Mass-produced by corporate watchmaking giants today, one would not likely guess that the first solar-powered watch was developed by a talented American engineer out of a humble room in his house. Roger W. Riehl called his invention the Synchronar 2100, bringing it to market in the 1970s and going on to build a successful company around it. An appropriately space-age-looking thing, this innovative gadget brought together multiple fields and technologies for the first time and was the first step in the development and subsequent commercialization of the solar-powered watch.

Electrical clocks and solar cells were both already over a hundred years old in the early 1970s (no, we’re not counting sundials as the first solar-powered timepieces for the purposes of this article), and electronics and solar technology were both developing quickly around the mid-20th century. The first solar cells efficient enough to be practically usable came out of Bell Laboratories in the 1950s, and the first electric wristwatch was born in 1957 with the Hamilton Electric 500. Showing just how cutting-edge Riehl’s work was, the Synchronar was released the same year as Hamilton’s Pulsar, which made the claim as the first watch with a digital LED display.

Just a couple of years before the Synchronar, Seiko introduced the world’s first quartz watch in 1969, and it would go on to revolutionize the watch industry over the following decades and converge with solar-power tech. This was already a time of some wild watch design, and it’s easy to imagine the Synchronar having been visually inspired by early seasons of Star Trek, with an angular body displaying the time in red digital LED glory on the side facing the wearer (like a “driver’s watch”). The “top” was dominated by windows to the photovoltaic cells in order to maximize light absorption.

Selling for $500 around the time of its debut, the Synchronar 2100 was most notable as the first solar-powered wristwatch, but it was more ambitious than that alone. The name Synchronar is a perfectly retro-futuristic portmanteau of “SYNchronous CHRONometer calenAR,” and “2100” refers to the perpetual calendar, programmed using an integrated circuit to be accurate until the year 2100 — Riehl chose to highlight the watch’s calendar even more than its solar-charging. The various functions and displays are all controlled by two sliders on top of the watch rather than via the more common solution of buttons.

The watch also featured advanced capabilities like three brightness levels for the display, which adjusted automatically according to ambient lighting conditions. The Synchronar was apparently bright enough to also claim the title of the first LED watch display legible in full sunlight. Also impressive were a high shock-resistance of 5,000G, water-resistance to 750 feet, and wide temperature tolerances. It’s almost like a proto-G-Shock with looks reminiscent of, say, Bulova’s Computron. Not least of all, the Synchronar claimed an accuracy to within one minute per year. These various technical features continued to improve with subsequent models following the original Synchronar 2100 “Mark I.”

Riehl’s achievements are all the more impressive since he was solely responsible for all design and development, more or less as a one-man operation — though he partnered with various companies for production. Riehl’s company changed names through the years, but the Synchronar continued to be produced in varying forms and quantities. The watch was a genuine phenomenon, and notable wearers included the likes of Paul McCartney and Egypt’s president Mohamed Hosni Mubarak. Riehl was working on a Mark V version with even more features when he passed away in 2005 and production ceased.

Most solar-powered watches today are relatively simple and combine various other energy-conserving features (the technology is still not efficient enough to power, for example, smartwatches with full-color screens). While modern solar watches can be relatively inexpensive, combined with quartz technology they offer some of the most practical and user-friendly watch-wearing experiences possible.

Notable modern solar-powered watches like Citizen‘s Eco Drive, Casio‘s Tough Solar, and Seiko Solar all use proprietary technology and take a slightly different approach. They charge from any kind of light (not just the sun), and the convenience of never needing to worry about replacing a dead battery negates one of the most common complaints about battery-powered watches. It also helps such watches feel more enduring.