Week to Sidereal Day Converter
Convert weeks to sidereal days with our free online time converter.
Quick Answer
1 Week = 7.019165 sidereal days
Formula: Week × conversion factor = Sidereal Day
Use the calculator below for instant, accurate conversions.
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All conversion formulas on UnitsConverter.io have been verified against NIST (National Institute of Standards and Technology) guidelines and international SI standards. Our calculations are accurate to 10 decimal places for standard conversions and use arbitrary precision arithmetic for astronomical units.
Week to Sidereal Day Calculator
How to Use the Week to Sidereal Day Calculator:
- Enter the value you want to convert in the 'From' field (Week).
- The converted value in Sidereal Day will appear automatically in the 'To' field.
- Use the dropdown menus to select different units within the Time category.
- Click the swap button (⇌) to reverse the conversion direction.
How to Convert Week to Sidereal Day: Step-by-Step Guide
Converting Week to Sidereal Day involves multiplying the value by a specific conversion factor, as shown in the formula below.
Formula:
1 Week = 7.019165 sidereal daysExample Calculation:
Convert 60 weeks: 60 × 7.019165 = 421.1499 sidereal days
Disclaimer: For Reference Only
These conversion results are provided for informational purposes only. While we strive for accuracy, we make no guarantees regarding the precision of these results, especially for conversions involving extremely large or small numbers which may be subject to the inherent limitations of standard computer floating-point arithmetic.
Not for professional use. Results should be verified before use in any critical application. View our Terms of Service for more information.
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View all Time conversions →What is a Week and a Sidereal Day?
The week (symbol: wk or w) is a unit of time equal to 7 days, 168 hours, or 10,080 minutes.
Official status: The week is not an SI unit, but it is accepted for use with the SI due to its universal cultural importance. The SI base unit of time is the second, and the day is the fundamental accepted non-SI unit.
Standard conversions:
- 1 week = 7 days (exact)
- 1 week = 168 hours (7 × 24)
- 1 week = 10,080 minutes (7 × 24 × 60)
- 1 week = 604,800 seconds (7 × 24 × 60 × 60)
- 1 year ≈ 52.14 weeks (365 ÷ 7)
- 1 month ≈ 4.35 weeks (30 ÷ 7)
The 7-day structure: The week consists of seven consecutive days, typically organized as:
International (Monday-first) convention:
- Monday (Moon's day) - Start of work week
- Tuesday (Tiw's day, Norse god of war)
- Wednesday (Woden's day, Odin)
- Thursday (Thor's day, god of thunder)
- Friday (Frigg's day, goddess of love)
- Saturday (Saturn's day)
- Sunday (Sun's day) - Traditional day of rest
US (Sunday-first) convention:
- Sunday considered first day of the week on US calendars
- Work week runs Monday-Friday
- Weekend is Saturday-Sunday
ISO 8601 standard:
- Monday is officially day 1 of the week
- Sunday is day 7
- Week numbering: Week 1 contains first Thursday of year
Workweek vs. weekend:
- Workweek/weekdays: Monday-Friday (5 days) in Western tradition
- Weekend: Saturday-Sunday (2 days) in Western tradition
- Varies by culture: Friday-Saturday in Muslim countries, Sunday only historically
Why 7 days, not 5, 8, or 10? Unlike the day (Earth rotation) or year (orbital period), the week has no astronomical basis. It's purely a human cultural construct that gained universal adoption through:
- Ancient Babylonian astronomy (7 visible celestial bodies)
- Jewish religious tradition (Genesis creation, Sabbath commandment)
- Christian adoption and spread (Sunday worship)
- Islamic adoption (Friday as holy day)
- Roman Empire standardization (321 CE Constantine decree)
- Deep cultural entrenchment making change impractical
What Is a Sidereal Day?
A sidereal day is the time required for Earth to complete one full rotation (360 degrees) on its axis relative to the fixed background stars.
Precise value: 1 sidereal day = 86,164.0905 seconds (mean sidereal day) = 23 hours, 56 minutes, 4.0905 seconds
Sidereal vs. Solar Day
Sidereal day (stellar reference):
- Earth's rotation relative to distant stars
- Duration: 23h 56m 4.091s
- Used by astronomers for telescope pointing
Solar day (Sun reference):
- Earth's rotation relative to the Sun
- Duration: 24h 00m 00s (mean solar day)
- Used for civil timekeeping (clocks, calendars)
The difference: ~3 minutes 56 seconds
Why Are They Different?
The sidereal-solar day difference arises from Earth's orbital motion around the Sun:
- Start position: Earth completes one full 360° rotation relative to stars (1 sidereal day)
- Orbital motion: During that rotation, Earth has moved ~1° along its orbit around the Sun
- Extra rotation needed: Earth must rotate an additional ~1° (~4 minutes) to bring the Sun back to the same position in the sky
- Result: Solar day = sidereal day + ~4 minutes
Analogy: Imagine walking around a merry-go-round while it spins. If you walk one full circle relative to the surrounding park (sidereal), you'll need to walk a bit farther to return to the same position relative to the merry-go-round center (solar).
One Extra Day Per Year
A surprising consequence: There is one more sidereal day than solar day in a year!
- Solar year: 365.242199 solar days
- Sidereal year: 365.256363 sidereal days
- Extra sidereal days: 366.256363 - 365.242199 ≈ 1 extra day
Why? Earth makes 366.25 full rotations relative to the stars during one orbit, but we only experience 365.25 sunrises because we're moving around the Sun.
Note: The Week is part of the imperial/US customary system, primarily used in the US, UK, and Canada for everyday measurements. The Sidereal Day belongs to the imperial/US customary system.
History of the Week and Sidereal Day
of the Week
Ancient Babylonian Origins (c. 2000-1000 BCE)
The 7-day week's roots lie in ancient Mesopotamian astronomy and astrology:
Babylonian astronomy:
-
Observed seven "wandering stars" (planets) visible to naked eye:
- Sun (Shamash) - brightest object
- Moon (Sin) - most obviously changing
- Mercury (Nabu) - messenger god
- Venus (Ishtar) - morning/evening star
- Mars (Nergal) - red planet, war god
- Jupiter (Marduk) - king of gods
- Saturn (Ninurta) - slow-moving
-
Each celestial body "ruled" one day
-
Seven was considered mystical/sacred number
-
Used in astrological predictions and religious rituals
Why 7 was special:
- Seven visible "planets" (including Sun and Moon)
- Seven days between moon phases (~7.4 days per quarter)
- Mathematical: 7 is prime, making it special
- Religious significance in Near Eastern cultures
Note: The moon's phases (29.5 days ÷ 4 ≈ 7.4 days) may have influenced the 7-day cycle, though it doesn't align perfectly.
Jewish Religious Codification (c. 1500-500 BCE)
The Hebrew Bible (Torah) embedded the 7-day week in religious law:
Genesis creation narrative (Genesis 1:1-2:3):
- Day 1: Light and darkness
- Day 2: Sky and waters
- Day 3: Land, seas, plants
- Day 4: Sun, moon, stars
- Day 5: Fish and birds
- Day 6: Land animals and humans
- Day 7: God rested → Sabbath (Shabbat)
Fourth Commandment (Exodus 20:8-11):
"Remember the Sabbath day, to keep it holy. Six days you shall labor and do all your work, but the seventh day is a Sabbath to the Lord your God."
Sabbath observance:
- Saturday (7th day) as mandatory day of rest
- No work permitted (cooking, travel, commerce)
- Synagogue worship and family meals
- Violations carried severe penalties (death in ancient times)
- Core to Jewish identity for 3,000+ years
Jewish week structure:
- Days numbered: Yom Rishon (Day 1) through Yom Shishi (Day 6)
- Only Shabbat (Sabbath, Day 7) has a name
- Week begins Saturday evening (sunset) and ends following Saturday sunset
Greek and Roman Adoption (300 BCE - 400 CE)
Greek influence:
- Hellenistic astronomers (post-Alexander) adopted Babylonian astrology
- Each day associated with a planet/deity
- Week spread through Greek-speaking world
- Ptolemy's astrology (2nd century CE) codified planetary hours and days
Roman nundinal cycle (753 BCE - 321 CE):
- Romans initially used 8-day market week (nundinae)
- Days labeled A through H
- Markets held every 8th day
- Used for agricultural and commercial scheduling
Planetary week adoption (1st-3rd century CE):
- 7-day planetary week entered Rome from Near East
- Coexisted with 8-day nundinal cycle
- Gradually replaced nundinal week for religious/astrological reasons
- Days named after planets/gods:
- Dies Solis (Sun) → Sunday
- Dies Lunae (Moon) → Monday
- Dies Martis (Mars) → Tuesday (Tiw = Germanic Mars)
- Dies Mercurii (Mercury) → Wednesday (Woden = Germanic Mercury)
- Dies Jovis (Jupiter) → Thursday (Thor = Germanic Jupiter)
- Dies Veneris (Venus) → Friday (Frigg = Germanic Venus)
- Dies Saturni (Saturn) → Saturday
Constantine's decree (321 CE):
- Emperor Constantine I officially recognized the 7-day week
- Declared Sunday (Dies Solis) a day of rest
- Aligned with Christian practice (resurrection day)
- Marked official end of nundinal cycle
- Made 7-day week legal standard across Roman Empire
Christian Transformation (1st-5th century CE)
Early Christian practice:
- Jewish Christians initially observed Saturday Sabbath
- Gradually shifted to Sunday (Dies Dominica, "Lord's Day")
- Commemorated Jesus's resurrection (Sunday morning)
- Sunday worship established by 100 CE
Christian week structure:
- Sunday: Lord's Day, primary worship
- Monday-Saturday: Workdays
- No Sabbath work prohibition (unlike Judaism)
- Sunday rest became custom, not religious law initially
Church influence:
- Constantine's decree (321 CE) made Sunday official rest day
- Christian terminology replaced pagan planet names in some languages:
- Portuguese: Domingo (Sunday = Lord's Day), Segunda-feira (Monday = Second day)
- Some Slavic languages: similar pattern
- Christian calendar organized around Sunday as "first day of week" (Western tradition)
Medieval Christian week:
- Elaborate liturgical calendar
- Different saints' days on specific weekdays
- Friday fasting (commemorating crucifixion)
- Sunday mandatory Mass attendance
- Week structured around religious observances
Islamic Adoption (7th century CE)
Islamic week (al-usbūʿ):
- Adopted existing 7-day week structure
- Friday (Jumu'ah) designated as day of congregational prayer
- Not a "day of rest" like Sabbath/Sunday—work permitted
- Friday midday prayer (Jumu'ah prayer) mandatory for men
Islamic day names:
- Days numbered similar to Hebrew tradition
- Saturday: Yawm as-Sabt (Day of the Sabbath—Hebrew influence)
- Sunday: Yawm al-Ahad (First day)
- Monday: Yawm al-Ithnayn (Second day)
- ...
- Friday: Yawm al-Jumu'ah (Day of Congregation)
Spread of Islamic week:
- Islamic expansion (7th-15th centuries) spread 7-day week to:
- North Africa
- Middle East
- Central Asia
- Parts of Southeast Asia
- Reinforced 7-day week as global standard
Global Standardization (1500-1900)
European colonialism:
- Spanish, Portuguese, French, British empires spread 7-day week
- Christian Sunday observance imposed in colonies
- Replaced indigenous time-keeping systems:
- Aztec 13-day and 20-day cycles
- Mayan complex calendar system
- Various Asian lunar-based systems
East Asia adoption:
- China: Adopted 7-day week in early 20th century (previously used 10-day xún divisions)
- Japan: Officially adopted 7-day week in 1873 during Meiji Restoration
- Korea: Adopted with modernization in late 19th/early 20th century
International commerce:
- Global trade required synchronized schedules
- Shipping and maritime schedules used 7-day week
- Telegraph and later telecommunications standardized weekly communications
Failed Reform Attempts
Despite universal adoption, several attempts to "improve" the week failed:
1. French Revolutionary Calendar (1793-1805):
- Replaced 7-day week with 10-day décade
- Aligned with metric system (10 days per week, 3 weeks per month)
- Days numbered Primidi through Décadi
- Only Décadi was rest day (1 in 10 vs. 1 in 7)
- Failed because:
- Less frequent rest days unpopular with workers
- Conflicted with Christian Sunday observance
- Disrupted social and family patterns
- Napoleon abolished it in 1805
2. Soviet 5-day and 6-day weeks (1929-1940):
-
1929-1931: 5-day "continuous week"
- Days numbered 1-5
- Each worker got one of five days off (rotating)
- Goal: Continuous factory production
- Problem: Families/friends couldn't synchronize time off
-
1931-1940: 6-day week
- Days numbered 1-6
- Day 6 was universal rest day
- Goal: Improve on 5-day system
- Problem: Still disrupted religious observance, traditional patterns
-
1940: Return to 7-day week
- Abandoned experiments
- Restored traditional Sunday rest
- 7-day week too culturally embedded to change
3. International Fixed Calendar (1923-present, never adopted):
- Proposed by Moses B. Cotsworth
- 13 months of 28 days each (4 perfect weeks per month)
- Extra month called "Sol" between June and July
- One "Year Day" outside the weekly cycle
- Never adopted because:
- Would disrupt all existing calendars
- Breaking the continuous 7-day cycle unacceptable religiously
- Massive economic costs
- Resistance from established institutions
4. Other proposals:
- Decimal weeks (10 days)
- 5-day weeks (aligned with work week)
- 8-day weeks (better divides into month)
- All failed: Cultural inertia too strong
Modern Universal Adoption
Current status:
- All 195+ countries use the 7-day week
- Synchronized globally despite cultural differences
- ISO 8601 standard (Monday = day 1, week 1 contains first Thursday)
- Different weekend patterns:
- Saturday-Sunday: Most of world (Christian tradition)
- Friday-Saturday: Many Muslim countries (Saudi Arabia, UAE until 2022)
- Friday only: Iran
- Sunday only: Historical in some countries
Why 7-day week succeeded:
- Religious universality: Judaism, Christianity, Islam all use 7-day week
- Ancient origins: 3,000+ years of continuity
- Global colonization: European powers spread it worldwide
- Economic integration: International commerce requires synchronization
- Cultural entrenchment: Too deeply embedded to change
- Mathematical convenience: Fits reasonably with months (4-5 weeks)
- Work-rest balance: 5-2 or 6-1 work-rest ratio culturally accepted
Modern cultural significance:
- Phrase "work week" universal
- "Weekend" concept global (even if different days)
- Weekly planning horizon standard
- Pay periods often weekly or bi-weekly
- Television programming on weekly schedules
- Religious observances every 7 days
- Social rhythms organized weekly
Ancient Observations (2000-300 BCE)
Babylonian astronomy (circa 2000-1500 BCE):
- Babylonian astronomers tracked stellar positions for astrological and calendrical purposes
- Noticed stars rose earlier each night relative to the Sun's position
- Created star catalogs showing this gradual eastward drift
Greek astronomy (circa 600-300 BCE):
- Thales of Miletus (624-546 BCE): Used stellar observations for navigation
- Meton of Athens (432 BCE): Discovered the 19-year Metonic cycle, reconciling lunar months with solar years
- Recognized that stellar year differed from seasonal year
Hipparchus and Precession (150 BCE)
Hipparchus of Nicaea (circa 190-120 BCE), one of history's greatest astronomers:
Discovery: By comparing ancient Babylonian star catalogs with his own observations, Hipparchus discovered precession of the equinoxes—the slow westward drift of the vernal equinox against the stellar background
Sidereal measurements: To detect this subtle effect (1 degree per 72 years), Hipparchus needed precise sidereal positions, implicitly understanding the sidereal day concept
Legacy: His work established the difference between:
- Sidereal year: One orbit relative to stars (365.256363 days)
- Tropical year: One cycle of seasons (365.242199 days)
The ~20-minute difference between these years arises from precession.
Ptolemy's Almagest (150 CE)
Claudius Ptolemy compiled Greek astronomical knowledge in the Almagest, including:
- Star catalogs with sidereal positions
- Mathematical models for predicting stellar rising times
- Understanding that stars complete one full circuit of the sky slightly faster than the Sun
Though Ptolemy's geocentric model was wrong, his sidereal observations were accurate and useful for centuries.
Islamic Golden Age (800-1400 CE)
Islamic astronomers refined sidereal timekeeping:
Al-Battani (850-929 CE):
- Measured the tropical year to high precision
- Created improved star catalogs using sidereal positions
Ulugh Beg (1394-1449 CE):
- Built the Samarkand Observatory with advanced instruments
- Produced star catalogs accurate to ~1 arcminute using sidereal measurements
Copernican Revolution (1543)
Nicolaus Copernicus (De revolutionibus orbium coelestium, 1543):
Heliocentric model: Placing the Sun (not Earth) at the center explained the sidereal-solar day difference:
- Earth rotates on its axis (sidereal day)
- Earth orbits the Sun (creating solar day difference)
- The 4-minute discrepancy results from Earth's ~1° daily orbital motion
This was strong evidence for heliocentrism, though it took decades for acceptance.
Kepler's Laws (1609-1619)
Johannes Kepler formulated laws of planetary motion using sidereal periods:
Third Law: The square of a planet's orbital period is proportional to the cube of its orbit's semi-major axis
Application: Calculating planetary positions required precise sidereal reference frames, not solar time
Rise of Telescopic Astronomy (1600s-1700s)
Galileo Galilei (1609):
- Telescopic observations required tracking celestial objects as they moved across the sky
- Sidereal time became essential for predicting when objects would be visible
Royal Observatory, Greenwich (1675):
- Founded by King Charles II with John Flamsteed as first Astronomer Royal
- Developed accurate sidereal clocks to time stellar transits
- Greenwich Mean Sidereal Time (GMST) became the astronomical standard
Paris Observatory (1667):
- French astronomers developed precision pendulum clocks for sidereal timekeeping
- Cassini family produced detailed planetary observations using sidereal coordinates
Precision Timekeeping (1800s)
19th century: Mechanical sidereal clocks achieved second-level accuracy:
Sidereal clock design: Modified to tick 366.2422/365.2422 times faster than solar clocks (accounting for the extra sidereal day per year)
Observatory operations: Major observatories (Greenwich, Paris, Harvard, Lick, Yerkes) used sidereal clocks as primary timekeeping for scheduling observations
Photography: Long-exposure astrophotography required tracking objects at the sidereal rate to prevent star trailing
IAU Standardization (1900s)
International Astronomical Union (IAU) formalized definitions:
Mean sidereal day: 86,164.0905 seconds (exactly, by definition)
Greenwich Mean Sidereal Time (GMST): Standard sidereal time referenced to Greenwich meridian
Vernal equinox reference: Traditional sidereal time measures Earth's rotation relative to the vernal equinox (intersection of celestial equator and ecliptic)
Modern Era: ICRF (1997-Present)
International Celestial Reference Frame (ICRF):
Problem: The vernal equinox shifts due to precession, making it an imperfect reference
Solution: ICRF uses ~300 distant quasars (billions of light-years away) as fixed reference points
Accuracy: Defines celestial positions to milliarcsecond precision
Atomic time: Sidereal time is now calculated from International Atomic Time (TAI) and Earth orientation parameters measured by Very Long Baseline Interferometry (VLBI)
Modern sidereal clocks: Digital, GPS-synchronized, automatically updated for Earth rotation variations
Common Uses and Applications: weeks vs sidereal days
Explore the typical applications for both Week (imperial/US) and Sidereal Day (imperial/US) to understand their common contexts.
Common Uses for weeks
When to Use sidereal days
1. Telescope Pointing and Tracking
Professional observatories use sidereal time to point telescopes:
Right Ascension (RA): Celestial equivalent of longitude, measured in hours of sidereal time (0h to 24h)
Local Sidereal Time (LST): The current RA crossing the meridian
Pointing formula: If LST = 18h 30m, objects with RA ≈ 18h 30m are currently at their highest point (zenith)
Tracking rate: Telescope motors rotate at the sidereal rate (1 rotation per 23h 56m 4s) to follow stars across the sky as Earth rotates
Example:
- Vega: RA = 18h 37m
- When LST = 18:37, Vega crosses the meridian (highest in sky)
- Observer can plan observations when object will be optimally placed
2. Astrophotography
Long-exposure astrophotography requires tracking at the sidereal rate:
Problem: Earth's rotation makes stars trail across the image during long exposures
Solution: Equatorial mounts with sidereal drive motors:
- Rotate at exactly 1 revolution per sidereal day
- Keep stars fixed in the camera's field of view
- Enables exposures of minutes to hours without star trailing
Adjustment: Solar rate ≠ sidereal rate; photographers must use sidereal tracking for stars, solar tracking for Sun/Moon
3. Satellite Orbit Planning
Satellite engineers use sidereal time for orbit design:
Sun-synchronous orbits: Satellites that always cross the equator at the same local solar time
- Orbital period is chosen to precess at the solar rate, not sidereal rate
Geosynchronous orbits: Satellites that hover over one point on Earth
- Orbital period = 1 sidereal day (23h 56m 4s)
- NOT 24 hours! Common misconception.
Molniya orbits: High-eccentricity orbits with period = 0.5 sidereal days for optimal high-latitude coverage
4. Very Long Baseline Interferometry (VLBI)
Radio astronomers use VLBI to achieve ultra-high resolution:
Technique: Combine signals from radio telescopes across continents
Timing requirement: Sidereal time must be synchronized to nanosecond precision across all telescopes
Result: VLBI can resolve features 1,000 times smaller than Hubble Space Telescope (angular resolution ~0.0001 arcseconds)
Application: Measures Earth's rotation variations by observing quasars at precise sidereal times
5. Navigation and Geodesy
Sidereal time is used for precise Earth orientation measurements:
Earth Orientation Parameters (EOPs):
- Polar motion (wobble of Earth's axis)
- UT1 (Earth rotation angle, related to Greenwich sidereal time)
- Length of day variations
GPS accuracy: GPS navigation requires knowing Earth's orientation to ~1 meter precision, necessitating sidereal time corrections
Tidal forces: Moon and Sun create tidal bulges that affect Earth's rotation, causing sidereal day variations at the millisecond level
6. Space Navigation
Spacecraft use sidereal reference frames:
Star trackers: Autonomous spacecraft orientation using star patterns
- Compare observed stellar positions with catalog
- Catalog uses sidereal coordinates (RA/Dec)
Interplanetary navigation: Voyager, New Horizons, and other deep-space probes navigate using sidereal reference frames (ICRF)
Mars rovers: Use Martian sidereal time ("sols") for mission planning
- 1 Mars sol = 24h 39m 35s (Mars rotates slower than Earth)
7. Amateur Astronomy
Amateur astronomers use sidereal time for planning:
Planispheres: Rotating star charts that show which constellations are visible at any given sidereal time and date
Computerized telescopes: GoTo mounts require accurate sidereal time for automatic star finding
Observation logs: Record sidereal time of observations for repeatability
Additional Unit Information
About Week (wk)
How many days are in a week?
Exactly 7 days in every week, universally across all cultures and countries worldwide.
This has been standard for over 2,000 years, originating from:
- Ancient Babylonian astronomy (7 visible celestial bodies)
- Jewish religious tradition (Genesis 7-day creation + Sabbath)
- Roman adoption and global spread
The 7-day week has no astronomical basis (unlike day or year) but achieved universal cultural adoption.
How many hours are in a week?
Exactly 168 hours in one week.
Calculation: 7 days × 24 hours/day = 168 hours
Context:
- Work week: 40 hours (standard full-time) out of 168 total
- Sleep: 56 hours per week (8 hours/night × 7 nights)
- Leisure: 168 - 40 (work) - 56 (sleep) = 72 hours
- Work-life balance: Only ~24% of week spent working (40/168)
Why does a week have 7 days?
The 7-day week has cultural and religious origins, not astronomical:
Three main reasons:
-
Babylonian astronomy (c. 2000 BCE):
- Seven visible "planets": Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn
- Each day dedicated to one celestial body
- Seven considered sacred number
-
Jewish religious tradition (c. 1500 BCE):
- Genesis: God created world in 6 days, rested on 7th (Sabbath)
- Fourth Commandment: "Remember the Sabbath day"
- Embedded in religious law for 3,000+ years
-
Global adoption:
- Christianity spread Sunday worship (resurrection day)
- Islam adopted 7-day week with Friday prayers
- Roman Empire standardized it (321 CE Constantine decree)
- Colonial expansion made it universal
Why not 5, 8, or 10 days? All attempts to change it failed (French 10-day, Soviet 5/6-day weeks) due to deep cultural and religious entrenchment.
How many weeks are in a year?
52.14 weeks in a standard 365-day year.
Calculation: 365 days ÷ 7 days/week = 52.14 weeks (52 weeks + 1 day)
More precisely:
- Common year (365 days): 52 weeks + 1 day
- Leap year (366 days): 52 weeks + 2 days
Practical implications:
- 52 "full weeks" per year
- Extra 1-2 days cause annual calendar drift
- Same date falls on different day of week each year
ISO week-numbering:
- Most years: Weeks 1-52
- Some years: Weeks 1-53 (when year has 53 Thursdays)
What is a work week?
A work week is the 5-day period from Monday-Friday when most businesses operate, totaling 40 hours (8 hours/day × 5 days) in the US standard.
Work week structure:
- Weekdays: Monday-Friday (5 days) - work/school days
- Weekend: Saturday-Sunday (2 days) - rest days
- 5-2 split: 5 days work, 2 days rest
Hours:
- US full-time: 40 hours per week standard
- France: 35 hours per week legal standard
- Part-time: 20-30 hours per week
- Overwork: 50-60+ hours per week
Variations:
- 4-day work week: Emerging trend (32-40 hours over 4 days)
- 6-day work week: Historical standard, still common in some countries
- Muslim countries: Friday-Saturday weekend (work Sunday-Thursday)
Origins:
- Industrial Revolution: Standardized factory schedules
- Labor movements: Won 5-day, 40-hour week (1926-1940 in US)
- Henry Ford: Pioneered 5-day, 40-hour week (1926)
- Fair Labor Standards Act (1938): Codified 40-hour week in US
What is the weekend?
The weekend is the 2-day period of rest at the end of the work week, typically Saturday and Sunday in Western countries.
Weekend structure:
- Saturday: First day off
- Sunday: Second day off, traditional Christian day of worship
- Purpose: Rest, recreation, family time, errands
Global variations:
- Western countries: Saturday-Sunday (majority of world)
- Muslim countries: Friday-Saturday or Friday-Sunday (historically)
- Saudi Arabia, UAE: Switched to Saturday-Sunday in 2022
- Iran: Friday only
- Israel: Friday-Saturday (aligns with Jewish Sabbath)
- Brunei, Bangladesh: Friday-Saturday
Origins:
- Jewish Sabbath: Saturday rest day (biblical commandment)
- Christian Sunday: Lord's Day (resurrection observance)
- Industrial era: Originally only Sunday off
- 1920s-1940s: Saturday added, creating "weekend"
- Labor advocacy: "Saturday half-day" became full day off
Cultural significance:
- "Thank God It's Friday" (TGIF)
- "Weekend warrior" (active on weekends)
- "Monday blues" (dreading return to work)
- Weekend social events, sports, entertainment
How many weeks are in a month?
Approximately 4.35 weeks in an average month.
Calculation:
- Average month = 30.44 days (365 ÷ 12)
- 30.44 days ÷ 7 days/week = 4.35 weeks
Actual variation:
- February: 4.0 weeks (28 days), 4.14 weeks (29 days, leap year)
- 30-day months: 4.29 weeks (April, June, September, November)
- 31-day months: 4.43 weeks (January, March, May, July, August, October, December)
Why not exactly 4 weeks?
- 4 weeks = 28 days
- Most months = 30-31 days
- 2-3 days "extra" per month
Implications:
- "Monthly" ≠ "every 4 weeks"
- Monthly salary ≠ 4 weekly salaries
- Rent is monthly (12 times/year), not 4-weekly (13 times/year)
What is a fortnight?
A fortnight is a period of 14 days or 2 weeks.
Origin:
- Old English: fēowertīene niht = "fourteen nights"
- Common in British English
- Less common in American English
Usage:
- UK: "I'll see you in a fortnight" (2 weeks from now)
- Australia/New Zealand: Common term
- Pay periods: "Fortnightly pay" = paid every 2 weeks
- Planning: "Fortnight holiday" = 2-week vacation
Related terms:
- Bi-weekly: Every 2 weeks (26 times per year)
- Semi-monthly: Twice per month (24 times per year)
- Fortnight = bi-weekly interval, not semi-monthly
Why do weekends exist?
Weekends exist due to religious tradition and labor reform:
Religious origins:
- Jewish Sabbath: Saturday rest day (biblical commandment, ~3,000 years old)
- Christian Sunday: Lord's Day, resurrection observance (2,000 years old)
- Both religions mandate one day of rest per week
Industrial era (1800s-1900s):
- Initially: 6-day work week, only Sunday off (Christian influence)
- Workers labored Monday-Saturday, 10-16 hours per day
- Exhausting, no family time
Labor reform (1900s):
- 1908: First 5-day work week proposed
- 1926: Henry Ford adopted 5-day, 40-hour week (factory efficiency + consumer spending)
- 1929: Great Depression led to work-sharing (reduce hours to employ more)
- 1938: Fair Labor Standards Act (US) established 40-hour week with overtime
- 1940: 5-day work week became US standard
Why 2-day weekend prevailed:
- Productivity: Workers more productive with adequate rest
- Consumer economy: Workers with free time spend money
- Family time: Social benefits
- Religious observance: Accommodates both Saturday (Jewish) and Sunday (Christian)
- Union advocacy: Labor movements fought for it
Modern trends:
- 4-day work week experiments (same hours, compressed)
- Flexible schedules: "Weekend" varies by individual
- Remote work blurs work-weekend boundaries
Can weeks start on different days?
Yes, weeks can start on either Sunday or Monday depending on cultural convention, though the 7-day cycle remains constant.
Two main systems:
1. Sunday-first (traditional Christian):
- Used in: United States, Canada, parts of Latin America
- Rationale: Sunday is the Lord's Day, "first day of week" in Christian tradition
- Calendars: US calendars show Sunday as leftmost column
- Biblical: Genesis creation starts with Sunday
2. Monday-first (ISO standard):
- Used in: Europe, Asia, Africa, Australia (most of world)
- ISO 8601 standard: Monday = day 1, Sunday = day 7
- Rationale: Work week starts Monday, weekend (Saturday-Sunday) grouped together
- Calendars: International calendars show Monday as leftmost column
Which is "correct"?
- Both are valid cultural conventions
- ISO 8601 standardizes Monday-first for international business/computing
- Work week universally Monday-Friday regardless
Computing:
- Programming: ISO 8601 standard (Monday = 1)
- Excel/Google Sheets: Can be configured either way
- Date/time libraries: Often use ISO standard
Practical impact:
- Minimal—everyone uses same 7-day cycle
- Only affects calendar layout and "first day" reference
- "Weekend" always means Saturday-Sunday (or local equivalent)
About Sidereal Day (sidereal day)
How long is a sidereal day in standard time?
Answer: 23 hours, 56 minutes, 4.091 seconds (or 86,164.091 seconds)
This is the time for Earth to rotate exactly 360 degrees relative to distant stars.
Precise value: 1 mean sidereal day = 86,164.0905 seconds
Comparison to solar day:
- Solar day: 86,400 seconds (24 hours)
- Sidereal day: 86,164.091 seconds
- Difference: ~236 seconds shorter (~3 min 56 sec)
Important: This is the mean sidereal day. Earth's actual rotation rate varies slightly (milliseconds) due to tidal forces, atmospheric winds, earthquakes, and core-mantle coupling.
Why is a sidereal day shorter than a solar day?
Answer: Because Earth orbits the Sun while rotating—requiring extra rotation to bring the Sun back to the same sky position
Step-by-step explanation:
-
Starting point: The Sun is directly overhead (noon)
-
One sidereal day later (23h 56m 4s): Earth has rotated exactly 360° relative to stars
- But Earth has also moved ~1° along its orbit around the Sun
- The Sun now appears slightly east of overhead
-
Extra rotation needed: Earth must rotate an additional ~1° (taking ~4 minutes) to bring the Sun back overhead
-
Result: Solar day (noon to noon) = sidereal day + ~4 minutes = 24 hours
Orbital motion causes the difference: Earth moves ~1°/day along its 365-day orbit (360°/365 ≈ 0.986°/day). This ~1° requires ~4 minutes of extra rotation (24 hours / 360° ≈ 4 min/degree).
Consequence: Stars rise ~4 minutes earlier each night relative to solar time, shifting ~2 hours per month, completing a full cycle annually.
Is sidereal time the same everywhere on Earth?
Answer: No—Local Sidereal Time (LST) depends on longitude, just like solar time zones
Key concepts:
Local Sidereal Time (LST): The Right Ascension (RA) currently crossing your local meridian
- Different at every longitude
- Changes by 4 minutes for every 1° of longitude
Greenwich Mean Sidereal Time (GMST): Sidereal time at 0° longitude (Greenwich meridian)
- Global reference point, like GMT/UTC for solar time
Conversion: LST = GMST ± longitude offset
- Positive (add) for east longitudes
- Negative (subtract) for west longitudes
Example:
- GMST = 12:00
- New York (74°W): LST = 12:00 - (74°/15) = 07:04
- Tokyo (139.75°E): LST = 12:00 + (139.75°/15) = 21:19
Duration is universal: A sidereal day (23h 56m 4s) is the same length everywhere—only the current sidereal time differs by location.
Do geosynchronous satellites orbit every 24 hours or 23h 56m?
Answer: 23h 56m 4s (one sidereal day)—NOT 24 hours!
This is one of the most common misconceptions about satellites.
The physics: For a satellite to remain above the same point on Earth's surface, it must orbit at Earth's rotational rate relative to the stars, not relative to the Sun.
Why sidereal?
- Earth rotates 360° in one sidereal day (23h 56m 4s)
- Satellite must complete 360° orbit in the same time
- This keeps satellite and ground point aligned relative to the stellar background
If orbit were 24 hours: The satellite would complete one orbit in one solar day, but Earth would have rotated 360° + ~1° (relative to stars) during that time. The satellite would drift ~1° westward per day, completing a full circuit westward in one year!
Geostationary orbit specifics:
- Altitude: 35,786 km above equator
- Period: 23h 56m 4.091s (1 sidereal day)
- Velocity: 3.075 km/s
Common examples: Communications satellites, weather satellites (GOES, Meteosat)
How many sidereal days are in a year?
Answer: Approximately 366.25 sidereal days—one MORE than the number of solar days!
Precise values:
- Tropical year (season to season): 365.242199 mean solar days
- Sidereal year (star to star): 365.256363 mean solar days
- Sidereal days in tropical year: 366.242199 sidereal days
One extra day: There is exactly one more complete rotation relative to stars than we experience sunrises.
Why?
- Earth makes 366.25 complete 360° rotations relative to stars per year
- But we experience only 365.25 sunrises because we orbit the Sun
- One rotation is "used up" by Earth's orbit around the Sun
Thought experiment: Stand on a rotating platform while walking around a lamp. If you walk one complete circle around the lamp (1 orbit), you'll have spun around 2 complete times relative to the room walls (2 rotations): 1 from walking the circle + 1 from the platform spinning.
Can I use a regular clock to tell sidereal time?
Answer: Not directly—sidereal clocks run about 4 minutes faster per day than solar clocks
Clock rate difference:
- Solar clock: Completes 24 hours in 1 solar day (86,400 seconds)
- Sidereal clock: Completes 24 sidereal hours in 1 sidereal day (86,164.091 seconds)
- Rate ratio: 1.00273791 (sidereal clock ticks ~0.27% faster)
Practical result: After one solar day:
- Solar clock reads: 24:00
- Sidereal clock reads: 24:03:56 (3 min 56 sec ahead)
Modern solutions:
- Sidereal clock apps: Smartphone apps calculate LST from GPS location and atomic time
- Planetarium software: Stellarium, SkySafari show current LST
- Observatory systems: Automated telescopes use GPS-synchronized sidereal clocks
Historical: Mechanical sidereal clocks used gear ratios of 366.2422/365.2422 to run at the correct rate
You can calculate: LST from solar time using formulas, but it's complex (requires Julian Date, orbital mechanics)
Why do astronomers use sidereal time instead of solar time?
Answer: Because celestial objects return to the same position every sidereal day, not solar day
Astronomical reason:
Stars and galaxies are so distant they appear "fixed" in the sky:
- A star at RA = 18h 30m crosses the meridian at LST = 18:30 every sidereal day
- Predictable, repeatable observations
If using solar time: Stars would cross the meridian ~4 minutes earlier each night, requiring daily recalculation of observation windows
Practical advantages:
1. Simple telescope pointing:
- Object's RA directly tells you when it's overhead (LST = RA)
- No date-dependent calculations needed
2. Repeatable observations:
- "Observe target at LST = 22:00" means the same sky position regardless of date
3. Right Ascension coordinate system:
- Celestial longitude measured in hours/minutes of sidereal time (0h to 24h)
- Aligns naturally with Earth's rotation
4. Tracking rate:
- Telescopes track at sidereal rate (1 revolution per 23h 56m 4s)
- Keeps stars fixed in the field of view
Historical: Before computers, sidereal time made astronomical calculations much simpler
What is the difference between a sidereal day and a sidereal year?
Answer: A sidereal day measures Earth's rotation; a sidereal year measures Earth's orbit
Sidereal Day:
- Definition: Time for Earth to rotate 360° on its axis relative to stars
- Duration: 23h 56m 4.091s (86,164.091 seconds)
- Reference: Distant "fixed" stars
- Use: Telescope tracking, astronomy observations
Sidereal Year:
- Definition: Time for Earth to orbit 360° around the Sun relative to stars
- Duration: 365.256363 days (365d 6h 9m 9s)
- Reference: Position relative to distant stars (not seasons)
- Use: Orbital mechanics, planetary astronomy
Key distinction:
- Day = rotation (Earth spinning)
- Year = revolution (Earth orbiting)
Tropical vs. Sidereal Year:
- Tropical year: 365.242199 days (season to season, used for calendars)
- Sidereal year: 365.256363 days (star to star)
- Difference: ~20 minutes, caused by precession of Earth's axis
The 20-minute precession effect: Earth's axis wobbles with a 26,000-year period, causing the vernal equinox to shift ~50 arcseconds/year westward against the stellar background. This makes the tropical year (equinox to equinox) slightly shorter than the sidereal year (star to star).
Does the Moon have a sidereal day?
Answer: Yes—the Moon's sidereal day is 27.322 Earth days, but it's tidally locked to Earth
Moon's sidereal rotation: Time for Moon to rotate 360° relative to stars = 27.322 days
Tidal locking: The Moon's rotation period equals its orbital period around Earth (both 27.322 days)
Consequence: The same face of the Moon always points toward Earth
- We only see ~59% of Moon's surface from Earth (libration allows slight wobbling)
- The "far side" never faces Earth
Moon's "solar day" (lunar day):
- Time from sunrise to sunrise on Moon's surface: 29.531 Earth days
- Different from Moon's sidereal day (27.322 days) for the same reason Earth's solar day differs from sidereal day
- Moon orbits Earth while rotating, requiring extra rotation to bring the Sun back to the same position
Lunar missions: Apollo missions and rovers used "lunar days" for mission planning—each day-night cycle lasts ~29.5 Earth days (2 weeks daylight, 2 weeks night)
How is sidereal time measured today?
Answer: Using atomic clocks, GPS, and Very Long Baseline Interferometry (VLBI) observations of distant quasars
Modern measurement system:
1. International Atomic Time (TAI):
- Network of ~450 atomic clocks worldwide
- Defines the second with nanosecond precision
- Provides base timescale
2. UT1 (Universal Time):
- Earth's rotation angle (actual rotation measured continuously)
- Monitored by VLBI observations of quasars
3. VLBI technique:
- Radio telescopes across continents simultaneously observe distant quasars
- Time differences reveal Earth's exact orientation
- Accuracy: ~0.1 milliseconds (0.005 arcseconds rotation)
4. ICRF (International Celestial Reference Frame):
- Defines "fixed" stellar background using ~300 quasars billions of light-years away
- Replaces older vernal equinox reference (which shifts due to precession)
5. GPS satellites:
- Amateur astronomers and observatories use GPS for accurate time and location
- Software calculates LST from UTC, GPS coordinates, and Earth orientation parameters
Calculation chain:
- Atomic clocks provide UTC
- Earth orientation parameters (EOP) give UT1
- Sidereal time formulas convert UT1 → GMST
- Longitude correction gives LST
Accuracy: Modern systems know Earth's orientation to ~1 centimeter (as a position on Earth's surface), requiring sidereal time precision of ~0.001 seconds
Why so complex? Earth's rotation is not uniform:
- Tidal forces (Moon/Sun) slow rotation by ~2.3 ms/century
- Atmospheric winds cause daily variations (milliseconds)
- Earthquakes can shift rotation by microseconds
- Core-mantle coupling affects long-term drift
Continuous monitoring ensures astronomical observations remain accurate.
Will sidereal time ever be replaced by something else?
Answer: Unlikely—it's fundamental to astronomy, tied directly to Earth's rotation and stellar positions
Why sidereal time persists:
1. Physical basis: Directly tied to Earth's rotation relative to the universe
- Not an arbitrary human convention like time zones
- Essential for understanding celestial mechanics
2. Coordinate system: Right Ascension (celestial longitude) is measured in sidereal hours
- All star catalogs, telescope systems, and astronomical databases use RA/Dec
- Replacing it would require re-cataloging billions of objects
3. Telescope tracking: All telescope mounts track at the sidereal rate
- Mechanically and electronically built into equipment
- Solar tracking is used only for Sun/Moon
4. International standards: IAU, observatories, space agencies globally use sidereal time
- Standardized formulas and software
5. No alternative needed: Sidereal time does its job perfectly for astronomy
Evolution, not replacement:
- Old reference: Vernal equinox (shifts due to precession)
- New reference: ICRF quasars (effectively fixed)
- Future: Increasingly precise atomic timescales and Earth rotation monitoring
Non-astronomical contexts: Civil society will continue using solar time (UTC) for daily life—there's no need for most people to know sidereal time
Conclusion: Sidereal time is here to stay as long as humans do astronomy from Earth. Even space-based observatories use sidereal coordinate systems for consistency with ground observations.
Conversion Table: Week to Sidereal Day
| Week (wk) | Sidereal Day (sidereal day) |
|---|---|
| 0.5 | 3.51 |
| 1 | 7.019 |
| 1.5 | 10.529 |
| 2 | 14.038 |
| 5 | 35.096 |
| 10 | 70.192 |
| 25 | 175.479 |
| 50 | 350.958 |
| 100 | 701.917 |
| 250 | 1,754.791 |
| 500 | 3,509.583 |
| 1,000 | 7,019.165 |
People Also Ask
How do I convert Week to Sidereal Day?
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Can I convert Sidereal Day back to Week?
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Learn more →What are common uses for Week and Sidereal Day?
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National Institute of Standards and Technology — Official time standards and definitions
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Last verified: December 3, 2025