u/dcforce

Credit: JAXA / NHK Kaguya Orbiter archive / Seán Doran

There are about 12.37 lunar cycles per year. For cultures that use one lunar cycle for each month, this creates making a regular calendar a challenge. Some years have 12 lunar months, while the leap years have 13 lunar months. However, many ancient cultures, from Babylon, to Greece and China noticed that the 235 lunar cycle repeats every 19 years. This is called the Metonic cycle. With this regularity, you can create rules of what years to add the 13th leap month, and keep a decentralized kingdom's calendars synchronized.

The first picture shows the whole Metonic cycle as a torus. Each yearly turning creates a spoke, with the winter solstice in the center and the summer solstice on the outside. Then within each year is a helix of the moon phases, with the new moon as a black dot and the full moon as a white dot. Then a tighter helix is wound around the moon phases to show the day night cycle. I set the sunrise and sunset times based on Denver Colorado, the nearest big city to me. If you go further from the equator, the winter to summer day and night ratios in the summer and winter become more extreme.

The second image is a cross section of the torus, showing +/- 1 year.

He wrote on his post:

>Timelapse of the ISS Optimal Propellent Maneuver (OPM)! We use this to reorient station's attitude in preparation for docking events. This one occurred over some magnificent Mediterranean city lights!

Credit: NASA astronaut Don Pettit

View from Boston, MA due to Canadian wildfires.

The BOHR (Betavoltaic Orbital High-Reliability) CubeSat, developed by City Labs, has become the world's first commercial nuclear-powered satellite, launching aboard a SpaceX Falcon 9 during the Transporter-17 mission on July 7, 2026. The mission is testing the company's proprietary NanoTritium betavoltaic power source, which converts the natural decay of tritium into continuous electricity without relying on sunlight, moving parts, or conventional batteries. Designed as a proof-of-concept, BOHR aims to demonstrate the viability of compact nuclear power for long-duration missions in deep space and permanently shadowed regions of the Moon, potentially enabling future spacecraft to operate reliably where solar power is unavailable: https://world-nuclear-news.org/articles/first-commercial-nuclear-powered-satellite-launched

More to read here:

(1). https://www.ans.org/news/article-8188/first-commercial-nuclear-satellite-launched-on-spacex-mission/

(2). https://citylabs.net/first-commercial-nuclear-powered-satellite-aboard-spacex-transporter-17/

(3). https://arstechnica.com/space/2026/07/miami-based-city-labs-achieves-a-first-for-commercial-nuclear-power-in-space/

(4). https://www.space.com/space-exploration/launches-spacecraft/spacex-just-launched-the-1st-ever-nuclear-powered-commercial-satellite

credit - space_facts7