A question about speed of light and sunrise time

  • Thread starter AlexLAV
  • Start date
  • #51
D H
Staff Emeritus
Science Advisor
Insights Author
15,415
686
No. Sunrise is due to the rotation of the Earth, not due to the sun turning on and off.
right answer and I read all of this
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.
 
  • #52
phinds
Science Advisor
Insights Author
Gold Member
17,055
8,067
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.
 
  • #53
DaveC426913
Gold Member
19,753
2,996
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.
 
Last edited:
  • #54
sophiecentaur
Science Advisor
Gold Member
2020 Award
26,132
5,355
IS there any point in taking a non-ideal model for this? Surely the case of a regular rotation and a circular orbit is the one to sort out first. Here, the speed of Earth through space wrt c may be looked on as relevant because the path of the light that is seen will not be a straight line, as drawn on a map of the situation. However, the apparent sunrise event will still be regular.
Other effects (elliptical orbit etc) may be of interest but I don't think they can involve any paradoxes.
 
  • #55
D H
Staff Emeritus
Science Advisor
Insights Author
15,415
686
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).
That's correct. That angular deviation results from the aberration of light, and this is a function solely of the relative velocity of the Sun with respect to the Earth.

Annual aberration is something for which astronomers need to account when they aim their sub-arcsecond telescopes. Certainly the stars don't really move by ±20 arcseconds over the course of a year. That motion is a relativistic optical illusion. That same illusion applies to the Sun, but for the Sun the annual aberration is a nearly constant ±20 arcsecond angular deviation.
 

Related Threads on A question about speed of light and sunrise time

Replies
14
Views
2K
  • Last Post
Replies
1
Views
726
Replies
2
Views
2K
  • Last Post
Replies
7
Views
2K
  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
17
Views
3K
Replies
4
Views
1K
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
14
Views
26K
Replies
4
Views
4K
Top