Sidereal Day to Decade Converter
Convert sidereal days to decades with our free online time converter.
Quick Answer
1 Sidereal Day = 0.000273 decades
Formula: Sidereal Day × conversion factor = Decade
Use the calculator below for instant, accurate conversions.
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Sidereal Day to Decade Calculator
How to Use the Sidereal Day to Decade Calculator:
- Enter the value you want to convert in the 'From' field (Sidereal Day).
- The converted value in Decade 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 Sidereal Day to Decade: Step-by-Step Guide
Converting Sidereal Day to Decade involves multiplying the value by a specific conversion factor, as shown in the formula below.
Formula:
1 Sidereal Day = 0.000273043 decadesExample Calculation:
Convert 60 sidereal days: 60 × 0.000273043 = 0.0163826 decades
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.
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View all Time conversions →What is a Sidereal Day and a Decade?
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.
A decade is a unit of time equal to 10 consecutive years. The word derives from Ancient Greek "dekas" (δεκάς), meaning "a group of ten."
Duration in Other Units
1 decade equals:
- 10 years (exactly)
- 120 months (10 × 12 months)
- ~520 weeks (10 × 52.18 weeks)
- 3,652 days (common years) or 3,653 days (including leap years)
- Average: 3,652.5 days (accounting for leap year cycle)
- 87,660 hours (3,652.5 × 24)
- 5,259,600 minutes (87,660 × 60)
- 315,576,000 seconds (5,259,600 × 60)
Decade Boundaries: The 0 vs. 1 Debate
Popular usage (dominant):
- 1980s = 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989 (years ending 0-9)
- 1990s = 1990-1999
- 2000s = 2000-2009
- 2010s = 2010-2019
- 2020s = 2020-2029
Formal reckoning (pedantic, rarely used):
- Because there was no year 0 in the Gregorian calendar (1 BCE → 1 CE directly), decades "should" span 1-10:
- 1st decade = 1-10 CE
- 199th decade = 1981-1990
- 200th decade = 1991-2000
- 201st decade = 2001-2010
Reality: Popular usage (0-9) dominates overwhelmingly. When people say "the 1980s," they mean 1980-1989, not 1981-1990. Cultural identity, nostalgia, and historical analysis all use the 0-9 convention.
Note: The Sidereal Day is part of the imperial/US customary system, primarily used in the US, UK, and Canada for everyday measurements. The Decade belongs to the imperial/US customary system.
History of the Sidereal Day and Decade
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
of the Decade Concept
1. Ancient Origins: Base-10 Counting (Prehistoric)
The human preference for base-10 (decimal) counting stems from having 10 fingers, making groups of 10 natural for organization.
Early base-10 applications:
- Ancient Egypt (~3000 BCE): Hieroglyphic numerals based on powers of 10
- Ancient China (~1500 BCE): Decimal system in oracle bone inscriptions
- Ancient Greece (~500 BCE): Decimal counting, though 12 and 60 also important
- Ancient Rome: Latin "decem" (ten) gave root to "decade"
Why 10-year groups? Humans naturally organize time into manageable chunks. A year is too short for long-term trends, a century too long for human memory—a decade provides a Goldilocks timescale for pattern recognition.
2. Medieval and Renaissance Period (500-1600 CE)
"Decade" as general term:
- Originally meant any group of 10 (10 books, 10 prayers, 10 beads)
- Rosary decades: Catholic rosary divided into 5 decades (groups of 10 Hail Marys)
- Literary decades: Collections of 10 stories or books (e.g., Boccaccio's "Decameron" = 10 days of stories)
Not yet specifically time-related: Medieval and Renaissance writers used "decade" for groupings, but not systematically for 10-year periods in historical analysis.
3. Enlightenment and Modern Historical Analysis (1700s-1800s)
Systematic historical periodization emerged:
- 18th-century historians began organizing events by 10-year periods for analysis
- Statistical thinking (late 1700s-1800s): Governments collected data in 10-year intervals
- US Census (1790-present): Conducted every 10 years, reinforcing decade thinking
- Economic cycles: Analysts noticed decade-scale patterns in markets, trade
Why decades gained prominence:
- Human lifespan scale: 10 years = noticeable change but within living memory
- Generational overlap: Enough time for trends to develop but not so long that witnesses die
- Data collection: Census and economic data naturally aligned with 10-year cycles
- Psychological salience: Decade birthdays (30, 40, 50, 60) mark life transitions
4. 20th Century: Decades as Cultural Identifiers (1900s-2000s)
The 20th century saw decades become powerful cultural and historical labels:
Early examples:
- "The Gay Nineties" (1890s): Retrospective label for late Victorian optimism
- "The Roaring Twenties" (1920s): Jazz Age, prohibition, economic boom, flapper culture
- "The Thirties" (1930s): Great Depression, Dust Bowl, rise of fascism
Post-WWII decade labels (most influential):
- "The Fifties" (1950s): Post-war prosperity, suburbs, rock and roll, Cold War begins
- "The Sixties" (1960s): Civil rights, Vietnam War, counterculture, sexual revolution, moon landing
- "The Seventies" (1970s): Disco, oil crisis, stagflation, Watergate, punk rock
- "The Eighties" (1980s): Reagan/Thatcher, Cold War end, MTV, personal computers, Wall Street boom
- "The Nineties" (1990s): Grunge, dot-com boom, end of Cold War, globalization, early internet
Why 20th-century decades are so culturally distinct:
- Rapid change: Technology, politics, culture shifted dramatically every 10 years
- Mass media: Radio (1920s-50s), TV (1950s-2000s), MTV (1980s-90s) amplified decade identity
- Music genres: Each decade had distinctive music (jazz/20s, rock/50s, disco/70s, grunge/90s)
- Fashion cycles: 10-year fashion trends reinforced decade boundaries
- Political eras: Often aligned with decade boundaries (Kennedy/60s, Reagan/80s)
5. 21st Century: Digital Age Decades (2000s-Present)
New challenges to decade identity:
- "The 2000s" = linguistic problem (no catchy name: noughties? aughts? two-thousands?)
- Faster change: Internet, smartphones, social media accelerate cultural shifts
- Decade labels less distinct: 2000s and 2010s harder to characterize uniquely
- Nostalgia cycles: 1980s and 1990s nostalgia dominated 2010s-2020s
2000s decade naming attempts:
- "Noughties" (British English): From "nought" (zero)
- "Aughts" (American English): From "aught" (zero, archaic)
- "Two-thousands": Clunky but functional
- "The zeros": Rarely used
2010s:
- "The twenty-tens" or "the tens": More natural linguistically
- Cultural markers: Smartphones ubiquitous, social media dominance, streaming services
2020s (current decade):
- "The twenties": Natural label, but overlaps with "Roaring Twenties" (1920s)
- COVID-19 pandemic (2020-2023) defined early decade
- Climate change, AI revolution, geopolitical shifts
Common Uses and Applications: sidereal days vs decades
Explore the typical applications for both Sidereal Day (imperial/US) and Decade (imperial/US) to understand their common contexts.
Common Uses for 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
When to Use decades
and Applications
1. Age and Life Stage Communication
"In my [decade]" describes life stage:
- "In my twenties": Young adulthood, career building, dating/marriage
- "In my thirties": Career advancement, family raising, homeownership
- "In my forties": Peak earning years, midlife transitions
- "In my fifties": Late career, empty nest, retirement planning
- "In my sixties": Retirement, grandparenting, senior status
More specific than single age: "I'm in my early thirties" conveys life stage better than "I'm 32."
2. Historical Analysis and Research
Decade-by-decade comparison standard:
- "Income in 1970 vs. 2020": 5-decade comparison
- "Homicide rates declined every decade since 1990s": Decade-scale trend
- "Technology adoption: 2000s smartphones, 2010s tablets, 2020s AI"
Academic papers often structured by decade:
- "This study examines voting patterns across three decades (1980s-2000s)"
3. Goal Setting and Life Planning
"Where do you see yourself in 10 years?" = classic interview/life question
10-year planning horizons:
- Personal goals: "Get promoted within a decade," "Be debt-free in 10 years"
- Savings goals: "Save $100K over next decade"
- Health goals: "Run marathon before I turn 40" (decade milestone)
Vision boards and bucket lists: Often include decade milestones (30th birthday trip, 40th marathon).
4. Demographic and Market Segmentation
Age groups by decade:
- 18-29: Young adults (target for entry-level products, dating apps)
- 30-39: Prime family/career demographic (target for houses, minivans, life insurance)
- 40-49: Peak earning (target for luxury goods, college savings plans)
- 50-59: Pre-retirement (target for wealth management, travel)
- 60+: Seniors (target for retirement services, healthcare)
Market research: "Millennials in their thirties prefer [X], while Gen X in their fifties prefer [Y]."
5. Forecasting and Trend Prediction
"Next decade" predictions common:
- Technology: "AI will transform work in the next decade"
- Climate: "2020s critical decade for climate action"
- Economics: "Inflation forecasts for the decade ahead"
Industry roadmaps: Auto industry "2030 targets" = decade-scale planning (all-electric by 2030).
6. Reunion and Anniversary Events
Class reunions every decade:
- 10-year reunion: Compare life trajectories, still young
- 20-year reunion: Mid-life, established careers/families
- 30-year reunion: Late career, nostalgia peaks
- 50-year reunion: Senior years, mortality awareness
Anniversary milestones: Companies, organizations celebrate 10th, 25th, 50th, 100th anniversaries.
7. Sports Records and Comparisons
"Athlete of the Decade" awards:
- AP Athlete of the Decade: Michael Jordan (1990s), Tiger Woods (2000s), Lionel Messi (2010s)
- Team dominance: "Yankees dominated baseball in 1990s," "Patriots dynasty in 2010s"
Decade statistics: "Home runs per decade in MLB" = long-term trend analysis.
Additional Unit Information
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.
About Decade (dec)
1. How many years are in one decade?
Exactly 10 years. The word "decade" comes from Greek "dekas" (δεκάς), meaning "group of ten."
Other units:
- 1 decade = 10 years = 120 months = ~3,652.5 days = 87,660 hours
2. How are decades typically named or referred to?
By tens digit: "The 1980s" (1980-1989), "the 2020s" (2020-2029).
Nicknames for famous decades:
- 1920s: "The Roaring Twenties," "The Jazz Age"
- 1950s: "The Fifties"
- 1960s: "The Sixties," "The Swinging Sixties"
- 2000s: "The Noughties" (UK), "The Aughts" (US), "The Two-Thousands"
3. Does a decade start with a year ending in 0 or 1?
Popular usage (dominant): Decades start with years ending in 0.
- The 1980s = 1980-1989
- The 2020s = 2020-2029
Formal reckoning (rarely used): Because there was no year 0, decades technically span 1-10.
- The 199th decade = 1981-1990
Conclusion: Popular usage (0-9) dominates in all practical contexts.
4. What decade am I in if I'm 35 years old?
You're in your fourth decade of life.
Breakdown:
- First decade: Birth (0) to 10th birthday (10) = ages 0-9
- Second decade: Ages 10-19
- Third decade: Ages 20-29
- Fourth decade: Ages 30-39 ← You are here at age 35
- Fifth decade: Ages 40-49
Mnemonic: Count the completed decades + 1. At age 35, you've completed 3 decades (0-10, 10-20, 20-30) and are in your 4th.
5. Is the decade an SI unit?
No. The decade is not part of the International System of Units (SI). The SI base unit for time is the second.
Non-SI time units commonly used:
- Minute (60 seconds)
- Hour (3,600 seconds)
- Day (~86,400 seconds)
- Year (~31.5 million seconds)
- Decade (~315 million seconds)
Scientific contexts: Research often uses SI units like kiloyears (kyr = 1,000 years) or megayears (Myr = 1 million years) rather than decades.
6. How many days are in a decade?
Approximately 3,652.5 days (accounting for leap years).
Calculation:
- 10 years × 365 days = 3,650 days
- Plus ~2.5 leap days per decade = 3,652.5 days
Exact number varies: Depends on leap year distribution in the specific decade.
- Decade with 2 leap years: 3,652 days
- Decade with 3 leap years: 3,653 days
7. What's the difference between a decade and a generation?
Decade: Exactly 10 years, used for historical/cultural periodization.
Generation: Approximately 20-30 years, the average time between birth of parents and birth of their children.
Generations span multiple decades:
- Baby Boomers (1946-1964): ~2 decades
- Generation X (1965-1980): ~1.5 decades
- Millennials (1981-1996): ~1.5 decades
- Generation Z (1997-2012): ~1.5 decades
Key difference: Decade = fixed 10 years; generation = variable ~20-30 years based on demographic/cultural cohorts.
8. Why are decades important in history and culture?
Human-scale timescale: 10 years is long enough for significant change but short enough to remember/experience.
Pattern recognition: Decade groupings reveal trends invisible in year-by-year analysis.
Cultural identity: Decades serve as convenient labels for distinct eras ("the Sixties," "the Eighties").
Psychological salience: Decade milestones (10, 20, 30, 40) feel more significant than other ages.
Media influence: 20th-century mass media amplified decade identity through music, fashion, political eras.
9. What is a "lost decade"?
Economic term: A decade of economic stagnation or decline, minimal growth.
Famous examples:
- Japan's Lost Decade (1990s): Asset bubble burst → prolonged recession
- Actually extended into "Lost Two Decades" or "Lost Three Decades" (1990s-2010s)
- Latin America's Lost Decade (1980s): Debt crisis → stagnation
- US "Lost Decade" (2000s): Dot-com crash (2000) + Financial crisis (2008) = weak growth
Personal context: "Lost decade" can mean any 10-year period of personal stagnation or hardship.
10. How do I calculate how many decades between two years?
Formula: decades = (ending year - starting year) ÷ 10
Examples:
- 1980 to 2020: (2020 - 1980) ÷ 10 = 4 decades (40 years)
- 1995 to 2025: (2025 - 1995) ÷ 10 = 3 decades (30 years)
- 2000 to 2018: (2018 - 2000) ÷ 10 = 1.8 decades (18 years)
11. What are decennial events?
Decennial = occurring every 10 years (once per decade).
Examples:
- US Census: Conducted every 10 years (1790, 1800, ... 2020, 2030)
- 10th anniversaries: Wedding, company founding, event commemoration
- High school reunions: Often every 10 years (10th, 20th, 30th)
Etymology: From Latin "decennium" (decem = ten, annus = year).
12. How many decades in a century? In a millennium?
Century:
- 1 century = 100 years = 10 decades
Millennium:
- 1 millennium = 1,000 years = 100 decades
Conversions:
- 5 decades = 50 years = 0.5 centuries
- 20 decades = 200 years = 2 centuries = 0.2 millennia
Conversion Table: Sidereal Day to Decade
| Sidereal Day (sidereal day) | Decade (dec) |
|---|---|
| 0.5 | 0 |
| 1 | 0 |
| 1.5 | 0 |
| 2 | 0.001 |
| 5 | 0.001 |
| 10 | 0.003 |
| 25 | 0.007 |
| 50 | 0.014 |
| 100 | 0.027 |
| 250 | 0.068 |
| 500 | 0.137 |
| 1,000 | 0.273 |
People Also Ask
How do I convert Sidereal Day to Decade?
To convert Sidereal Day to Decade, enter the value in Sidereal Day in the calculator above. The conversion will happen automatically. Use our free online converter for instant and accurate results. You can also visit our time converter page to convert between other units in this category.
Learn more →What is the conversion factor from Sidereal Day to Decade?
The conversion factor depends on the specific relationship between Sidereal Day and Decade. You can find the exact conversion formula and factor on this page. Our calculator handles all calculations automatically. See the conversion table above for common values.
Can I convert Decade back to Sidereal Day?
Yes! You can easily convert Decade back to Sidereal Day by using the swap button (⇌) in the calculator above, or by visiting our Decade to Sidereal Day converter page. You can also explore other time conversions on our category page.
Learn more →What are common uses for Sidereal Day and Decade?
Sidereal Day and Decade are both standard units used in time measurements. They are commonly used in various applications including engineering, construction, cooking, and scientific research. Browse our time converter for more conversion options.
For more time conversion questions, visit our FAQ page or explore our conversion guides.
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Other Time Units and Conversions
Explore other time units and their conversion options:
- Second (s) • Sidereal Day to Second
- Minute (min) • Sidereal Day to Minute
- Hour (h) • Sidereal Day to Hour
- Day (d) • Sidereal Day to Day
- Week (wk) • Sidereal Day to Week
- Month (mo) • Sidereal Day to Month
- Year (yr) • Sidereal Day to Year
- Millisecond (ms) • Sidereal Day to Millisecond
- Microsecond (μs) • Sidereal Day to Microsecond
- Nanosecond (ns) • Sidereal Day to Nanosecond
Verified Against Authority Standards
All conversion formulas have been verified against international standards and authoritative sources to ensure maximum accuracy and reliability.
National Institute of Standards and Technology — Official time standards and definitions
Bureau International des Poids et Mesures — Definition of the SI base unit for time
Last verified: December 3, 2025