﻿ Sun-Earth Travel Time For Light

### 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.