2. Light's trip to Neptune from the Sun

Morsel:

It takes about four hours for light from the Sun to reach Neptune.

Meal:

Like all other planets, Neptune follows an elliptical orbit around the Sun. As such, its distance from the Sun is not constant. The average distance a planet is from the Sun is known as the semi-major axis, which is 4.495 billion km. The closest point that a planet is to the Sun is called the perihelion, whilst the farthest is the aphelion. In the case of Neptune, the distance between the two is about 101 million km. A massive distance to astronomical crumbs such as us, but to light, moving at 299,792,458 m/s through a vacuum, that's just less than 6 minutes travel time – nearly as long as it takes for sunlight to reach Earth.

The distances between objects in space are unimaginably vast and the numbers can become cumbersomely large when using kilometres or the like. As a convenience, astronomers use the unit of length known as the “astronomical unit” or “au”. This is defined exactly as 149,597,870,700 m, which is roughly the distance between the Sun and the Earth. For distances on a galactic scale, they use “parsecs” or “pc”, which is roughly about 206,000 aus (or, more precisely, “as the distance at which one au subtends an angle of one arcsecond”).

Here is a table for the planets, plus the demoted Pluto and the Sun's closest star:

Celestial Object	Distance from the Sun (au)	Sunlight's travel time
Mercury	0.4	3.2 minutes
Venus	0.7	6 minutes
Earth	1	8.3 minutes
Mars	1.5	12.7 minutes
Jupiter	5.2	43.2 minutes
Saturn	9.5	79 minutes
Uranus	19.2	2.7 hours
Neptune	30	4.2 hours
Pluto	39.5	5.5 hours
Proxima Centauri	268774.25	37,255 hours or 4.2 years

As aforementioned, the elliptical orbits of the planets cause these values to vary slightly (plus minute effects such as the tidal effect and orbits becoming proportionally bigger as the Sun loses mass). These values are also from a light photon that has left the Sun's surface. It is a different story for photons produced in the Sun's core.

When a photon is emitted from a nuclear reaction in the Sun's core, it travels at the speed of light in a random direction. Due to the density of the Sun's lead core, it travels only about 0.01cm before it collides with a charged particle and is absorbed, before subsequently being re-emitted in a random direction, and this process repeats billions upon billions of time. The density of the Sun towards the surface decreases and thus the collisions are fewer as time goes on. The number of collisions, or steps, that an initial photon takes to get from the core to the surface is known as the Random Walk Problem. Thanks to this process, the estimates for the age of the sunlight range from 10,000 to 170,000 years.

Recipe:

Cain, Frasier. n.d. “How Far Is Neptune’s from the Sun?” Universe Today.

Anon. n.d. “Speed of Light in Vacuum.” The NIST Reference on Constants, Units, and Uncertainty. Retrieved July 15, 2017.

Springob, Christopher. n.d. “Is the Distance from the Earth to the Sun Changing?” Astronomy Department at Cornell University. Retrieved July 15, 2017.

Anon. n.d. “Measuring the Universe - The IAU and Astronomical Units.” International Astronomical Union. Retrieved July 15, 2017.

Anon. 2006. “Ancient Sunlight.” NASA. Retrieved July 15, 2017.

Mitalas, R. & Sills, K. R. 1992. “On the Photon Diffusion Time Scale for the Sun.” Astrophysical Journal 401(2):759–60.