Definition
A shallow tray of oil, or a mercury bath in older equipment, provides a truly horizontal reflecting surface. The observer brings the body's direct image into coincidence with its reflection, and the measured angle is twice the apparent altitude. Halve the reading, then apply index error and the normal corrections, but apply no dip correction because no sea horizon is used. Bubble horizons built into aircraft sextants serve the same purpose but are less precise at sea.
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