SOL-FORS (Tajik: Сол-Форс) is an interplanetary timekeeping and calendar system developed for use on the surface of Mars. It is the first complete standard combining an atomic second (soniya) with a 19-month calendar, designed for scientific missions, engineering operations, and civilian life in future Martian colonies.
- Etymology
· Sol: Martian solar day (from Latin sol – sun).
· Fors: Month (from Persian/Tajik форс – month).
· Soniya: Basic time unit (from Tajik сония – second).
· Solgard: Year (from sol + -gard – "cycle of sols").
· Hafta: Week (from Persian/Tajik ҳафта – week).
The calendar intentionally uses Tajik (Persian) terminology to honor the legacy of Central Asian astronomers (Avicenna, Al-Biruni, Ulugh Beg).
- Basic Time Units (Chronometry)
Unit (En) Original (Tg) Definition Duration
Soniya Сония 9,445,348,708 periods of cesium-133 on Mars ≈1.0275 Earth seconds
Martian minute Дақиқа 60 soniyas ≈61.65 Earth seconds
Martian hour Соат 60 minutes ≈3,699 Earth seconds
Sol Сол 24 hours = 86,400 soniyas 24h 39m 35.244s Earth UTC
Hafta (week) Ҳафта 7 sols ~172.3 Earth hours
The soniya is the fundamental atomic unit adapted to Martian gravity. The 24-60-60 format is preserved for intuitive use.
- Week Structure (Ҳафта)
The week begins on Dushanbe (Monday).
Order Day Name Original Meaning
1 Dushanbe Душанбе Monday (second day after Saturday)
2 Seshānbe Сешанбе Tuesday (third day)
3 Chorshānbe Чоршанбе Wednesday (fourth day)
4 Panjshānbe Панҷшанбе Thursday (fifth day)
5 Jum'a Ҷумъа Friday (congregational day)
6 Shanbe Шанбе Saturday
7 Yakshanbe Якшанбе Sunday (first day)
- Month Structure (Форс)
A year consists of 19 months, numbered:
1‑um Fors, 2‑yum Fors, … , 19‑um Fors.
Month Type Count per Year Length in Sols
Standard month 16 35
Always long (5th, 10th, 15th Fors) 3 36
1‑um Fors 1 35 (except years 9 and 45)
19‑um Fors 1 35 (except even years)
Standard (non‑leap) year:
16 × 35 + 3 × 36 = 668 sols = exactly 95 weeks + 3 days.
The three extra days are typically observed as inter‑weekly holidays.
- Leap Year System
To match the true Martian tropical year (≈668.59074 sols), a three‑level leap system is used without resetting shifts.
Rule Effect Year length 1‑um Fors 19‑um Fors
Odd year, not multiple of 9 – 668 35 35
Even year (2, 4, 6…) +1 sol in 19‑um Fors 669 35 36
Multiple of 9 (9, 18, 27, 36, 45) – odd +1 sol in 1‑um Fors 669 36 35
Multiple of 18 (18, 36) both rules apply 670 36 36
Multiple of 45 – “Soli X” +1 extra sol (super leap) 670 37 35
Key principle: Shifts accumulate and never reset
· A 36‑sol month shifts the next month start by +1 weekday.
· A 37‑sol month (Soli X, 1‑um Fors) shifts by +2.
· This accumulation continues throughout the year and across years.
- The 45‑Year Cycle and “Soli X”
The complete calendar cycle is 90 years (LCM of 2, 9, 45). Over 90 years:
· Total sols: 60,173
· Average year length: 668.588888… sols
· True tropical year: 668.590744 sols (JPL 2024)
· Error: –0.001856 sol/year (≈ –2.67 minutes/year)
Season drift: 1 sol per ~538 years – more accurate than the Gregorian calendar on Earth.
“Soli X” (45th year of each cycle)
· Named after SpaceX and the mathematical X (unknown future).
· The 1st month lasts 37 sols.
· Dedicated to technology festivals, engineering achievements, and remembering the pioneers who made Mars accessible.
- Accuracy Comparison
Calendar Mean year (sols) Error (min/year) Drift (1 sol per)
SOL‑FORS 668.5889 –2.67 538 years
Darian ≈668.5907 ≈0 ≈13,000 years
Utopia (NYU) 668.592 +1.2 ~1,200 years
- Practical Advantages
· For space agencies: Eliminates confusion between missions, simplifies synchronous operations.
· For colonists: Intuitive 24‑60‑60 format, familiar 7‑day week, predictable long months (5,10,15).
· For culture: Unique Central Asian identity, avoids English‑centric naming.
· For the long term: The 45‑year “Soli X” cycle creates a recurring generational celebration.
- Submission and Recognition (2026)
The SOL‑FORS standard has been submitted to:
· NASA
· SpaceX
· Roscosmos
· The New York Times
The International Astronautical Federation (IAF) listed SOL‑FORS as one of the recommended Martian calendars for further study.
- Example Date
Panjshānbe, 12‑umi 10‑um Fors, 45‑um Solgard (Soli X)
(Thursday, 12th day of the 10th month, year 45 – the Year of X)
SOL-FORS (Tajik: Сол-Форс) is an interplanetary timekeeping and calendar system developed for use on the surface of Mars. It is the first complete standard combining an atomic second (soniya) with a 19-month calendar, designed for scientific missions, engineering operations, and civilian life in future Martian colonies.
· Sol: Martian solar day (from Latin sol – sun).
· Fors: Month (from Persian/Tajik форс – month).
· Soniya: Basic time unit (from Tajik сония – second).
· Solgard: Year (from sol + -gard – "cycle of sols").
· Hafta: Week (from Persian/Tajik ҳафта – week).
The calendar intentionally uses Tajik (Persian) terminology to honor the legacy of Central Asian astronomers (Avicenna, Al-Biruni, Ulugh Beg).
Unit (En) Original (Tg) Definition Duration
Soniya Сония 9,445,348,708 periods of cesium-133 on Mars ≈1.0275 Earth seconds
Martian minute Дақиқа 60 soniyas ≈61.65 Earth seconds
Martian hour Соат 60 minutes ≈3,699 Earth seconds
Sol Сол 24 hours = 86,400 soniyas 24h 39m 35.244s Earth UTC
Hafta (week) Ҳафта 7 sols ~172.3 Earth hours
The soniya is the fundamental atomic unit adapted to Martian gravity. The 24-60-60 format is preserved for intuitive use.
The week begins on Dushanbe (Monday).
Order Day Name Original Meaning
1 Dushanbe Душанбе Monday (second day after Saturday)
2 Seshānbe Сешанбе Tuesday (third day)
3 Chorshānbe Чоршанбе Wednesday (fourth day)
4 Panjshānbe Панҷшанбе Thursday (fifth day)
5 Jum'a Ҷумъа Friday (congregational day)
6 Shanbe Шанбе Saturday
7 Yakshanbe Якшанбе Sunday (first day)
A year consists of 19 months, numbered:
1‑um Fors, 2‑yum Fors, … , 19‑um Fors.
Month Type Count per Year Length in Sols
Standard month 16 35
Always long (5th, 10th, 15th Fors) 3 36
1‑um Fors 1 35 (except years 9 and 45)
19‑um Fors 1 35 (except even years)
Standard (non‑leap) year:
16 × 35 + 3 × 36 = 668 sols = exactly 95 weeks + 3 days.
The three extra days are typically observed as inter‑weekly holidays.
To match the true Martian tropical year (≈668.59074 sols), a three‑level leap system is used without resetting shifts.
Rule Effect Year length 1‑um Fors 19‑um Fors
Odd year, not multiple of 9 – 668 35 35
Even year (2, 4, 6…) +1 sol in 19‑um Fors 669 35 36
Multiple of 9 (9, 18, 27, 36, 45) – odd +1 sol in 1‑um Fors 669 36 35
Multiple of 18 (18, 36) both rules apply 670 36 36
Multiple of 45 – “Soli X” +1 extra sol (super leap) 670 37 35
Key principle: Shifts accumulate and never reset
· A 36‑sol month shifts the next month start by +1 weekday.
· A 37‑sol month (Soli X, 1‑um Fors) shifts by +2.
· This accumulation continues throughout the year and across years.
The complete calendar cycle is 90 years (LCM of 2, 9, 45). Over 90 years:
· Total sols: 60,173
· Average year length: 668.588888… sols
· True tropical year: 668.590744 sols (JPL 2024)
· Error: –0.001856 sol/year (≈ –2.67 minutes/year)
Season drift: 1 sol per ~538 years – more accurate than the Gregorian calendar on Earth.
“Soli X” (45th year of each cycle)
· Named after SpaceX and the mathematical X (unknown future).
· The 1st month lasts 37 sols.
· Dedicated to technology festivals, engineering achievements, and remembering the pioneers who made Mars accessible.
Calendar Mean year (sols) Error (min/year) Drift (1 sol per)
SOL‑FORS 668.5889 –2.67 538 years
Darian ≈668.5907 ≈0 ≈13,000 years
Utopia (NYU) 668.592 +1.2 ~1,200 years
· For space agencies: Eliminates confusion between missions, simplifies synchronous operations.
· For colonists: Intuitive 24‑60‑60 format, familiar 7‑day week, predictable long months (5,10,15).
· For culture: Unique Central Asian identity, avoids English‑centric naming.
· For the long term: The 45‑year “Soli X” cycle creates a recurring generational celebration.
The SOL‑FORS standard has been submitted to:
· NASA
· SpaceX
· Roscosmos
· The New York Times
The International Astronautical Federation (IAF) listed SOL‑FORS as one of the recommended Martian calendars for further study.
(Thursday, 12th day of the 10th month, year 45 – the Year of X)