You plan to take a star fix at morning twilight. Explain how you select suitable stars and what astronomical conditions make this possible.

A. Select the three brightest stars visible without any calculation — they will automatically provide the best fix. Star selection requires only brightness, not azimuth spread.

B. Star sight planning at morning twilight: (1) WHY TWILIGHT: stars are visible because the sky is dark enough to see the stars BUT it is light enough to see the sea horizon — required to measure the altitude with the sextant. This window is approximately 15-30 minutes at nautical twilight (sun 12° below horizon). Outside this window: either too dark to see the horizon or too light to see the stars; (2) STAR SELECTION CRITERIA: (a) Spread of azimuths — select stars that give position lines crossing at approximately 60°-120° apart to minimise the effect of errors; ideally three stars giving an approximately 120° spread (equilateral triangle) for a strong three-position-line fix; (b) Magnitude — select 1st or 2nd magnitude stars (bright); standard star sights use the 57 navigational stars in the Almanac; (c) Altitude range — ideally between 15° and 75° altitude: below 15° refraction correction becomes large and unpredictable; above 75° the azimuth line is nearly vertical, giving a poor cut angle with other bodies; (3) PLANNING TOOLS: the Nautical Almanac star finder (2102-D star finder or equivalent), or the "Selected Stars" list on the daily Almanac pages, gives azimuth and altitude of recommended stars from the approximate DR position; (4) PRE-SET: pre-calculate each star's approximate altitude and azimuth so you can set the sextant drum and find the star quickly in the brief twilight window.

C. Take star sights at midnight when the sky is darkest. The sea horizon cannot be seen at night but the stars can be measured against each other instead.

D. Morning twilight star sights are outdated — GPS provides greater accuracy. Star sights are only examined for historical knowledge, not practical application.

A. Step-by-step tidal calculation using the Rule of Twelfths (or use Admiralty Tide Tables co-tidal and range factor): (1) Range at standard port = HW 4.8m − LW 0.6m = 4.2m; (2) Duration of rise: LW to HW — need LW time. Assume LW was 0730 (HW 1400 = roughly 6h 30m duration — typical semi-diurnal). Duration ≈ 6h 30m; (3) Time from LW to 1030 = 3 hours; (4) Using Rule of Twelfths — first hour rises 1/12, second 2/12, third 3/12: in 3h the tide has risen (1+2+3)/12 × 4.2m = 6/12 × 4.2m = 2.1m; (5) Height at 1030 = LW + rise = 0.6 + 2.1 = 2.7m; (6) Charted MHWS = 8.2m = height of bridge above Chart Datum (CD). Bridge clearance at 1030 = 8.2 − 2.7 = 5.5m (height of bridge above 1030 water level); (7) Apply seasonal correction: +0.1m raises water level slightly: clearance = 5.5 − 0.1 = 5.4m; (8) Air draught 7.4m vs clearance 5.4m — NOT SAFE by 2.0m. The vessel cannot safely pass at 1030.

B. The charted clearance of 8.2m is at MHWS — since the tide at 1030 is not at MHWS, the clearance will always be greater than 8.2m. The vessel with 7.4m air draught is always safe.

C. Apply the seasonal correction directly to the air draught: 7.4 + 0.1 = 7.5m. Since 7.5m < 8.2m clearance, the passage is safe at any state of tide.

D. Without access to the full Admiralty Tide Tables the calculation cannot be completed. Use the bridge clearance at chart datum (which is always the stated value) as the worst-case figure.

A. The charted depth of 8.5m is greater than the vessel's 7.8m draught — the vessel always has positive UKC in this channel regardless of tide.

B. Depth of water calculation: (1) Time from HW to 0800 = 2 hours — the tide is FALLING; (2) Range: HW 4.2m, assume LW (next) approximately 0.8m (typical UK 3.4m range). Falling tide — using Rule of Twelfths from HW: first hour falls 1/12, second hour falls 2/12; in 2 hours tide has fallen (1+2)/12 × 3.4m = 3/12 × 3.4 = 0.85m; (3) Height at 0800 = HW − fall = 4.2 − 0.85 = 3.35m above CD; (4) Actual depth of water = charted depth + tidal height = 8.5 + 3.35 = 11.85m; (5) UKC = depth of water − draught = 11.85 − 7.8 = 4.05m; (6) Required UKC = 0.6m; available UKC = 4.05m — SAFE with considerable margin. Note: in a real tidal approach, the pilot would calculate on the minimum tidal window and dynamic UKC accounting for squat at approach speed.

C. The tide is falling — entering on a falling tide is never permitted. The master must wait for the next high water.

D. At 0800 the tidal height is 4.2m (same as HW at 0600) because it takes at least 3 hours for the tide to begin falling.