Calendarium Solaris

The Calendarium Solaris is a universal solar calendar grounded in the movement of the Earth around the Sun. Every year begins at the precise astronomical moment of Equinox 0°, when the Sun's ecliptic longitude reaches exactly 0°, as determined in Coordinated Universal Time (UTC). The calendar consists of 12 months of exactly 30 days each, a permanent 5-day week, and a four-tier intercalation system that keeps it aligned with the tropical solar year to within approximately one day per 16,000 years.

Yes, and that is one of its core principles. The Calendarium Solaris is anchored to the actual movement of the Earth around the Sun. The year begins at the precise astronomical moment of Equinox 0°, and the four Anchor Days mark the equinoxes and solstices, the true turning points of Earth's orbit. This means the calendar does not impose an artificial structure on time; it reflects the natural cycle that governs the seasons, the light, and life on Earth.

The Gregorian calendar was not designed from scientific principles, as it evolved from centuries of historical convention and political negotiation. The Calendarium Solaris was built from first principles, using the Earth's actual orbit as its foundation. The goal is a calendar that is structurally simple, astronomically accurate, culturally neutral, and stable for thousands of years.

It is a serious proposal, fully specified in a formal document (Full Specification v3.08.1) accompanied by a scientific foundations document. The trial period commenced at Equinox 0°, 20 March 2026 (UTC). The calendar is live, documented, and available for use today at solariscalendar.org.

The official epoch is Equinox 0°, Year 0, corresponding to the equinox of 2028 (20 March 2028, approximately 02:17 UTC). A trial period runs from Equinox 0° 2026 alongside existing calendars to allow tools, communities, and documentation to develop before the official start.

The Calendarium Solaris is an independent calendar project developed and maintained under the trade name Calendarium Solaris, registered with the Dutch Chamber of Commerce. Contact: [email protected].

A standard year contains 365 days: 12 months of 30 days each (360 days), 4 Anchor Days marking the equinoxes and solstices, and 1 Yearday closing the year. In leap years, an Intercalary Day is added after Yearday, bringing the total to 366 days. The Anchor Days, Yearday, and Intercalary Day fall outside the regular week and month structure, and are called Outside Days.

This is one of the most common questions about calendar reform. The 13-month structure has genuine appeal, but the Calendarium Solaris chooses 12 months of 30 days for reasons that are astronomical, mathematical, and practical.

The moon — the obvious argument for 13
The most natural basis for a 13-month calendar is the lunar cycle. A synodic month, the time from one new moon to the next, averages 29.53059 days. Since there are approximately 12.37 such cycles in a tropical year, neither 12 nor 13 fits the moon exactly, and you cannot simultaneously align a calendar with both the sun and the moon without a separate correction system. This is precisely why virtually all historical calendars are either purely lunar (the Islamic calendar), purely solar (the Gregorian calendar), or complex lunisolar hybrids (the Hebrew and Chinese calendars). The Calendarium Solaris makes a deliberate choice: it anchors solely to the sun. (Source: Encyclopaedia Britannica, "Synodic month"; Wikipedia, "Lunar month", citing Chapront-Touzé & Chapront, 1988.)

13 is a prime number — and that matters
12 is a highly composite number, the smallest positive integer with exactly six divisors: 1, 2, 3, 4, 6 and 12. This makes it straightforward to divide a year into halves, thirds, quarters and sixths without fractions. 13 is a prime number with no divisors other than 1 and itself. A 13-month year cannot be divided into equal quarters, halves, or thirds in whole months, which creates a structural mismatch with virtually every planning cycle used by businesses, governments, and institutions. (Source: Wolfram MathWorld, "Highly Composite Number"; Ramanujan, S., "Highly Composite Numbers", Proceedings of the London Mathematical Society, 1915.)

The four seasons are a natural human reality
Virtually all human cultures have independently organised the year around four cardinal points: the spring equinox, summer solstice, autumn equinox, and winter solstice. These are precise, calculable astronomical events that govern agriculture, ecology, and the distribution of daylight. A 13-month calendar ignores these four divisions entirely. The Calendarium Solaris places those four moments explicitly into the structure as Anchor Days.

Week stability is achieved a different way
The main practical attraction of 13×28 is that every month contains exactly four seven-day weeks, eliminating week drift. The Calendarium Solaris achieves the same result, namely that every date always falls on the same weekday, through a five-day week in a 30-day month. The problem is solved without sacrificing the four-season structure or the divisibility of 12.

A note on the menstrual cycle
The 13-month calendar has sometimes been promoted on the grounds that the average menstrual cycle is approximately 28 days. This claim is weaker than it appears. Clinical research shows that menstrual cycle length varies considerably between individuals and across a lifetime, typically ranging from 23 to 35 days, with an average closer to 29 days than to 28. The 28-day figure is a statistical approximation, not a biological constant, and it applies to roughly half of the world's population. It does not constitute a universal human rhythm on which to base a global calendar. (Sources: Gorrindo et al., American Journal of Obstetrics and Gynecology, 2007; Grieger & Clifton, Human Reproduction Update, 2017; Liang et al., npj Digital Medicine, 2023.)

The 12×30 structure of the Calendarium Solaris rests on multiple independent foundations, astronomical, mathematical, and practical, rather than optimising for a single principle.

The 12 months use Latin ordinal roots combined with the suffix -sol: Primisol, Secundisol, Tertisol, Quartisol, Quintisol, Sextisol, Septisol, Octisol, Nonisol, Decisol, Undecisol, and Duodecisol. The naming is deliberate: neutral, sequential, and free of any cultural, religious, or hemispheric association.

The Calendarium Solaris uses a 5-day week: Solcycli (initiation), Luxcycli (growth), Maxcycli (apex), Descycli (descent), and Paxcycli (rest). The suffix -cycli refers to a cycle or phase of the solar day. The names are derived from solar and cyclical concepts rather than from any cultural, religious, or political tradition.

The 7-day week is an ancient historical construct with no astronomical basis, as it originates in Babylonian tradition and was later adopted via the Jewish and Roman calendars into the Gregorian system. (Source: Zerubavel, E., The Seven Day Circle: The History and Meaning of the Week, University of Chicago Press, 1985.) Because it does not divide evenly into months or years, it causes constant misalignment, causing dates to fall on different weekdays every year, making long-term planning unnecessarily complex. The 5-day week divides perfectly into 30-day months (exactly 6 weeks per month), producing a calendar in which every date always falls on the same weekday, every year, without exception.

Anchor Days are the four Outside Days that mark the precise astronomical moments of the equinoxes and solstices. They carry no weekday designation, and they pause the week cycle for one day, after which the week resumes exactly where it left off. This mechanism keeps the week permanently stable while ensuring the calendar remains faithful to real astronomical events.

Yearday is the closing Outside Day of every standard year, positioned after Duodecisol 30. Like Anchor Days, it carries no weekday or month designation. It functions as a moment of annual closure, marking the end of the solar cycle.

The Intercalary Day follows Yearday in leap years and constitutes day 366. It serves as the intercalation point, the mechanism that keeps the calendar synchronised with the tropical solar year over long timescales.

Four rules applied in order: years divisible by 4 receive an Intercalary Day; years divisible by 100 do not; years divisible by 400 do; years divisible by 4,000 do not. This four-tier system produces a mean year of 365.24225 days, aligned with the tropical solar year to within approximately one day per 16,000 years.

The year is divided into four quarters, each containing one Anchor Day plus three months of 30 days, totalling 91 days per quarter. Q1 opens with Equinox 0°, followed by Primisol, Secundisol, and Tertisol. Q2 opens with Solstice 90°, followed by Quartisol, Quintisol, and Sextisol. Q3 begins with the first three days of Septisol, then Equinox 180° falls between Septisol 3 and Septisol 4, after which Septisol continues through Octisol and Nonisol. Q4 begins with the first day of Decisol, then Solstice 270° falls between Decisol 1 and Decisol 2, after which Decisol continues through Undecisol and Duodecisol. The year closes with Yearday, and in leap years the Intercalary Day follows.

The calendar is based on the tropical year, the time it takes the Earth to complete one full orbit around the Sun, measured from one Equinox 0° to the next. This is the natural cycle that governs the seasons and is the scientifically appropriate basis for any civil solar calendar.

The Gregorian calendar has a mean year of 365.24250 days, compared to the tropical year of 365.24219 days, a difference of 0.00031 days per year, accumulating to one full day approximately every 3,226 years. The Calendarium Solaris has a mean year of 365.24225 days, a difference of only 0.00006 days per year, accumulating to one day approximately every 16,000 years. This makes the Calendarium Solaris approximately five times more accurate than the Gregorian calendar over the long term.

Equinox 0° is the zero-point of the standard ecliptic coordinate system used in astronomy worldwide. It is the most precisely determinable of the four cardinal solar points, and it is hemispherically neutral, unlike solstices, which mark seasonal extremes that are opposite between the Northern and Southern Hemispheres. Starting the year at this moment ensures the calendar begins at a point of genuine global balance.

UTC — Coordinated Universal Time — is the primary international time standard, maintained by the Bureau International des Poids et Mesures (BIPM) in Paris. It is based on atomic clocks and belongs to no nation, culture, or religion. The Calendarium Solaris uses UTC to determine the precise moment of Equinox 0° each year, ensuring that the start of the calendar year is astronomically uniform and geographically neutral worldwide.

No fixed intercalation scheme can remain perfectly accurate indefinitely, because the tropical year itself shortens very slowly over geological timescales. The Calendarium Solaris therefore includes an explicit provision for a formal review at year 16,000 P.O., at which point the astronomical knowledge available at that time will be used to assess whether any adjustment is necessary. This is a scientifically sound approach: rather than claiming permanent perfection, the calendar acknowledges the limits of present knowledge and builds in the means to adapt.

The Gregorian calendar has months of unequal length, a 7-day week that drifts relative to months and years, and no structural connection to the astronomical events it nominally tracks. The Calendarium Solaris has months of exactly equal length, a 5-day week that never drifts, and four Anchor Days that formally embed the equinoxes and solstices into the calendar structure. It is also approximately five times more accurate over the long term.

Yes. Calendar reform has been proposed and debated for centuries. Notable proposals include the World Calendar, the International Fixed Calendar, and the Hanke-Henry Permanent Calendar. (Source: Steel, D., Marking Time: The Epic Quest to Invent the Perfect Calendar, Wiley, 2000; Bromberg, I., "The Symmetry454 Calendar", University of Toronto, 2004.) Most proposals did not gain widespread adoption. The Calendarium Solaris differs from all of these in one fundamental respect: it does not attempt to modify the Gregorian framework but replaces it with a system grounded in astronomical science from the outset.

The calendar draws on the same intercalation logic as the Gregorian calendar, specifically the four-tier leap year system, and extends it with a fourth correction rule (divisible by 4,000). In all other respects, year structure, week length, month names, and astronomical anchoring, the Calendarium Solaris is an original system.

No. Adoption is entirely voluntary. The Calendarium Solaris is designed to run alongside any existing calendar system. During the trial period (from Equinox 0° 2026), each Solaris date is displayed together with the corresponding Gregorian date in smaller notation to ease the transition.

Visit the calendar. The live calendar shows today's Solaris date alongside the Gregorian date. You can explore months and years, add appointments, and export them to any external calendar application.

Yes. Any Gregorian date can be converted to a Calendarium Solaris date using the date converter in the calendar. Because the Solaris date for any given solar position is fixed permanently, unlike Gregorian dates which drift relative to weekdays, and converted dates will always fall on the same weekday every year.

The Calendarium Solaris does not prescribe how the working week is organised, which remains a matter for individuals, organisations, and societies to determine. However, the calendar naturally lends itself to a working rhythm of 3.5 days on and 1.5 days rest: for example, Solcycli through Descycli as working days and Paxcycli as a half-rest day. The Outside Days, Anchor Days, Yearday, and Intercalary Day, fall entirely outside the week structure and are natural candidates for collective pause or celebration.

Outside Days carry no weekday designation. They are best treated as exceptional days outside the normal working or planning cycle: moments of pause marking the major turning points of the solar year. How any individual, community, or organisation observes or utilises these days is entirely their own choice.

Yes. There is no legal or institutional barrier to any organisation choosing to use the Calendarium Solaris for internal planning, scheduling, or record-keeping, alongside their existing calendar obligations. The Full Specification is publicly available and provides everything needed to implement the system.

No. The calendar carries no religious content, does not redefine existing religious observances, and does not require anyone to abandon their existing calendar. Religious holidays, observances, and dates can continue to be observed exactly as before. The Calendarium Solaris simply offers a parallel framework for civil timekeeping.

No. Equinox 0° is one of the two points in the year at which neither hemisphere is favoured, as day and night are approximately equal worldwide. Starting the year at this moment is the most hemispherically neutral choice available. The month and weekday names are based on Latin ordinal sequences and solar concepts, with no reference to seasons, directions, or cultural events tied to either hemisphere.

Latin is a historically neutral language in international contexts, as it belongs to no living nation or culture, is broadly familiar across scientific and academic traditions, and carries no contemporary political or religious associations. The Latin ordinal root system also produces names that are immediately comprehensible in sequence (Primisol = first, Secundisol = second, and so on), reducing the learning curve considerably.

No. The Calendarium Solaris is a scientific and structural proposal. Its only purpose is accurate, neutral, and stable timekeeping grounded in the observable movement of the Earth around the Sun.

The trial period runs from Equinox 0° 2026 (20 March 2026, UTC) until the official epoch at Equinox 0°, Year 0 (20 March 2028, UTC), approximately two years. During this period the calendar runs alongside existing systems.

After Intercalary Day in Year 4, the Calendarium Solaris is intended to be displayed independently, without the accompanying Gregorian date. At that point, a community evaluation will take place to determine whether the Gregorian coupling needs to be extended or whether the calendar can stand fully independently.

The trial period uses negative year counts relative to the official epoch. The official Year 0 begins at Equinox 0° 2028. The current Solaris date, including the corresponding Gregorian date, is always displayed at solariscalendar.org.

Years before the official epoch are marked A.O. (Ante Originem, before the origin). Years after are marked P.O. (Post Originem, after the origin). This is analogous to BC/AD or BCE/CE in the Gregorian system, but anchored to an astronomical event rather than a historical or religious one.

Support from major calendar software providers is a long-term goal. In the meantime, the calendar at solariscalendar.org allows appointment export to standard calendar formats compatible with Google Calendar, Apple Calendar, and Microsoft Outlook. Developers interested in integration are welcome to contact [email protected].

A suite of Solaris apps is also in active development. See the Apps page for the current beta builds.

Four apps are currently in beta development, all available for Android unless noted:

• Calendarium Solaris — the full web calendar as a native Android app, including appointments, sync, and import/export.
• Solaris Clock — a clock app showing the current Solaris date, with alarms, a countdown timer, stopwatch, meditation timer, and breathwork guide.
• Solaris Launcher — an Android home screen replacement that keeps the Solaris date front and centre. The Gregorian date remains visible in system notifications for easy conversion.
• Solaris Chat — a Matrix protocol client (Android & iOS) that displays all dates in Solaris format, with real-time in-chat translation for English, Dutch, Portuguese, French, and German.

See the Apps page for download links and beta tester sign-up.

Use the Beta Tester Sign-up button on the Apps page, or go directly to the feedback form and select Apps & Beta. Mention which app(s) you want to test and your device model. We will be in touch when a test slot is available.

Yes. When the Solaris Launcher is installed, both the Calendar app and the Clock app can be set as additional home screen overlays. This lets you swipe between the launcher, the full calendar, and the clock — all showing Solaris dates.

Solaris Chat is a messaging app built on the Matrix open standard — a federated, end-to-end encrypted protocol for real-time communication. Unlike centralised services, Matrix has no single owner: anyone can run their own server while still communicating with users on other servers. All message timestamps in Solaris Chat are displayed in Calendarium Solaris format. The app also supports real-time translation of chat messages between English, Dutch, Portuguese, French, and German.

The scientific foundations document is grounded in established IAU standards, BIPM time metrology definitions, and published ephemeris data. Formal peer review by independent astronomers and academic institutions is actively being pursued. The full documentation is available for review upon request.

The Scientific Foundations document is available for download at solariscalendar.org. It covers the tropical year, equinoxes and solstices, intercalation and long-term accuracy, UTC, long-period orbital variations, and historical context. All key values are traceable to primary sources including the IAU, BIPM, NASA JPL, and the published work of Meeus and Savoie.

Submission to relevant bodies including the ISO, the IAU, and the UN is part of the planned communication strategy. These are long-term processes. The specification and scientific documentation are prepared in a form suitable for formal institutional review.

Contact [email protected]. Contributions are welcome from astronomers, historians, software developers, translators, educators, and anyone with a serious interest in calendar science or reform.