When taking a sextant altitude of a celestial body, what corrections must be applied to the raw sextant reading before it can be used for position fixing, and why?

A. No corrections are needed once the reading is taken

B. Index error (a sextant calibration error, unique to that instrument), dip (correcting for the height of eye above sea level, since the visible horizon is below the true horizontal at any height), and refraction (the bending of light through the atmosphere, which displaces the body's apparent position, particularly near the horizon) — plus semi-diameter for the Sun/Moon since the limb, not the centre, is normally observed

C. Only index error needs correcting; the rest are negligible in practice

D. Corrections are applied by the chart plotter automatically and require no manual calculation

A. The tidal height at the time of transit to confirm sufficient under-keel clearance

B. The yacht's fuel consumption for that leg

C. The colour of the seabed shown on the chart

D. Only the distance to the next waypoint

A. Switch the ECDIS to standby mode and navigate by GPS position alone

B. Ignore it — ECDIS alarms are frequently false and can be left running

C. Assess the situation against the planned route and charted depths, take action to maintain safe water, and inform the Master if there is any doubt

D. Acknowledge and silence the alarm immediately so it does not distract the helmsman