The practice of precomputing astronomical sights in the air, so reducing the delay between taking the observations and plotting the position lines on the chart, appears to be gaining in popularity. The observer selects a future instant in time, and calculates the altitude and azimuth of a suitable body or bodies, for an assumed position near the D.R. position at that time. Then, if the observation is taken at the time chosen, the intercept is obtained directly by comparing the sextant reading with the precomputed altitude, corrections having previously been made for systematic errors (e.g. dome refraction, personal error, &c.)