# A question about speed of light and sunrise time

D H
Staff Emeritus
No. Sunrise is due to the rotation of the Earth, not due to the sun turning on and off.
That is the wrong answer. Nobody other than DaveC said anything about the sun turning on and off. If you had "read all of this" you would have seen why DaveC's answer is the wrong answer.

Going back to DaveC's post,
Regardless of whether any particular photons of light coming from the sun took 8 milliseconds, 8 minutes or 8 days to reach Earth, the photons making up the sun's rays are continuously streaming from the sun - essentially a solid line. The Earth turns and, when the spot where you are standing has a clear line of sight with the Sun, that's sunrise.

It is that last sentence that is erroneous. Here is a corrected version: The Earth turns and, when the spot where you are standing has a clear line of sight with where the Sun was 8.3 minutes ago, that's sunrise.

Due to the finite speed of light we don't see what is. We see what was. Those "what is" and "what was" positions of the Sun would coincide if either the speed of light was infinite or if the Earth and Sun were not moving with respect to one another. However, the speed of light is finite, and the Earth and Sun are moving with respect to one another. There is a slight angular deviation, about 20.5 arcseconds, between those "what is" and "what was" positions of the Sun.

This angular offset, aka the solar annual aberration, makes sunrise occur a tiny bit earlier (note: not later) than it would if the speed of light was infinite.

phinds
Gold Member
That is the wrong answer. Nobody other than DaveC said anything about the sun turning on and off. If you had "read all of this" you would have seen why DaveC's answer is the wrong answer.

Going back to DaveC's post,

It is that last sentence that is erroneous. Here is a corrected version: The Earth turns and, when the spot where you are standing has a clear line of sight with where the Sun was 8.3 minutes ago, that's sunrise.

Due to the finite speed of light we don't see what is. We see what was. Those "what is" and "what was" positions of the Sun would coincide if either the speed of light was infinite or if the Earth and Sun were not moving with respect to one another. However, the speed of light is finite, and the Earth and Sun are moving with respect to one another. There is a slight angular deviation, about 20.5 arcseconds, between those "what is" and "what was" positions of the Sun.

This angular offset, aka the solar annual aberration, makes sunrise occur a tiny bit earlier (note: not later) than it would if the speed of light was infinite.

I agree that your statement is correct but ONLY if the speed of light is what it is. Dave's statement, unlike yours, is correct REGARDLESS of whether the speed of light is c or infinite. Your statement does not answer the OPs question.

DaveC426913
Gold Member
Those "what is" and "what was" positions of the Sun would coincide if either the speed of light was infinite or if the Earth and Sun were not moving with respect to one another. However, the speed of light is finite, and the Earth and Sun are moving with respect to one another. There is a slight angular deviation, about 20.5 arcseconds, between those "what is" and "what was" positions of the Sun.
For my own edification, this angular deviation is due to the Earth's movement in its orbit around the sun, correct? In 8 minutes the Earth has moved almost 9,000 miles around its orbit, (this is unrelated to its own axial rotation).

I accept this will have an impact on the time of sunrise (which changes over the course of a year).

I discounted this as it did not seem to be what the OP was asking.

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sophiecentaur