### 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:

- minimum: 490.7s (perihelion, first week in January)
- maximum: 507.3s (aphelion, first week in July)
- mean: 499.0s

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:

- the mean distance from the Sun to the Earth: 108 times the diameter of the Sun
- the diameter of the Earth: 1/108 times the diameter of the Sun

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