Success_Machine asked:
What is the longest wavelength photon ever measured?

First, in my opinion, one must define the word "photon".

As I understand it, the word "photon" was coined in quantum physics to represent a "corpuscle" of light. In other words, light or a "photon" is an electromagnetic fluctuation that has a "dipole" motion. If the fluctuation is not a dipole then it can not be polarized. Radio transmissions, same as light, can also be polarized, therefore they have a dipole component to them, since the "dipole" component is part of the "photon" the transmitted radio waves can also be called "photons".
There are radio transmitters that work on or below 20 kilohertz, I have shown experimentally that electromagnetic frequencies down to 200 hertz can be successfully transmitted short distances. The 60 hertz (50 hertz in Europe) power lines also generate electromagnetic waves, but can these frequencies be called photons. To measure a dipole electromagnetic wave from a 50 hertz power line you would like to have two sensors half a wavelength apart. Since 50 hertz = 6,000 kilometers in wave length, the sensors would need to be 3,000 kilometers apart and synchronized to indicate the electromagnetic fluctuation at the same moment in time. Not a very convenient way to do it. It is far simpler to measure the frequency and then calculate the wavelength from that.
There are very low frequencies associated with the human brain for instance, if I remember correctly one frequency is about 4 hertz which would have a wave length of 75,000 kilometers.
I really do not know what the lowest frequency is that has been measured, I suppose it would depend on how long you wanted to wait for the wave to complete its cycle.

I can imagine electronic or gravitational fluctuations in space that have very long time duration's.
If one could remain stationary in space just outside the Earth's orbit around the Sun, it would be possible to detect a gravitational, and perhaps electronic, fluctuation caused by the Earth's field as it passes by once every year.
The Galaxy would behave similarly I suppose and the whole universe may rotate around some unknown axis, so the frequencies could be extremely low.

Johannes