANDAR) at 18.916667° N, 72.833333° E, station ID 43 and station code 41.
The PSMSL record describes a long historical series extending from 1878 to 2024, with a documented gap in 2012-2014 because the tide gauge was not functioning. It also identifies the Survey of India Geodetic and Research Branch as the data authority for the station. Those facts make Apollo Bunder a more defensible reference than an unverified web tide widget located somewhere else along the Mumbai waterfront.
Apollo Bunder is adjacent to the Gateway of India waterfront, but “Apollo Bunder,” “Mumbai Port,” and a particular waterfront structure are not automatically the same measurement point. A tide gauge measures water level at a specific installation, while the Gateway shoreline includes quay walls, steps, harbour structures, local shoaling, and small-scale bathymetric differences. The Mumbai Port Authority page describes harbour-wide tidal information rather than publishing a separate coordinate for a Gateway-of-India gauge.
The diagram should therefore be read as a station-representative Apollo Bunder tide chart, suitable for geography, oceanography, and civil-engineering explanation. It should not be used as a construction datum, navigation datum, flood-design level, or safe-clearance value without checking the current port notice, chart, benchmark transfer, and project-specific survey.
The diagram sets the radial centre, labelled CD, at 0 m above Mumbai Chart Datum. Chart Datum is the low-water reference used for nautical charting and harbour depth information; it is not the same thing as the long-term average level of the sea. The Mumbai Port Authority explicitly states that its tidal levels are elevations above or below Chart Datum. Its published extremes are Highest High Water recorded +5.39 m and Lowest Low Water recorded -0.46 m, while the statistical reference levels lie between them.
Mean Sea Level, or MSL, is a statistical average level over an adopted observation period. For Mumbai Port, the published MSL is +2.51 m relative to Chart Datum. In the diagram it is shown by a blue dashed circle, while the tide predictions are plotted as individual red high-water and blue low-water points. MSL is therefore a reference line, not a daily tide event and not a replacement for Chart Datum.
The Mumbai Port Authority gives:
| Reference level | Elevation relative to Chart Datum |
|---|---|
| Highest high water recorded | +5.39 m |
| Mean high water spring tides (MHWS) | +4.42 m |
| Mean high water neap tides (MHWN) | +3.30 m |
| Mean sea level (MSL) | +2.51 m |
| Mean low water neap tides (MLWN) | +1.86 m |
| Mean low water spring tides (MLWS) | +0.76 m |
| Lowest low water recorded | -0.46 m |
| Highest low water | +2.74 m |
The implied mean spring range is 4.42 - 0.76 = 3.66 m, and the implied mean neap range is 3.30 - 1.86 = 1.44 m. These are reference statistics, not a claim that every individual spring or neap tide will equal those values. The Port Authority describes Mumbai Harbour as semi-diurnal, with a period of approximately 12 hours 40 minutes, so two high waters and two low waters commonly occur in a day, with unequal successive highs and lows possible.
The PSMSL station record provides important local-reference history. Up to 1936, the datum was recorded as 9.132 m below BM 2PP1. From 1937 onward, the station datum was Chart Datum, with BM 2PP1 listed as 8.522 m above Chart Datum. In 1966 the benchmark changed to BM 5/88, listed as 7.44 m above Chart Datum. The record also notes a historical RLR relationship and a 31 mm adjustment for part of the 1931-1958 mean-tidal-level series.
These benchmark changes matter because a water-level number is meaningful only with its vertical reference. The present diagram does not convert the published values into a geodetic height above the national vertical datum, WGS 84 ellipsoidal height, or a project benchmark. It preserves the hydrographic convention: metres relative to the local Chart Datum reference used for the tide table and harbour tidal levels.
Survey of India publishes annual tide predictions with daily times and heights of high and low waters for Indian ports. Its description of the Indian Tide Table states that predictions are generated from tidal analysis and used to predict hourly heights and the times and heights of high and low waters. Mumbai is included in the Indian-responsibility port list.
For this diagram, the May 2025 monthly PDF was selected because the Survey of India page provides a complete official monthly download for May 2025, and the archive contains a dedicated MUMBAI.pdf sheet. The table header identifies the station, coordinates, year, month, units in metres, and time zone -0530, which is Indian Standard Time, UTC+05:30.
The plotted event values are:
| Lunar window | Date | High/low event times and heights, IST | Daily plotted range |
|---|---|---|---|
| Near First Quarter | 5 May 2025 | 00:30 low 2.03 m; 05:49 high 2.98 m; 12:17 low 1.78 m; 19:17 high 3.68 m | 1.90 m |
| Full Moon | 12 May 2025 | 05:18 low 0.70 m; 12:01 high 4.20 m; 17:57 low 1.86 m; 23:21 high 3.73 m | 3.50 m |
| Last Quarter | 20 May 2025 | 00:20 low 2.03 m; 06:04 high 3.07 m; 11:54 low 1.74 m; 18:35 high 3.79 m | 2.05 m |
| New Moon | 27 May 2025 | 05:21 low 0.04 m; 12:13 high 4.88 m; 18:13 low 1.60 m; 23:56 high 4.18 m | 4.84 m |
All heights in the table are retained exactly to the precision printed by Survey of India. The daily range is a simple descriptive calculation from the highest and lowest printed turning points for that date; it is not a harmonic reconstruction and does not imply a continuous water-level curve between events.
Two features are especially clear. First, the Full Moon and New Moon windows have substantially larger daily ranges than the quarter-phase windows, consistent with the spring-neap cycle. Second, the May 2025 New Moon day has a particularly low low-water value, 0.04 m above Chart Datum, and a high water of 4.88 m, giving a plotted range of 4.84 m, which is higher than the long-term mean spring range of 3.66 m. That is not a contradiction: MHWS is a mean statistical level, while an individual spring tide can be larger or smaller because of constituent phase, lunar distance, weather, and local hydrodynamics.
The radial angle represents elapsed time through May 2025, increasing clockwise from 1 May 00:00 IST. The phase lines are positioned using the U.S. Naval Observatory primary-phase times converted from Universal Time to IST:
| Phase | Phase time in IST | Diagram meaning |
|---|---|---|
| First Quarter | 4 May 2025, 19:22 | Quadrature; reduced average tidal range expected |
| Full Moon | 12 May 2025, 22:26 | Syzygy; spring-tide enhancement expected |
| Third/Last Quarter | 20 May 2025, 17:29 | Quadrature; reduced average tidal range expected |
| New Moon | 27 May 2025, 08:32 | Syzygy; spring-tide enhancement expected |
The phase times are astronomical annotations, not tide-table events. They do not mean that the local maximum tide occurs exactly at the instant of the phase. Tidal response has phase lags, and the largest spring range may occur before or after the exact new or full Moon.
The physical explanation is the standard lunar-phase effect. Around New Moon and Full Moon, the Sun, Earth, and Moon are approximately aligned; the lunar and solar tidal-producing forces reinforce one another. NOAA calls these aligned positions syzygy and the resulting greater-than-average range spring tides. At First and Third Quarter, the Sun and Moon are approximately at right angles as viewed from Earth; the principal semidiurnal contributions partly offset in range, producing neap tides.
The Mumbai observations are consistent with that expectation: the Full Moon and New Moon windows have the two largest plotted ranges, while the quarter-phase windows are much smaller. The relationship is statistical rather than deterministic. Mumbai’s tide is not a pure lunar tide: solar semidiurnal constituents, shallow-water harmonics, basin geometry, harbour resonance, and local meteorological forcing all alter the exact height and timing.
The figure uses two independent visual systems:
The radial distance is a vertical water-level scale in metres above Chart Datum. Concentric rings at 1 m intervals provide scale guidance. The MSL, MHWS, MHWN, MLWN, and MLWS rings are reference levels. The outer ring is an annotation boundary, not a water level.
The “N” arrow is deliberately labelled as a diagram-orientation marker. It points to the start-of-month temporal axis, not to geographic north at the Gateway of India. A true site map would require a shoreline orientation, a north arrow tied to geographic coordinates, a map projection, and a separate plan-view scale. Mixing geographic direction with lunar-cycle angle would create a misleading hybrid graphic.
The CSV companion file contains the plotted high- and low-water entries, date, IST time, height above Chart Datum, and water-type label. It is useful for checking the diagram numerically or importing the selected points into GIS, spreadsheet, or plotting software.
The principal cross-check is between two official systems. Survey of India supplies dated, station-specific May 2025 predictions for Mumbai (Apollo Bandar). Mumbai Port Authority supplies the local Chart Datum reference levels and the semi-diurnal harbour description. The May values fall within the published local regime: most high waters lie around the neap-to-spring reference envelope, while the May 27 high water of 4.88 m is above MHWS but below the recorded highest high water of 5.39 m. The May 27 low water of 0.04 m is below MLWS but above the recorded lowest low water of -0.46 m.
This is a reference-level cross-check, not a duplicate independent observation of every May 2025 turning point. The one-page Survey of India extract identifies the station, month, units, and time zone, but does not itself print a separate datum paragraph. The diagram therefore states the Chart Datum convention explicitly from the Mumbai Port Authority page and avoids claiming a geodetic conversion that is not supplied in the extracted tide-table sheet.
Small differences may arise because the Port Authority statistics describe Mumbai Harbour generally, whereas the Survey of India table is station-specific to Apollo Bandar. The Gateway of India waterfront may experience local effects from quay walls, harbour entrances, shallow patches, dredged channels, and the exact position and elevation of the gauge. A value transferred from the Apollo Bunder station to a construction site must be checked by levelling or GNSS/benchmark connection and by the applicable port chart.
The chosen month, May, is useful because it is a complete official monthly prediction period and precedes the main southwest monsoon season. It is not a guarantee of calm conditions. Observed water level equals astronomical tide plus non-tidal residuals. Low atmospheric pressure can raise sea level through the inverse-barometer effect; high pressure can depress it. Strong winds can drive local setup or setdown, and storms can add surge and wave setup. These effects can cause an observed high water to differ from a predicted astronomical high water even when the tidal constituents are correct.
Monsoon conditions are especially important for operational use. During the southwest monsoon, wind stress, waves, rainfall, runoff, atmospheric pressure, and surge can dominate short-term departures from the astronomical prediction. A tide table is therefore not a storm-surge forecast. The diagram intentionally labels its red and blue points as Survey of India predictions, not as claimed real-time gauge observations.
The New Moon and Full Moon labels identify phase, not lunar distance. Perigean spring tides can be stronger than other spring tides because the Moon is closer to Earth. Indian coastal research using Survey of India tide-gauge records has documented substantial seasonal and perigean modulation, including a reported difference of about 87 cm between selected maximum tidal heights at Mumbai in January and August 1974. That result illustrates why spring-tide amplitude cannot be predicted from phase alone.
The exact timing of high and low water also changes across a month. Tidal constituents have different astronomical periods and phases; shallow-water overtides and the geometry of Mumbai Harbour make the tide waveform asymmetric. The May 2025 table’s unequal successive highs and lows are therefore expected, not errors.
Use the Survey of India Apollo Bandar May 2025 tide table as the primary dated dataset, with Mumbai Port Authority levels as the independent Chart Datum and spring-neap cross-check. Mumbai’s official reference levels are MSL +2.51 m CD, MHWS +4.42 m CD, MLWS +0.76 m CD, MHWN +3.30 m CD, and MLWN +1.86 m CD. The May representative ranges are 1.90 m near First Quarter, 3.50 m at Full Moon, 2.05 m at Last Quarter, and 4.84 m at New Moon. New and Full Moon are syzygies, so solar and lunar tidal effects reinforce; First and Last Quarter are quadratures, so the range tends to be smaller. The figure keeps those lunar annotations visibly separate from the official predicted high- and low-water points.
