Solar Cross Calendar
A symmetrical 13 month calendar based on equinoxes and solstices.
1. Introduction
I made this page primarily to share an ongoing art project: a symmetrical 13-month calendar structured around the equinoxes and solstices. The project is released as a Creative Commons, open-source framework, intended to establish a mathematical, geometrical, and philosophical foundation that others can use, adapt, and evolve over time. Rather than
inheriting its structure from a specific culture or historical moment, the calendar attempts to remain as culturally neutral as possible, grounding itself in mathematical relationships and astronomical phenomena that are objective and shared by everyone on Earth.
The first choice behind the calendar is mathematical. It begins with a simple question: what number divides the solar year of approximately 365.24 days with the smallest remainder? One elegant answer is 13 equal months of 28 days — four weeks of seven days each — creating a year of 364 days with a clean, repeating rhythm.
The second principle is astronomical. The calendar is aligned with the equinoxes and solstices, natural events that mark the turning of the seasons and are experienced globally, regardless of background or belief. These moments define the fundamental geometry of the system. The 13th month is divided into four individual weeks, each one assigned to an equinox or solstice, anchoring the four seasons. The remaining 12 months are arranged as three months per season, while the solstice and equinox weeks function as gateways, guiding the transition from one season to the next.
The third choice addresses the remaining 1.24 days of the solar year. To account for this remainder, the calendar introduces a leap-year mechanism. Each year includes one additional day — called the Day Out of Time — placed on the last new moon before the New Year. Every four years, a second Day Out of Time is added, creating a leap year, and is placed on the first full moon of that year. Together, these adjustments preserve the long-term alignment between the calendar and the solar year.
The philosophical foundation of the project is an attempt to design a calendar that is more closely attuned to the natural cycles of the Earth. In this system, New Year’s Day is set at the Spring Equinox, emphasizing renewal and emergence as the starting point of the year. The initial reference point for the calendar is the Spring Equinox of 2026. Until then, the project remains in an open, exploratory phase, with further stages planned to unfold gradually after that moment, allowing the calendar to continue evolving over time.