Extremely bright flash occuring overhead every thirteen seconds.

My brother and I witnessed a small but extremely bright flash of light occuring overhead every thirteen seconds around 9:22 pm on August 13, 2023. The flash had no visible cause and occurred at different locations in the sky moving slowly south. It did not appear to be moving in a straight line and no vehicle of any kind was visable.. At the same time, we were able to see airplanes with flashing lights moving across the night sky. We could not follow whatever it was, but had to simply wait for it to flash to know it's location. It did not appear to be a satellite, plane, or military flare. It was a clear night with stars visible and a really bright moon.