Based on measurements made over the years by the Lunar Laser Ranging Experiment (LLRE), the Moon is slowly drifting away from the Earth..

at the rate of **3.82 +/- .07 centimeters every year!**

However, what if the Moon isn’t moving away from us or at least not as quickly as we might think? What if the speed of light is not Constant but is changing over time? Next year a new experiment hopes to shine some light on the speed of light and help determine whether the speed is constant or is actually slowing down. According to a new theory by Louise Riofio, the speed of light is getting slower at a rate of 1 centimeter per second each year.

Please keep in mind that this is a teeny tiny amount shorter each year. Since light can travel at nearly 300,000 km/s, a centimeter less is hardly noticeable. If true and because this a cumulative effect, this change will be become more noticeable over time. In ten years it’d be 10 cm less distance traveled in a second, in a hundred years it’d be 100 cm less / s. This also indicates that the speed of light might have been much faster in our past.

If you shine a laser beam at the Moon and bounce it off the mirror you can figure out how long it takes for the light to get there and back. Spoiler: it’s less than 3 seconds.

The distance to the Moon from earth is roughly **384,400 km**

[384,400,000 meters away from us] and the speed of light is **299 792 458 m / s**

The time it takes for light to travel to the moon is

384,400,000 m / 299,792,458 m = 1.2822203.. seconds

Now keep in mind that I’m using a single number for the distance to the moon and that the moon actually gets closer to earth and sometimes further away. At times as close as 356,761 km or it can be as far as 406,555 km [*see moon distance time table below]

It takes about 1.3 seconds for light to get to the Moon bounce off the mirror and then come back to us so that’s another 1.3 s for the return trip. Unfortunately, the distance to the Moon changes based on its orbit. So the time it takes depends on when you measure it and from where on earth you measure it.

Closest: ~356,761 km divided by the distance light travels in one second = 1.1900266.. seconds

Furthest: ~406,555 km divided by the distance light travels in one second = 1.35612150.. seconds

*I’m not sure about the precision on this, since I’m rounding the numbers to start with, the precision isn’t really the important part, as much as to know that there’s a range of time that light can take to reach the Moon which is between ~1.19 to ~1.35 seconds.*

Despite the fact that the distance changes depending on where the Moon is in its orbit, on average these measurements indicate that the distance to the moon is increasing every year by 3.8 cm but what if the speed of light is not constant? What if the moon isn’t getting further away but the speed of light is actually slowing down? Is this possible and how could we tell?

If the Moon is moving away from earth at this speed then it would have been much closer to us not that long ago assuming that the rate of change is constant and has always been constant. In fact, the Moon should have been too close much too recently for current theories of the age of the Moon. Also we do not know at this point if this is cyclical, will it slow down in ten years or a thousand years?

*Only time will tell*

Upcoming experiments on the International Space Station may demonstrate whether or not the speed of light is always the same over time. The current hypothesis is that light is in fact getting slower every year, but the experimental data will hopefully tell us whether it’s the same, slowing down or speeding up. The suspicion is that it’s a rate of 1 cm / s per year but what if it’s more? What if it’s less? What effect will that have on the Moon drift rate?

### Let’s take a look

If the rate of change for the speed of light turns out to be: **1 cm / s per year**

Then this would mean that the apparent moon drift of 3.82 cm each year is actually..

If I understand this correctly, since the time it takes for light to reach the Moon from earth is 1.2822203 seconds (when the Moon is 384,400 km from us) and if the speed of light is slowing down by 1 cm / s then shouldn’t the effect be 1.2822203 cm? So, 3.82 cm subtract 1.28 cm I get 2.54 cm of actual Moon drift. Still on its way but not as quickly as we thought.

If it turns out that the speed of light is slowing down at a greater rate,

let’s say **3 cm / s per year** then..

Distance to the Moon in light seconds: 1.2822203 x 3 cm = **3.8466.. cm** then the Moon would no longer be drifting away but is in fact drifting towards us!

I might be missing something obvious in these sloppy math estimates and if I am please let me know. For one, I expect that it’s very very unlikely to be a rate of 3 cm/s per year. That would have huge implications for the entire Universe! It makes me wonder if this change in speed is everywhere or if it’s a local phenomena?

When we look upon the Moon today we always see the same side but if we’re standing on the Moon looking back at Earth, the Earth spins and reveals all.

If the speed of light is constant over time (as most of us have thought it to be) then the Moon’s orbit really is increasing at the speed we observe, at least for the time being. At this rate, in a about 50 billion years from now, a month will be 47 days long as the moon locks into the same side of Earth. The Moon will see the Earth spin more and more slowly until it finally stops and shows only one side to the Moon. It is believed that at this point the Moon will stop moving away from Earth as the tidal forces and gravitational drag that caused the drift will have stopped.

If on the other hand what if the speed of light is speeding up by 1 cm / s per year? Then the Moon is going bye bye. It’d be flying away from us at the speed of 4.8 cm each year. Not good, mind you, probably nothing to worry about for few billion years.

One last note, if it turns out that the speed of light is slowing down exactly the same speed that the Moon is drifting away from us, well, that would be super freaky!

*References*

**Is the Moon moving away from us and when was this discovered?**

**GM=tc^3 Adventures in Space/Time**

http://riofriospacetime.blogspot.com/

**Moon distance time table**

https://www.timeanddate.com/astronomy/moon/distance.html

**Apollo Laser Ranging Experiments Yield Results August, 1994**

https://eclipse.gsfc.nasa.gov/SEhelp/ApolloLaser.html

**Measuring the distance to the moon with radio waves instead of lasers**

https://www.scienceinschool.org/content/moon-and-back-reflecting-radio-signal-calculate-distance