Time was once something you could see.
The day began when the light came and ended when it went. The year turned at the longest day and the shortest, and at the two moments between them when light and dark stand equal.
For most of human history the sky was the first calendar anyone consulted. Somewhere along the way the reckoning came loose from what it was measuring. Our months run to 28, 29, 30 or 31 days for reasons the sky no longer explains. The year begins on 1 January, a day on which nothing happens overhead. A week of seven days does not divide into a year, so every birthday wanders to a different weekday each time it comes around. And since the railways brought time zones, the clock no longer tells us where the sun is. It tells us which zone we are standing in.
None of this is inaccurate. It simply points nowhere. We have grown so used to it that the friction has become invisible: the counting on knuckles, the looking up, the dark winter morning that the clock insists is the same hour as a bright summer one.
The Calendarium Solaris points time back at the sky. A calendar that takes its shape from the Earth's orbit, a clock that takes its shape from the Earth's turning, and both of them read at the sun.
A year that begins in the sky
The Solaris year does not begin on a date somebody once chose. It begins at Equinox 0°, the moment the sun's longitude reaches exactly zero. That moment arrives at the same instant for everyone on Earth, north and south of the equator, and it belongs to no nation, no church and no hemisphere.
From there come twelve months of thirty days each. Their names say nothing but where they stand: Primisol, Secundisol, Tertisol and onwards to Duodecisol, the first to the twelfth month of the sun. No emperors, no gods, and no ninth month called the seventh.
A trial period has been running since Equinox 0° of 2026, alongside the calendars already in use. During it, every Solaris date carries the Gregorian date small and in brackets behind it, so nobody loses their footing. The official count begins at Equinox 0°, Year 0 (20 March 2028).
Five days, and then again
The Solaris week is five days long, and its names follow the arc of a single day, from the first light to rest.
Six of these weeks make a month exactly, with nothing left over. Every month begins on Solcycli and ends on Paxcycli, and a date keeps its weekday for good. If your birthday falls on a Maxcycli, it will fall on a Maxcycli every year of your life.
When the Earth turns a corner, the week stands still
Twelve months of thirty days make 360. The five or six days that remain are not squeezed into the weeks. They stand outside them.
When an equinox or a solstice arrives, the week simply stops.
That day has no weekday name. It belongs to the sky, to the moment the Earth reaches one of the four turning points of its orbit, on the same day for everyone. The next morning the week picks up exactly where it left off. The Yearday that closes the year stands outside in the same way, and in a leap year so does the Intercalary Day that follows it.
Because the days that would not divide are not part of the week, the week is never asked to divide into the year. That one decision keeps the five-day rhythm intact forever, and it gives every year a handful of shared pauses, most of them at the great turning points of the orbit.
The year is also precise. The Gregorian leap-year rules are kept and one is added: a year divisible by 4000 has no Intercalary Day. The mean year becomes 365.24225 days, and the calendar stays in step with the sun for more than sixteen thousand years. The specification even says what should happen then: at year 16,000 the astronomy of that time is to decide whether an adjustment is needed. The calendar states the limits of present knowledge instead of claiming permanence.
An hour that follows the light
Solaris Time divides the light into twelve hours and the dark into twelve, at the place where you actually stand. The day begins at 0:00 DT, at sunrise. Noon is 6:00 DT, when the sun stands highest over your own horizon rather than over the middle of a time zone. At sunset the night begins, at 0:00 NT. DT is diurno tempore, the time of day; NT is nocturno tempore, the time of night.
So the hours breathe with the year. In Porto a day hour lasts 76 minutes in the height of summer and 46 in the depth of winter. Near the equinoxes day and night hours are almost equal, and on the equator they hardly change at all.
This is not a new idea. It is the oldest form of the hour, the one people lived by in Egypt, Greece and Rome, and in Japan until 1873. The equal hour of sixty minutes took over when clocks came to be driven by falling weights and gear wheels, which have no way of knowing that the days are getting longer.
The equal hour won on mechanism, not on argument.
And this clock is honest about its limits. Beyond the polar circles there are days on which the sun does not rise, or does not set. Solaris Focus does not invent a sunrise for them. It says that there are no unequal hours to show.
What the body already knows
There is a reason to care beyond elegance. The human body keeps a clock of its own. It runs close to twenty-four hours but not exactly, so it has to be set again every day, and what sets it is light. In a study of tens of thousands of people, that inner clock followed the sun where they lived, not the number on the dial. The distance between the two, called social jetlag, can be measured, and being out of step with the inner clock is repeatedly associated with poorer metabolic and cardiovascular health.
No study has ever tested the Solaris clock, and we will not pretend that one has. What a solar clock can honestly claim is smaller, and still worth having: it makes the cue visible. When you can see that you are in the ninth hour of light, sunset does not take you by surprise, and the shape of your own day is in front of you. The research and its sources are further down this page.
Solaris Focus
A calendar you have to look up is a calendar you do not live by.
Solaris Focus puts the Calendarium Solaris where the day is actually spent. It is one Android app in which every date is a Solaris date and every hour can be an hour of the sun, and in which the clock, the calendar, the notes, the journal, the camera and the conversations all share the same day.
An alarm can be set on the third hour after sunrise instead of on half past seven, and it moves with the season. A photograph is filed under the Solaris day on which it was taken. The journal keeps a place every day for three things you are grateful for, the breathing exercise counts a slow box of four, and what you write in the Grow programmes carries the Solaris date on which you wrote it. Over time that becomes a history of yourself, kept in the same reckoning as everything else.
Most parts you do not need can be switched off, and their tile leaves the home screen. What you write is encrypted before it leaves the device, and you choose where it lives: on the phone alone, on the Solaris server, or on a server of your own. The server cannot read it, which also means that nothing can be handed back if you lose your key.
Solaris Focus is in beta on Android. There is no iPhone version; that line is served by the web application.
Perhaps the question is not what time the clock says, but where the sun is.
The calendar and the clock are both here, for today and for wherever you stand.
The Reckoning in Detail
The structure, the arithmetic, the history of the hour and the evidence, in full.
"The Calendarium Solaris is offered for voluntary adoption worldwide. It is designed to operate alongside any existing calendar system and carries no religious, political, or hemispheric associations."
What is the Calendarium Solaris?
The Calendarium Solaris is a universal solar calendar, precisely synchronised with the tropical year, the time from one Equinox 0° to the next, which is the cycle of the seasons. Unlike the Gregorian calendar, which is rooted in historical convention, the Calendarium Solaris is anchored to an observable astronomical event: Equinox 0°, the moment when solar longitude reaches exactly 0°.
Every year begins on this exact equinox, a moment that occurs simultaneously across the entire planet, making the Calendarium Solaris a truly universal, hemisphere-neutral timekeeping system. The official start is Equinox 0°, Year 0 (20 March 2028 UTC). A trial period runs from Equinox 0° 2026 alongside existing calendars. During this trial period, each Calendarium Solaris date is displayed alongside the corresponding Gregorian date. After Intercalary Day, Year 4, the calendar stands independently.
Perfect Structure
12 months of exactly 30 days each. Every month contains exactly 6 weeks, always starting on Solcycli and ending on Paxcycli.
Balanced Quarters
Four perfectly equal quarters of 91 days each, 90 weekdays (18 weeks) plus one floating Anchor Day marking an equinox or solstice.
Astronomically Anchored
The year always begins at Equinox 0°, a universal, observable astronomical event, separated from arbitrary cultural conventions.
Highly Accurate
With a four-rule intercalation system, the Calendarium Solaris stays aligned with the tropical year for approximately 16,000 years.
The Five-Day Week & Outside Days
The conventional seven-day week is an ancient historical construct that forces months and years out of alignment. The Calendarium Solaris introduces a highly efficient, permanently stable 5-day week. Because every month has exactly 30 days, each month contains exactly 6 weeks. The days of the week never shift, every month always begins on Solcycli and ends on Paxcycli.
Solcycli
Initiation, the rise
Start of the week
Luxcycli
Growth, the light
Building momentum
Maxcycli
Apex, the peak
Height of the week
Descycli
Descent, winding down
Transition day
Paxcycli
Rest and peace
End of the week
Outside Days
The days that fall outside the 360-day month structure are called Outside Days. Four of these are Anchor Days, each marking one of the four astronomical equinoxes or solstices; one Yearday closes the solar year; and in leap years an Intercalary Day follows. These days carry no weekday designation, they pause the week cycle without ever disrupting it. They serve as moments of global pause, marking the great turning points of Earth's orbit.
Year Structure
The year consists of 12 months of 30 days, following a permanent 5-day week cycle (Solcycli, Paxcycli). Four Anchor Days mark the precise equinoxes and solstices; a Yearday closes the cycle. In leap years, an Intercalary Day is appended. All outside days carry no weekday designation.
| # | Month / Anchor | Days | Position |
|---|---|---|---|
| — | Equinox 0° | 1 | Day 1, Equinox 0°, start of year |
| 1 | Primisol | 30 | |
| 2 | Secundisol | 30 | |
| 3 | Tertisol | 30 | |
| — | Solstice 90° | 1 | Day 94, Solstice 90° |
| 4 | Quartisol | 30 | |
| 5 | Quintisol | 30 | |
| 6 | Sextisol | 30 | |
| — | Equinox 180° | 1 | Day 187, anchor day within Septisol after day 4 |
| 7 | Septisol | 30 | |
| 8 | Octisol | 30 | |
| 9 | Nonisol | 30 | |
| — | Solstice 270° | 1 | Day 277, anchor day within Decisol after day 3 |
| 10 | Decisol | 30 | |
| 11 | Undecisol | 30 | |
| 12 | Duodecisol | 30 | |
| — | Yearday | 1 | Closes the year |
| — | Intercalary Day | 1 | Closes the leap year according to the intercalation rules |
Intercalation Rules
Because the tropical year is not exactly 365 days, the Calendarium Solaris uses a four-rule intercalation system. This produces an average year length of 365.24225 days, just a few seconds off from the tropical year, ensuring alignment for approximately 16,667 years.
Rule 1, Divisible by 4
Intercalary Day added.
Rule 2, Divisible by 100
No Intercalary Day.
Rule 3, Divisible by 400
Intercalary Day added.
Rule 4, Divisible by 4000
No Intercalary Day.
How Does It Compare?
The Calendarium Solaris is approximately 5 times more accurate than the Gregorian calendar in long-term synchronisation with the tropical year.
| Calendar | Avg. Year Length | Deviation | One-day drift after |
|---|---|---|---|
| Julian | 365.25000 days | +0.00781 days/year | ≈ 128 years |
| Gregorian | 365.24250 days | +0.00031 days/year | ≈ 3,226 years |
| Khayyam | 365.24242 days | +0.00023 days/year | ≈ 4,348 years |
| Calendarium Solaris | 365.24225 days | +0.00006 days/year | ≈ 16,667 years |
"The calendar is anchored to the sun. So is the day. Solaris Time divides the light into twelve and the dark into twelve, at the location where you actually stand."
What Solaris Time Is
A Solaris day has two halves. From sunrise to sunset there are twelve day hours, written DT, for diurno tempore. From sunset to the next sunrise there are twelve night hours, written NT, for nocturno tempore. That is where the two letters come from, and they are easy to hold on to in English as well: DT is day time, NT is night time. The minutes inside those hours are ordinary minutes; only how many of them fit into an hour changes with the season and with your latitude.
This is an old form of the hour, not a new one. It is usually called the temporal or seasonal hour, and it was the ordinary hour of daily life for most of recorded history. What is new is that a phone can now compute it exactly, for your own position, without a sundial and without a table.
Sunrise. The first hour of light begins, so the clock begins at nought, as clocks do.
The middle of the light. Solar noon, at your horizon, not at the centre of a time zone.
Sunset. The twelve hours of dark begin, and they run to the next sunrise.
Because the light and the dark are each cut into twelve, the two kinds of hour trade length through the year. A day hour and a night hour always add up to two hours of the clock. At the equinoxes they are both very close to sixty minutes; at the solstices they are as far apart as your latitude allows.
| Where you stand | Equinox 0° | Solstice 90° | Equinox 180° | Solstice 270° |
|---|---|---|---|---|
| Porto, 41° N | 61 min day / 59 night | 76 / 44 | 61 / 59 | 46 / 74 |
| The equator, 0° | 61 / 59 | 61 / 59 | 61 / 59 | 61 / 59 |
| Sydney, 34° S | 61 / 59 | 49 / 71 | 61 / 59 | 72 / 48 |
Lengths rounded to the minute, computed for sunrise and sunset at the standard horizon of −0.833°, which is why the equinox hours are not exactly sixty. The two halves of the world read the same table in opposite directions: at Solstice 90° the hours of light are at their longest in Porto and at their shortest in Sydney. On the equator they barely move at all.
Where the Hours Came From
The division of the day into twenty-four parts is Egyptian in origin, and it was not built out of equal pieces. Night was tracked by decans, star groups that rose in succession, and twelve of them served to mark the hours of darkness; daylight was then divided into twelve to match.1 Because the length of daylight changes through the year, so did the hour.
-
Antiquity
The temporal hour is the hour of daily life across the Greek and Roman world. In midsummer Alexandria a daylight hour ran roughly half as long again as a midwinter one.1 Water clocks and sundials were built to follow it, with scales that changed by season.
-
c. 150 BC, the astronomers split off
Hipparchus proposed dividing the day into twenty-four equinoctial hours, equal parts taken from the day on which light and dark are already equal.12 Astronomy needed a constant unit for its arithmetic. Ordinary life did not adopt it, and kept the seasonal hour for well over a thousand years.
-
14th century, the escapement decides
The mechanical clock arrives in Europe. It can be built to keep temporal hours, but it is naturally suited to equal ones: a wheel driven by a falling weight has no way of knowing that the days are getting longer. Fixed hours become common only once these clocks spread.1 The equal hour won on mechanism, not on argument.
-
1873, the last public system falls
Japan kept temporal hours throughout the Edo period, with clocks, the wadokei, engineered to run at two different rates for day and night and be reset with the seasons. The system was abolished with the calendar reform of 1873, when Japan moved to the Gregorian calendar and the twenty-four equal hours.3
-
1884, the clock leaves the horizon
Railways and telegraphs made local solar noon impossible to work with: every town had its own. The International Meridian Conference in Washington fixed a prime meridian and a universal day, and standard time zones followed.4 From then on the clock no longer told you where the sun was. It told you which administrative zone you were standing in.
That last step is worth being precise about. The equal hour is an excellent unit and there is nothing wrong with it. What was lost was not accuracy but reference: an hour used to be a fraction of your daylight, and it became a fraction of a day defined at a distant meridian. Within a single time zone, solar noon can fall well over an hour apart from one edge to the other, and the clock says nothing about it.
Your Body Runs on Sunlight
The reason to care is not nostalgia. Human physiology runs on a clock of its own, and that clock is set by light, not by the number on a dial. The following statements are each supported by published research; the sources are listed at the foot of this page.
The internal clock runs near, but not exactly at, twenty-four hours
Under carefully controlled lighting, the intrinsic period of the human circadian pacemaker averages about 24.18 hours, with a narrow spread across individuals and across age groups. Because it is not exactly twenty-four hours, it has to be reset every day by an external cue.
Czeisler, C.A. et al., "Stability, precision, and near-24-hour period of the human circadian pacemaker", Science 284(5423), 1999.5That cue is the sun, and the clock entrains to sun time
In a study of tens of thousands of people, the timing of the human clock tracked local solar time rather than local clock time. Within a single time zone, chronotype shifted progressively with longitude, that is, with the actual time of sunrise, and the effect was clearest in populations less shielded by artificial light.
Roenneberg, T., Kumar, C.J. and Merrow, M., "The human circadian clock entrains to sun time", Current Biology 17(2), 2007.6Natural light pulls the clock earlier, and pulls people together
After a week of living outdoors under sunlight alone, participants' internal timing advanced by about two hours relative to a week in ordinary electric lighting, and the differences between early and late types shrank. Electric light during the evening and too little daylight during the day delay the clock.
Wright, K.P. et al., "Entrainment of the human circadian clock to the natural light-dark cycle", Current Biology 23(16), 2013.7Living out of step with that clock is measurable
The gap between biological timing and socially imposed timing, called social jetlag, is associated with a higher body mass index in the general population, independently of how long people sleep. In the laboratory, deliberately shifting sleeping and eating away from internal time reduced leptin, raised glucose and insulin, reversed the daily cortisol rhythm and raised mean arterial pressure.
Roenneberg, T. et al., "Social jetlag and obesity", Current Biology 22(10), 2012.8 · Scheer, F.A.J.L. et al., "Adverse metabolic and cardiovascular consequences of circadian misalignment", PNAS 106(11), 2009.9Where the clock is set matters, and it can be measured at a border
At the eastern edge of a United States time zone the sun sets an hour later than at the western edge of the next zone, while schools and workplaces keep the same clock hours on both sides. Comparing households across those borders, later sunset was associated with less sleep and with worse health and earnings outcomes. The comparison isolates solar time from social schedule, because only the sun differs.
Giuntella, O. and Mazzonna, F., "Sunset time and the economic effects of social jetlag: evidence from US time zone borders", Journal of Health Economics 65, 2019.10Even a one-hour shove has consequences
In a national registry study, the incidence of acute myocardial infarction rose significantly in the first weekdays after the transition to daylight saving time, and fell after the transition back to standard time. On the strength of this and related evidence, the American Academy of Sleep Medicine holds that permanent standard time, the setting that keeps the clock closest to solar time, is the better choice for health and safety.
Janszky, I. and Ljung, R., "Shifts to and from daylight saving time and incidence of myocardial infarction", New England Journal of Medicine 359(18), 2008.11 · Rishi, M.A. et al., American Academy of Sleep Medicine position statement, Journal of Clinical Sleep Medicine, 2023.12What this does and does not show
No study has ever tested the Solaris clock. Nothing on this page should be read as a claim that counting in hours of the sun makes anyone healthier. That research does not exist, and it would be dishonest to imply otherwise.
What the evidence does support is narrower and still worth saying. The human clock is set by light. It follows the sun rather than the dial. The further a society's schedule drifts from solar time, the more misalignment is measured, and misalignment is repeatedly associated with poorer metabolic and cardiovascular outcomes. Those are findings about light and timing, not about any particular notation for the hour.
What a solar clock can honestly claim is this: it makes the cue visible. If you can see that you are in the ninth hour of light, sunset is not a surprise, and the shape of your own day at your own latitude is in front of you rather than hidden behind a number agreed at a distant meridian. What anyone does with that is their own business.
The Clock and the Calendar Together
The Calendarium Solaris is anchored to the equinox, an observable astronomical moment rather than a historical convention. Solaris Time applies the same principle one level down: the calendar takes its structure from the Earth's orbit, and the clock takes its structure from the Earth's rotation, both read at the sun.
The two meet at the Anchor Days. On the equinoxes, a day hour and a night hour are almost exactly equal, which is the astronomical fact the anchor day marks. On the solstices they stand at their yearly extreme. The calendar's four turning points are also the four turning points of the clock, and on those days the clock shows it without being told.
Hemisphere Neutral
The calendar names its anchor days by solar longitude rather than by season, so it favours neither half of the world. The clock inherits that: it counts from your own sunrise, wherever that is.
Same Reference
Date and hour come from one astronomical model. A day in the calendar and an hour in the clock are answers to the same question, asked at different scales.
Nothing Is Abolished
Ordinary clock time stays visible in brackets, exactly as the Gregorian date does behind the Solaris date. It is there to help you find your footing, not to be the answer.
In Practice
Solaris Time is implemented in Solaris Focus. Sunrise, solar noon and sunset are computed for the position you give it, or for a city from a list carried inside the app, using the standard solar position algorithm; nothing is looked up over the network and the full precision never leaves the device.
| Notation | 4:16 DT and 5:65 NT: the hour of the day or the night, the minutes within it, and which half you are in. DT is diurno tempore, between sunrise and sunset; NT is nocturno tempore, between sunset and sunrise. |
| Alarms | An alarm set on an hour of the sun is worked out again from sunrise every day, so it moves with the sun instead of standing still while the sun moves. An alarm that has to line up with the rest of the world is set to standard time and then it does stand still. |
| Timers | Ask for an hour and you get the hour you are actually in, which is long while the light lasts and short while it does not. |
| Weather | Given per part of the Solaris day and per hour of the sun, rather than per clock hour. |
| Turned off | The whole system can be switched off in one place, and then an hour is simply sixty minutes and nothing is called a sun hour that is not one. |
| Without a location | No location means no sunrise, and no sun hour is invented. The app falls back to ordinary hours and says so. |
| Other locations | The clock shows the hour of the sun in locations you add, so you can see what part of their day somebody else is in. |
| Faces | One ring of twenty-four, day and night as two half rings, one ring of twelve, or digital. |
Sources and References
Historical statements on this page rest on general reference works; the physiological statements rest on peer-reviewed primary research and on the position statement of a professional body.
- 1Lombardi, M.A., "A Chronicle of Timekeeping", Scientific American, February 2006. scientificamerican.com
- 2Wikipedia contributors, "Equinoctial hours"; "Hour". Wikipedia, The Free Encyclopedia. [Accessible overviews consistent with the primary historical literature.] en.wikipedia.org/wiki/Equinoctial_hours
- 3Hashimoto, T., "Japanese Clocks and the Origin of Punctuality in Modern Japan", University of Tokyo Center for Philosophy, UTCP Collection 6. utcp.c.u-tokyo.ac.jp
- 4International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day, October 1884: Protocols of the Proceedings. Washington, 1884.
- 5Czeisler, C.A., Duffy, J.F., Shanahan, T.L. et al., "Stability, precision, and near-24-hour period of the human circadian pacemaker", Science 284(5423), 1999, pp. 2177–2181. doi:10.1126/science.284.5423.2177
- 6Roenneberg, T., Kumar, C.J. and Merrow, M., "The human circadian clock entrains to sun time", Current Biology 17(2), 2007, pp. R44–R45. doi:10.1016/j.cub.2006.12.011
- 7Wright, K.P., McHill, A.W., Birks, B.R. et al., "Entrainment of the human circadian clock to the natural light-dark cycle", Current Biology 23(16), 2013, pp. 1554–1558. doi:10.1016/j.cub.2013.06.039
- 8Roenneberg, T., Allebrandt, K.V., Merrow, M. and Vetter, C., "Social jetlag and obesity", Current Biology 22(10), 2012, pp. 939–943. doi:10.1016/j.cub.2012.03.038
- 9Scheer, F.A.J.L., Hilton, M.F., Mantzoros, C.S. and Shea, S.A., "Adverse metabolic and cardiovascular consequences of circadian misalignment", PNAS 106(11), 2009, pp. 4453–4458. doi:10.1073/pnas.0808180106
- 10Giuntella, O. and Mazzonna, F., "Sunset time and the economic effects of social jetlag: evidence from US time zone borders", Journal of Health Economics 65, 2019, pp. 210–226. doi:10.1016/j.jhealeco.2019.03.007
- 11Janszky, I. and Ljung, R., "Shifts to and from daylight saving time and incidence of myocardial infarction", New England Journal of Medicine 359(18), 2008, pp. 1966–1968. doi:10.1056/NEJMc0807104
- 12Rishi, M.A. et al., "Permanent standard time is the optimal choice for health and safety: an American Academy of Sleep Medicine position statement", Journal of Clinical Sleep Medicine, 2023. doi:10.5664/jcsm.10898
Sunrise and sunset figures in the table were computed with the standard solar position algorithm at a horizon of −0.833°, the same model the applications use.
Experience the Calendar
See today's Calendarium Solaris date, explore months and years, and add your own appointments, exportable to any calendar app. The clock shows the hour of the sun where you stand, with four faces to choose from.
Open the Calendar ☀ Open the Clock ☀