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J. Perelman
"Entertaining physics". Book 1.
Chapter 7. Rays of light

THE PROBLEM OF SOLAR SUNRISE

You watched the sunrise at exactly 5 hours. As we know that light is not instantaneous: it takes some time for the rays managed to walk from the light source to the eye of the observer. We can therefore ask the question: what time would be watching you the same sunrise, if the light spread instantly?

The light runs a distance from the sun to the Earth in 8 minutes. It would seem that the instantaneous propagation of light, we would see the Sun for 8 minutes earlier, i.e. in 4 hours 52 minutes.

For many will likely be a complete surprise that this answer is completely wrong. After all, the Sun “rises” because the earth rotates in an already lit space new points of its surface. Therefore, when the instantaneous propagation of light you would see the sunrise in the same moment that the sequential distribution, i.e., exactly at 5 o'clock [If we take into account the so-called “atmospheric refraction”, the result will be even more unexpected. Refraction distorts the path of the rays in the air, and thus allows us to see the sunrise earlier its geometric appearance above the horizon. But when the instantaneous propagation of light refraction can not be, because the refraction due to the difference of the speed of light in different media. The lack of refraction will entail that the observer will see the Sun rise a little laterthan when nemnogone the propagation of light; the difference is, depending on the latitude of the observation place, air temperature and other conditions, ranging from 2 minutes to several days and even more (in polar regions). It turns out an interesting paradox: while instant (i.e. infinitely fast) the propagation of light we would be watching the sunrise later than nemnogone!].

Another thing, if you see (through a telescope) appear on the edge of the Sun any ledge (prominence): when the instantaneous propagation of light you would see him for 8 minutes earlier.

Entertaining physics J. Perelman

 




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