### Sun-Earth Travel Time

In most astronomical situations, the speed limit is slow, simply because the rel-distances are so great.
In the following, we'll use a grid comoving with the center of mass of the solar system.

Here's a simple illustration of a photon travelling from the Sun to the Earth.
It shows the slowness of the speed limit when large rel-distances are involved.
The rel-time it takes 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 rel-radius of the Sun and Earth.
The mean rel-time of photon travel 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 rel-distance equal to the Sun's diameter.)
This simulation uses 497 pixels to represent the rel-distance; it draws the line
representing the position of the photon at a rate of one pixel per second.
At the given scale, the rel-size of the Sun is approximately correct, but the rel-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 rel-distance from the Sun to the Earth: 108 times the rel-diameter of the Sun
- the rel-diameter of the Earth: 1/108 times the rel-diameter of the Sun

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