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Do We Observe the Sunrise Six Minutes After it Actually Occurs?

-- | February 2, 2016

Question: We know sunlight takes about 8 min to reach earth. So we see sunrise 8 min later. Also we see sunrise 2 min earlier due to atmospheric refraction. Does it mean that we see sunrise after about 6 min of actual sunrise?? — Priyanshu

Answer: I think that you are right, in the sense that if the Sun suddenly turned-off at sunrise one day, we would not notice this until about 8 minutes (of light travel time) minus 2 minutes (due to atmospheric refraction allowing us to see the Sun a bit below the horizon), or 6 minutes. The refraction bit amounts to about 34 arcminutes of bending of the light from the Sun, which is about 2 minutes of time. Note, though, that this ignores the difference between the definition of sunrise, which refers to the point at which the upper limb of the Sun is at the horizon, and the position of the Sun, which is referenced to its center. If you add the half-size of the Sun, or about 15 arcminutes, then sunrise occurs when its center is about 50 arcminutes below the horizon, which equates to about 3 minutes of time.

Jeff Mangum

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