Latitude by meridian altitude is one of the commonest position lines used in the Merchant Navy today, and the traditional method of obtaining it is to observe the Sun until it reaches maximum altitude, at which point it is said to dip. With the advent of power-driven vessels, however, this method became liable to an error of 5 minutes of arc, and as faster surface craft are developed it is reasonable to assume that the error could be even greater. This is because the rate of change of altitude of a body is related to the observer's speed over the Earth so that the body will dip either before or after meridian passage depending on whether the observer is moving towards or away from the geographical position of the Sun.