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Sun-Earth Travel Time

In most astronomical situations, the speed limit is slow, simply because the distances are so great.

Here's a simple illustration of a photon travelling from the Sun to the Earth. It shows the slowness of the speed limit, relative to such large distances. The travel-time of a theoretical photon going from the center of the Sun to the center of the Earth varies by 16.6s during the year:

To be more realistic, we will correct for the radius of the Sun and Earth. The mean travel-time from the center of the Sun's apparent disk to the surface of the Earth is:

499.0 - 2.32 - 0.02 = 496.7s
(A photon takes 4.64s to go a distance equal to the Sun's diameter.) This simulation uses 497 pixels to represent the distance; it draws the line representing the position of the photon at a rate of one pixel per second.

At the given scale, the size of the Sun is approximately correct, but the size of the Earth is much too large. If you're ever building a model solar system, start with the Sun's diameter as being your unit of measure. Then, remember the number 108:

These figures are a good approximation, to within 1% of the precise values.