Sidereal Day to Century Converter

Convert sidereal days to centuries with our free online time converter.

Quick Answer

1 Sidereal Day = 0.0000273 centuries

Formula: Sidereal Day × conversion factor = Century

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.

Last verified: December 2025|Reviewed by: Sam Mathew, Software Engineer

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How to Use the Sidereal Day to Century Calculator:

Enter the value you want to convert in the 'From' field (Sidereal Day).
The converted value in Century 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 Century: Step-by-Step Guide

Converting Sidereal Day to Century involves multiplying the value by a specific conversion factor, as shown in the formula below.

Formula:

1 Sidereal Day = 2.7304e-5 centuries

Example Calculation:

Convert 60 sidereal days: 60 × 2.7304e-5 = 0.00163826 centuries

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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What is a Sidereal Day and a Century?

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 century is a unit of time equal to 100 consecutive years. The word derives from Latin "centum" (one hundred).

Duration in Other Units

1 century equals:

100 years (exactly)
10 decades (100 ÷ 10)
1,200 months (100 × 12)
~5,217 weeks (100 × 52.17)
36,524 days (100 common years) or 36,525 days (accounting for ~25 leap years)
Average: 36,525 days (100 × 365.25)
876,600 hours (36,525 × 24)
52,596,000 minutes (876,600 × 60)
3,155,760,000 seconds (52,596,000 × 60)

Century Boundaries: The 1 vs. 0 Debate

Formal reckoning (technically correct):

1st century: 1-100 CE
18th century: 1701-1800
19th century: 1801-1900
20th century: 1901-2000
21st century: 2001-2100

Why? Because there was no year 0 in the Gregorian calendar (1 BCE → 1 CE directly), the first century was years 1-100, not 0-99.

Popular usage (dominant in practice):

18th century: "The 1700s" (1700-1799)
19th century: "The 1800s" (1800-1899)
20th century: "The 1900s" (1900-1999)
21st century: "The 2000s onward" (2000-2099)

Reality: Popular usage dominates. When people say "20th century," they typically mean 1900-1999, not 1901-2000. The millennium celebration happened January 1, 2000, not January 1, 2001, despite formal correctness.

Note: The Sidereal Day is part of the imperial/US customary system, primarily used in the US, UK, and Canada for everyday measurements. The Century belongs to the imperial/US customary system.

History of the Sidereal Day and Century

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 Century Concept

1. Ancient Origins: Roman Centuria (509 BCE - 27 BCE)

Roman military organization:

Centuria (plural: centuriae) = Roman military unit of approximately 100 soldiers (later reduced to 80)
Led by a centurion (centurio)
Latin "centum" = one hundred
6 centuries = 1 cohort; 10 cohorts = 1 legion (~6,000 soldiers)

Early timekeeping:

Romans used Ab urbe condita (AUC, "from the founding of the city") dating from Rome's legendary founding (753 BCE)
No systematic use of "century" for 100-year periods yet
Time organized by consulships, reigns, dynasties

2. Calendar Development and Anno Domini Dating (1 CE - 1582 CE)

Anno Domini (AD) system:

Dionysius Exiguus (c. 525 CE): Calculated years from Jesus Christ's birth
Introduced Anno Domini (AD, "in the year of the Lord") dating
Critical error: No year 0 (went directly from 1 BCE to 1 CE)
This creates century boundary confusion still debated today

Julian Calendar (46 BCE - 1582 CE):

Julius Caesar introduced 365.25-day year (leap year every 4 years)
Provided stable framework for long-term chronology
Enabled systematic dating of events over centuries

Gregorian Calendar Reform (1582 CE):

Pope Gregory XIII corrected Julian calendar drift
Established modern calendar system still used today
Removed 10 days (October 4, 1582 → October 15, 1582)
Century years divisible by 400 are leap years (1600, 2000), others not (1700, 1800, 1900)

3. Renaissance and Enlightenment: Historical Periodization (1400s-1700s)

Systematic historiography emerged:

15th-16th centuries: Renaissance scholars developed historical chronologies
17th-18th centuries: Enlightenment historians systematized century-based periodization
Edward Gibbon (Decline and Fall of the Roman Empire, 1776-1789): Used century-scale analysis

Why centuries became standard:

Generational scale: ~4 generations per century = intergenerational change visible but comprehensible
Administrative records: Tax records, census data, government documents accumulated over centuries
Pattern recognition: 100-year scale reveals structural changes invisible in decade-scale analysis
Round number psychology: Base-10 counting makes 100-year periods psychologically satisfying

Periodization labels emerged:

"The 16th century" = 1500s Renaissance, Reformation
"The 17th century" = 1600s Scientific Revolution, Baroque
"The 18th century" = 1700s Age of Enlightenment, Industrial Revolution begins

4. 19th Century: Century as Historical Framework (1801-1900)

The "long 19th century" concept:

Historians sometimes define as 1789-1914 (French Revolution to WWI)
Captures coherent historical era despite not matching formal century boundaries

Major 19th-century transformations:

Industrial Revolution (1760-1840): Steam power, factories, railroads
Urbanization: Rural → urban population shift
Imperialism: European colonial empires peak
Scientific progress: Darwin, Maxwell, Mendel
Political revolutions: 1848 Revolutions, unification of Germany/Italy
Technological: Telegraph, telephone, photography, electricity

Century consciousness:

People in 1800s increasingly thought in century-scale terms
"The spirit of the 19th century" = common phrase
Fin de siècle (end of century, 1890s-1900s) = cultural movement

5. 20th Century: Century of Extremes (1901-2000)

Eric Hobsbawm's "short 20th century" (1914-1991):

WWI start to Soviet Union collapse
Captures coherent historical narrative despite formal century boundaries

Major 20th-century transformations:

World Wars: WWI (1914-1918), WWII (1939-1945)
Ideological conflict: Fascism, Communism, Capitalism compete
Cold War (1947-1991): US vs. USSR, nuclear arms race
Decolonization: European empires disintegrate (1940s-1970s)
Technological revolutions:
- Automobiles, airplanes (early 1900s)
- Nuclear energy (1940s)
- Computers (1940s-1950s)
- Space exploration (1950s-1960s)
- Internet (1990s)
- Mobile phones (1990s-2000s)
Population explosion: 1.6 billion (1900) → 6.1 billion (2000)
Medical advances: Antibiotics, vaccines, life expectancy doubled
Environmental: Climate change, ozone depletion, biodiversity loss

Century labeling:

"The American Century" (Henry Luce, 1941): US dominance of 20th century
"The People's Century" (BBC, 1995): Mass politics, democracy spread

6. 21st Century: Digital Age and Beyond (2001-Present)

Millennium transition debate:

Popular celebration: January 1, 2000 (Y2K)
Formal start: January 1, 2001
Most people celebrated 2000 despite pedantic correctness

21st-century defining features (so far):

9/11 attacks (2001): "War on Terror" begins
Digital revolution: Smartphones ubiquitous (iPhone 2007)
Social media: Facebook, Twitter, Instagram, TikTok
Climate crisis: Accelerating global warming, extreme weather
COVID-19 pandemic (2020-2023): Global disruption
AI revolution: ChatGPT (2022), generative AI breakthroughs
Geopolitical shifts: Rise of China, multipolar world
Economic: 2008 Financial Crisis, wealth inequality

"21st-century skills": Digital literacy, critical thinking, adaptability

Common Uses and Applications: sidereal days vs centuries

Explore the typical applications for both Sidereal Day (imperial/US) and Century (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 centuries

and Applications

1. Historical Analysis and Research

Century-by-century comparison:

Economic growth: "19th-century industrialization vs. 20th-century information age"
Warfare evolution: "19th-century muskets → 20th-century machine guns → 21st-century drones"
Life expectancy trends: Analyzed century-by-century

Academic papers:

"This study examines voting patterns across two centuries (1800s-1900s)"
"Century-scale climate reconstructions"

2. Art, Literature, and Cultural Studies

Periodization:

"18th-century literature": Age of Enlightenment, Voltaire, Swift, Johnson
"19th-century novel": Dickens, Austen, Tolstoy, Dostoevsky
"20th-century art": Impressionism, Cubism, Surrealism, Abstract Expressionism

Art history courses: Often organized by century ("Art of the 17th Century")

3. Genealogy and Family History

Tracing ancestry:

4-5 generations per century = century scale ideal for family trees
"My great-great-grandfather lived in the 19th century"
Immigration records, census data organized by century

Life stages across centuries:

Born late 1800s, died mid-1900s = lived through two centuries

4. Climate and Environmental Science

Century-scale climate patterns:

Medieval Warm Period (10th-13th centuries): Warmer than average
Little Ice Age (14th-19th centuries): Cooler than average
20th-21st century warming: Anthropogenic climate change

Projections:

"By end of 21st century, sea level rise 1-2 meters"

5. Economic and Development Studies

Long-term economic trends:

19th century: Agricultural → industrial economies
20th century: Industrial → service/information economies
21st century: Digital/knowledge economies

Development indicators: Tracked over centuries (literacy, GDP, poverty)

6. Philosophy and Long-Term Thinking

"Think in centuries":

Long Now Foundation: 10,000-year thinking
Contrast with short-term thinking (quarterly earnings, election cycles)

Philosophical movements:

18th century: Enlightenment rationalism
19th century: Romanticism, existentialism emerges
20th century: Postmodernism

7. Legal and Property Rights

Land ownership records:

Property deeds reference century-old transactions
"Century farms": Farms in same family 100+ years

Copyright:

Life + 70 years = often extends into next century after author's death

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 Century (c)

1. How many years are in a century?

Exactly 100 years. The word "century" comes from Latin "centum" (one hundred).

Other units:

1 century = 100 years = 10 decades = 1,200 months = ~36,525 days

2. When did the 21st century begin?

Formal answer: January 1, 2001 (because there was no year 0, the 1st century was years 1-100, so the 21st century is 2001-2100).

Popular answer: January 1, 2000 (most people celebrated the new millennium in 2000, and colloquially refer to "the 2000s" as the start of the 21st century).

Reality: Both are used; formal definition is technically correct, but popular usage dominates in practice.

3. Why is the 1800s called the 19th century?

Because of how centuries are numbered:

1st century = years 1-100
2nd century = years 101-200
18th century = years 1701-1800
19th century = years 1801-1900 (the "1800s")
20th century = years 1901-2000 (the "1900s")

Rule: Century number = (hundreds digit + 1). So 1800s → century 18+1 = 19th century.

4. How many generations are in a century?

Approximately 4-5 generations, assuming ~20-25 years per generation.

Calculation:

If generation = 25 years → 100 ÷ 25 = 4 generations per century
If generation = 20 years → 100 ÷ 20 = 5 generations per century

Example: Great-great-grandparents often lived in a different century than you.

5. Is a century a standard unit in science?

No. The century is not part of the International System of Units (SI). The SI base unit for time is the second.

Scientific time units:

Years (Julian year = 365.25 days exactly)
Kiloyears (kyr): 1,000 years
Megayears (Myr): 1,000,000 years
Gigayears (Gyr): 1,000,000,000 years

Century usage: Common in history, demography, climate science, but not formal SI unit.

6. How many days are in a century?

Approximately 36,525 days (accounting for leap years).

Calculation:

100 years × 365 days = 36,500 days
Plus ~25 leap days per century = 36,525 days total

Exact number varies: Depends on leap year distribution (Gregorian calendar: 97 leap years per 400 years).

7. What is the difference between centennial and bicentennial?

Centennial: 100th anniversary (1 century)

Bicentennial: 200th anniversary (2 centuries)

Other -ennial terms:

Sesquicentennial: 150th anniversary (1.5 centuries)
Tercentennial/Tricentennial: 300th anniversary (3 centuries)
Quadricentennial: 400th anniversary (4 centuries)
Quincentennial: 500th anniversary (5 centuries)

8. Can a person live in three different centuries?

Yes, but extremely rare.

Requirements:

Born in one century (e.g., 1898, 19th century)
Live through next century (1900s, 20th century)
Live into third century (2000s, 21st century)
Requires living 102+ years if born in last years of century

Example: Born December 1898 (19th century) → lived through 20th century (1901-2000) → died January 2001 (21st century) = lived in 3 centuries despite being only 102 years old.

9. What is a "long century" in history?

Historical concept: Period longer than 100 years but representing a coherent historical era.

Famous examples:

"Long 19th century" (1789-1914): French Revolution to WWI start
"Long 18th century" (1688-1815): Glorious Revolution to Waterloo
"Short 20th century" (1914-1991): Conversely, WWI to USSR collapse = only 77 years but captures coherent era

Why useful: Historical eras don't align neatly with formal century boundaries; "long/short century" captures thematic unity.

10. How do I calculate how many centuries between two years?

Formula: centuries = (ending year - starting year) ÷ 100

Examples:

1500 to 2000: (2000 - 1500) ÷ 100 = 5 centuries (500 years)
1776 to 2024: (2024 - 1776) ÷ 100 = 2.48 centuries (~248 years)
1900 to 2100: (2100 - 1900) ÷ 100 = 2 centuries (200 years)

11. What is "fin de siècle"?

French phrase: "End of the century" (literally "end of era")

Historical meaning: Cultural period at end of 19th century (1890s-1900s) characterized by:

Cultural pessimism mixed with optimism
Decadence, Art Nouveau
Anxiety about modernity
Transition into new century

Modern usage: Any "end of century" cultural moment (Y2K was modern "fin de siècle").

12. How many centuries in a millennium?

10 centuries = 1 millennium

Conversions:

1 century = 0.1 millennia (100 years)
5 centuries = 0.5 millennia (500 years)
10 centuries = 1 millennium (1,000 years)
20 centuries = 2 millennia (2,000 years)

Conversion Table: Sidereal Day to Century

Sidereal Day (sidereal day)	Century (c)
0.5	0
1	0
1.5	0
2	0
5	0
10	0
25	0.001
50	0.001
100	0.003
250	0.007
500	0.014
1,000	0.027

How do I convert Sidereal Day to Century?

To convert Sidereal Day to Century, 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.

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What is the conversion factor from Sidereal Day to Century?

The conversion factor depends on the specific relationship between Sidereal Day and Century. 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 Century back to Sidereal Day?

Yes! You can easily convert Century back to Sidereal Day by using the swap button (⇌) in the calculator above, or by visiting our Century 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 Century?

Sidereal Day and Century 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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NIST Time and Frequency

National Institute of Standards and Technology — Official time standards and definitions

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Bureau International des Poids et Mesures — Definition of the SI base unit for time

Last verified: December 3, 2025

Sidereal Day to Century Converter

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Sidereal Day to Century Calculator

How to Use the Sidereal Day to Century Calculator:

How to Convert Sidereal Day to Century: Step-by-Step Guide

Formula:

Example Calculation:

What is a Sidereal Day and a Century?

What Is a Sidereal Day?

Sidereal vs. Solar Day

Why Are They Different?

One Extra Day Per Year

Duration in Other Units

Century Boundaries: The 1 vs. 0 Debate

History of the Sidereal Day and Century

Ancient Observations (2000-300 BCE)

Hipparchus and Precession (150 BCE)

Ptolemy's Almagest (150 CE)

Islamic Golden Age (800-1400 CE)

Copernican Revolution (1543)

Kepler's Laws (1609-1619)

Rise of Telescopic Astronomy (1600s-1700s)

Precision Timekeeping (1800s)

IAU Standardization (1900s)

Modern Era: ICRF (1997-Present)

1. Ancient Origins: Roman Centuria (509 BCE - 27 BCE)

2. Calendar Development and Anno Domini Dating (1 CE - 1582 CE)

3. Renaissance and Enlightenment: Historical Periodization (1400s-1700s)

4. 19th Century: Century as Historical Framework (1801-1900)

5. 20th Century: Century of Extremes (1901-2000)

6. 21st Century: Digital Age and Beyond (2001-Present)

Common Uses and Applications: sidereal days vs centuries

Common Uses for sidereal days

1. Telescope Pointing and Tracking

2. Astrophotography

3. Satellite Orbit Planning

4. Very Long Baseline Interferometry (VLBI)

5. Navigation and Geodesy

6. Space Navigation

7. Amateur Astronomy

When to Use centuries

1. Historical Analysis and Research

2. Art, Literature, and Cultural Studies

3. Genealogy and Family History

4. Climate and Environmental Science

5. Economic and Development Studies

6. Philosophy and Long-Term Thinking

7. Legal and Property Rights

Additional Unit Information

About Sidereal Day (sidereal day)

How long is a sidereal day in standard time?

Why is a sidereal day shorter than a solar day?

Is sidereal time the same everywhere on Earth?

Do geosynchronous satellites orbit every 24 hours or 23h 56m?

How many sidereal days are in a year?

Can I use a regular clock to tell sidereal time?

Why do astronomers use sidereal time instead of solar time?

What is the difference between a sidereal day and a sidereal year?

Does the Moon have a sidereal day?

How is sidereal time measured today?

Will sidereal time ever be replaced by something else?

About Century (c)

1. How many years are in a century?

2. When did the 21st century begin?

3. Why is the 1800s called the 19th century?

4. How many generations are in a century?

5. Is a century a standard unit in science?

6. How many days are in a century?

7. What is the difference between centennial and bicentennial?

8. Can a person live in three different centuries?

9. What is a "long century" in history?

10. How do I calculate how many centuries between two years?

11. What is "fin de siècle"?

12. How many centuries in a millennium?

Conversion Table: Sidereal Day to Century

People Also Ask

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Can I convert Century back to Sidereal Day?