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Sun's Lag and Advance

The sun's lag and advance with respect to our once-a-year rotating frame is very similar to watching the sun go round us in our own backyards, that is, the mathematics is the same but applied to a rotation that is once per day. If the sun arcs high overhead, and the observer rotates uniformly in the backyard once per day, then the sun is due south at noon but ahead of the rotating observer in the middle of the afternoon, only to fall back level at 6pm. Similarly, there is a lag in the middle of the morning at 9am. This effect makes it necessary to carry a compass at low latitudes since the sun is unreliable for finding directions.