The seven retroreflectors placed on the Moon's surface will bounce photons back to you if you have a laser powerful enough.
It's the 56th anniversary of Apollo 11 landing on the Moon today, and pretty much one of the first things that Buzz Alrdrin did after exiting the lunar module was to deploy a retroreflector on the surface.
The main idea was to measure the distance between the Earth and the Moon to the inch. It's a simple concept; use a powerful enough laser and you can bounce a beam off the array of corner cube prisms that make up the retroreflector and measure the time gap between light leaving your array and it arriving back. The speed of light is a constant, so that means you can work out the distance as precisely as you can measure the time (which is roughly 2.4 seconds; the key is in the number of decimal places you can go to). Corner cube prisms are forgiving of slight changes in orientation, so there's some wiggle room in the aiming.
As a result we now know that the Moon is moving away from the Earth at a rate of 3.8 cm per year. That might not sound like much, but it's roughly the same speed as continental drift, and look at what that's achieved over the ages.
A retroreflector left behind by Apollo 14
We also have a way of proving that the Moon landings took place for continuing skeptics, though you can't imagine that anyone thought that would have been really necessary back when Aldrin was placing it down at his feet. Yet here we are, merrily barrelling along on what you have to hope is the dumbest timeline. Conspiracy theorists are invited to get the funding together to build their own laser just in case the four current Lunar Laser Ranging Observatories (LLROs) are in on the cover-up.
They have plenty to aim at. There are seven retroreflectors on the Moon now. Five were landed by NASA and the Soviet Union in the 1960s and 70s, and their signal is starting to degrade as the reflectors get covered by lunar dust. Two new ones are in place, one on the Vikram lunar lander which landed in 2023, and another on Firefly Aerospace's Blue Ghost 1 which landed in March this year.
A prototype of the Blue Ghost Next Generation Laser Retroreflector back on Earth
Blue Ghost's Next Generation Laser Retroreflector (NGLR) has been designed to not only mitigate the accumulation of lunar dust, which is driven mainly by micrometeoroid impacts, but also contribute to improved accuracy, promising sub-millimeter range measurements.
More retroreflectors are scheduled too, maintaining the long line of science experiments that started with humanity's first footsteps on our nearest neighbour. Amongst other remote missions, these are also planned for the Artemis crewed missions back to the Moon as they continue to progress toward their goal, albeit in an increasingly haphazard manner as funding and technical issues conspire to undermine them.
It's also, of course, going to be difficult to book a big enough soundstage to fool everyone this time round. Just kidding...