Month to Sidereal Day Converter
Convert months to sidereal days with our free online time converter.
Quick Answer
1 Month = 30.520208 sidereal days
Formula: Month × 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.
Month to Sidereal Day Calculator
How to Use the Month to Sidereal Day Calculator:
- Enter the value you want to convert in the 'From' field (Month).
- 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 Month to Sidereal Day: Step-by-Step Guide
Converting Month to Sidereal Day involves multiplying the value by a specific conversion factor, as shown in the formula below.
Formula:
1 Month = 30.52021 sidereal daysExample Calculation:
Convert 60 months: 60 × 30.52021 = 1831.212 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.
Need to convert to other time units?
View all Time conversions →What is a Month and a Sidereal Day?
A month is a unit of time used with calendars, approximately based on the orbital period of the Moon around Earth. The word "month" derives from "Moon" (Proto-Germanic mǣnōth).
Modern Gregorian Calendar Months
In the Gregorian calendar (standard worldwide since 1582), months have irregular lengths:
| Month | Days | Hours | Weeks (approx) | |-----------|----------|-----------|-------------------| | January | 31 | 744 | 4.43 | | February | 28 (29 leap) | 672 (696 leap) | 4.00 (4.14 leap) | | March | 31 | 744 | 4.43 | | April | 30 | 720 | 4.29 | | May | 31 | 744 | 4.43 | | June | 30 | 720 | 4.29 | | July | 31 | 744 | 4.43 | | August | 31 | 744 | 4.43 | | September | 30 | 720 | 4.29 | | October | 31 | 744 | 4.43 | | November | 30 | 720 | 4.29 | | December | 31 | 744 | 4.43 |
Average Month for Conversions
For mathematical conversions, an average month is defined as:
- 1/12th of a year = 365.25 days ÷ 12 = 30.4375 days (often rounded to 30.44 days)
- 730.5 hours (30.4375 × 24)
- 43,830 minutes (730.5 × 60)
- 2,629,800 seconds (43,830 × 60)
- 4.35 weeks (30.4375 ÷ 7)
Lunar Month vs. Calendar Month
- Synodic month (lunar cycle, new moon to new moon): 29.53 days (29 days, 12 hours, 44 minutes, 3 seconds)
- Sidereal month (Moon's orbit relative to stars): 27.32 days
- Gregorian calendar month: 28-31 days (avg 30.44 days)
- Drift: Calendar months drift ~2 days per month from lunar phases
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 Month 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 Month and Sidereal Day
of the Month
1. Ancient Lunar Origins (Pre-3000 BCE)
The concept of the month originated from observing the lunar cycle—the period from one new moon to the next, approximately 29.53 days (synodic month).
Early lunar calendars:
- Babylonian calendar (c. 2000 BCE): 12 lunar months (~354 days per year), with periodic intercalary (13th) months added every 2-3 years to realign with seasons
- Egyptian calendar (c. 3000 BCE): 12 months of exactly 30 days each (360 days) + 5 epagomenal days = 365 days, detached from lunar cycle
- Hebrew/Jewish calendar (c. 1500 BCE): Lunisolar calendar with 12-13 months (29-30 days each), still used today for religious observances
- Chinese calendar (c. 1600 BCE): Lunisolar calendar with 12-13 months, determining Chinese New Year (late January to mid-February)
Why lunar months? Ancient civilizations without artificial lighting noticed the Moon's dramatic visual changes every ~29.5 days, making it an obvious natural timekeeper.
2. Roman Calendar Evolution (753 BCE - 46 BCE)
The Roman calendar underwent dramatic transformations:
Romulus Calendar (753 BCE - legendary):
- 10 months, 304 days total, starting in March (spring equinox)
- Months: Martius (31), Aprilis (30), Maius (31), Junius (30), Quintilis (31), Sextilis (30), September (30), October (31), November (30), December (30)
- Winter gap (~61 days) was unnamed, creating calendar chaos
Numa Pompilius Reform (c. 713 BCE):
- Added January and February to fill winter gap
- 12 months, 355 days total (still 10.25 days short of solar year)
- Required periodic intercalary months (Mercedonius) to realign with seasons
- Romans disliked even numbers, so most months had 29 or 31 days (February got unlucky 28)
Late Roman Republic (c. 100 BCE):
- Calendar administration corrupt—priests (pontifices) manipulated intercalary months for political gain (extending terms, delaying elections)
- Calendar drifted months out of sync with seasons (harvest festivals in wrong seasons)
3. Julian Calendar (46 BCE - 1582 CE)
Julius Caesar's reform (46 BCE):
- Consulted Egyptian astronomer Sosigenes of Alexandria
- Adopted solar year = 365.25 days (365 days + leap day every 4 years)
- Redesigned month lengths to solar-based 28-31 days:
- 31 days: January, March, May, July (Quintilis), September, November
- 30 days: April, June, August (Sextilis), October, December
- 28/29 days: February (unlucky month, kept short)
- 46 BCE = "Year of Confusion" (445 days long to realign calendar with seasons)
Later adjustments:
- 44 BCE: Quintilis renamed July (Julius Caesar, after his assassination)
- 8 BCE: Sextilis renamed August (Augustus Caesar)
- August given 31 days (stealing 1 from February) to match July's prestige, redistributing others
- Final pattern: Jan(31), Feb(28/29), Mar(31), Apr(30), May(31), Jun(30), Jul(31), Aug(31), Sep(30), Oct(31), Nov(30), Dec(31)
Problem with Julian calendar: Solar year = 365.2422 days (not exactly 365.25), so calendar gained ~11 minutes per year = 3 days every 400 years
4. Gregorian Calendar (1582 CE - Present)
Pope Gregory XIII's reform (1582):
- Corrected drift: Removed 10 days (October 4, 1582 → October 15, 1582) to realign with seasons
- New leap year rule:
- Leap year every 4 years (like Julian)
- EXCEPT century years (1700, 1800, 1900) NOT leap years
- EXCEPT century years divisible by 400 (1600, 2000, 2400) ARE leap years
- Result: 97 leap years per 400 years = 365.2425 days average (only 27 seconds/year error)
- Month lengths unchanged from final Julian pattern
Adoption:
- Catholic countries (Spain, Portugal, Italy): Immediately (1582)
- Protestant countries (Britain, colonies): 1752 (removed 11 days: Sept 2 → Sept 14)
- Russia: 1918 (removed 13 days, after October Revolution became November Revolution)
- China: 1912 (Republic of China adoption)
- Turkey: 1926 (secular reforms)
- Now universal for civil purposes worldwide
5. Lunar Calendars Continue
Despite Gregorian dominance, lunar/lunisolar calendars continue for religious/cultural purposes:
- Islamic Hijri calendar: 12 lunar months (354-355 days), cycles through seasons every 33 years, determines Ramadan
- Hebrew calendar: Lunisolar with 12-13 months, determines Jewish holidays
- Chinese calendar: Lunisolar, determines Chinese New Year, Mid-Autumn Festival
- Hindu calendars: Multiple regional lunisolar systems
- Buddhist calendars: Various lunisolar systems across Thailand, Sri Lanka, Myanmar
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: months vs sidereal days
Explore the typical applications for both Month (imperial/US) and Sidereal Day (imperial/US) to understand their common contexts.
Common Uses for months
and Applications
1. Financial Planning and Budgeting
Monthly budget framework:
- Income: Track monthly take-home pay (after taxes)
- Fixed expenses: Rent/mortgage, car payment, insurance (consistent monthly amounts)
- Variable expenses: Groceries, utilities, entertainment (varies month-to-month)
- Savings goals: "Save $500/month" = $6,000/year
- Debt repayment: "Extra $200/month toward credit card" = $2,400/year payoff
Monthly vs. annual thinking:
- $150/month subscription = $1,800/year (psychological impact: monthly feels smaller)
- "Latte factor": $5 daily coffee = $150/month = $1,800/year = $18,000/decade
Monthly financial ratios:
- Rent rule: Rent should be ≤30% of monthly gross income
- 50/30/20 rule: 50% needs, 30% wants, 20% savings (monthly breakdown)
2. Subscription and Membership Economy
Monthly Recurring Revenue (MRR) = business model foundation:
- SaaS (Software as a Service): Monthly subscription pricing (e.g., Adobe Creative Cloud $54.99/month)
- Streaming services: Netflix, Spotify, Disney+ (monthly billing standard)
- Gym memberships: Monthly dues (e.g., $30-100/month depending on gym)
- Amazon Prime: $14.99/month (or $139/year = $11.58/month, annual cheaper)
Monthly vs. annual pricing psychology:
- Annual = higher upfront cost, lower monthly rate, customer lock-in
- Monthly = lower barrier to entry, higher churn risk, higher effective rate
3. Project Management and Milestones
Standard project durations:
- 1-month sprint: Agile/Scrum often uses 2-4 week sprints (close to 1 month)
- 3-month project: Standard short-term project (1 quarter)
- 6-month project: Medium-term initiative (2 quarters, half-year)
- 12-month project: Long-term strategic initiative (full year)
Monthly milestones:
- Month 1: Planning and setup
- Month 2: Development/implementation
- Month 3: Testing and refinement
- Month 4: Launch and monitoring
4. Employment and Compensation
Pay period variations:
- Monthly (12 pay periods/year): Common internationally, especially Europe/Asia
- Pros: Aligns with monthly bills, simpler accounting
- Cons: Long gap between paychecks (especially if month has 31 days)
- Semi-monthly (24 pay periods/year): 1st and 15th of each month
- Pros: More frequent pay (twice per month), aligns with mid-month expenses
- Cons: Pay dates vary (weekends/holidays), inconsistent days between paychecks
- Bi-weekly (26 pay periods/year): Every 2 weeks (e.g., every other Friday)
- Pros: Consistent day of week, 2 "extra" paychecks per year
- Cons: Doesn't align with monthly bills, some months have 3 paychecks
Monthly salary vs. hourly:
- Salaried: Annual salary ÷ 12 = monthly salary (e.g., $72,000/year = $6,000/month)
- Hourly: (Hourly rate × hours/week × 52 weeks) ÷ 12 months (e.g., $25/hr × 40hrs × 52 ÷ 12 = $4,333/month)
5. Calendar Organization
Month as primary calendar unit:
- Monthly view: Standard calendar layout (7 columns × 4-6 rows = 28-42 cells)
- Month numbering: January = 1, February = 2, ... December = 12
- Date notation:
- US: MM/DD/YYYY (month first)
- International (ISO 8601): YYYY-MM-DD (year-month-day)
- European: DD/MM/YYYY (day first)
Month-based planning:
- Goals: "Read 2 books per month" = 24 books/year
- Habits: "Exercise 3 times per week" = 12-13 times per month
- Reviews: "Monthly review" of goals, finances, habits
6. Seasonal Business Cycles
Retail calendar:
- January: Post-holiday sales, fitness equipment (New Year's resolutions)
- February: Valentine's Day
- March-April: Spring cleaning, Easter, tax season
- May: Mother's Day, Memorial Day (unofficial summer start)
- June: Father's Day, graduations, weddings
- July-August: Summer travel, back-to-school shopping (late August)
- September: Labor Day, fall season begins
- October: Halloween
- November: Thanksgiving, Black Friday (biggest shopping day)
- December: Holiday shopping season (Christmas/Hanukkah)
Quarterly thinking (3-month periods):
- Q1 (Jan-Mar): New Year momentum, tax season
- Q2 (Apr-Jun): Spring/early summer, end of fiscal year for many companies
- Q3 (Jul-Sep): Summer slowdown, back-to-school
- Q4 (Oct-Dec): Holiday season, year-end push, budget planning
7. Age and Developmental Milestones
Infant/child development:
- 0-12 months: Tracked monthly (dramatic changes each month)
- 3 months: Lifts head, smiles
- 6 months: Sits up, starts solid foods
- 9 months: Crawls, says "mama/dada"
- 12 months: Walks, first words
- 12-24 months: Often still tracked monthly ("18 months old" vs. "1.5 years")
- 2+ years: Typically switch to years ("3 years old")
Age expression:
- Months (0-23 months): More precise for developmental tracking
- Years (2+ years): Standard for most purposes
- Decades (30s, 40s, etc.): Rough life stages
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 Month (mo)
1. How many days are in a month?
It varies by month:
- 31 days: January, March, May, July, August, October, December (7 months)
- 30 days: April, June, September, November (4 months)
- 28 days: February (non-leap year)
- 29 days: February (leap year, every 4 years with exceptions)
Average month = 30.44 days (365.25 ÷ 12), used for conversions.
Mnemonic: "30 days hath September, April, June, and November. All the rest have 31, except February alone, which has 28 days clear, and 29 in each leap year."
Knuckle trick: Make fists and count across knuckles (31 days) and valleys (30 days, except February).
2. Why do months have different lengths?
Historical reasons:
- Roman calendar origins: 10-month calendar (Romulus) had 304 days, leaving ~61-day winter gap
- Numa Pompilius added January and February (c. 713 BCE), creating 12 months with 355 days
- Julius Caesar (46 BCE): Julian calendar with 365.25-day year required distributing days across 12 months
- Political decisions: July (Julius Caesar) and August (Augustus Caesar) both given 31 days for prestige, shortening February to 28 days
Result: Irregular pattern (31-28-31-30-31-30-31-31-30-31-30-31) due to Roman politics, not astronomy.
3. What is an average month length used for conversions?
Average month = 30.4375 days (often rounded to 30.44 days)
Calculation: 365.25 days per year ÷ 12 months = 30.4375 days per month
- 365.25 accounts for leap year (365 × 3 years + 366 × 1 year = 1,461 days ÷ 4 years = 365.25)
When to use average month:
- Converting months to days/weeks/hours when specific month unknown
- Financial calculations (monthly interest rates, annual salary ÷ 12)
- Age approximations ("6 months old" ≈ 183 days)
When NOT to use average: Specific date calculations (use actual month lengths).
4. Is a month based on the Moon?
Historically, yes. Currently, only approximately.
Etymology: "Month" derives from "Moon" (Old English mōnað, Proto-Germanic mǣnōth).
Lunar cycle: 29.53 days (synodic month, new moon to new moon)
Gregorian calendar month: 28-31 days (avg 30.44 days)
- Drift: Calendar months drift ~2 days per month from lunar phases
- Example: Full moon on January 15 → next full moon ~February 13 (29.5 days later), not February 15
Modern lunar calendars:
- Islamic calendar: Strictly lunar (12 months × 29.5 days = 354 days), cycles through seasons every 33 years
- Hebrew/Chinese calendars: Lunisolar (12-13 months, adding extra month every 2-3 years to stay aligned with seasons)
Why detached? Solar year (365.24 days) and lunar year (354.37 days) are incompatible—12 lunar months = 10.87 days short of solar year.
5. How many weeks are in a month?
Average month = 4.35 weeks (30.44 days ÷ 7 days/week)
Common mistake: Assuming 1 month = 4 weeks (WRONG—actually 4 weeks = 28 days, most months are 30-31 days)
Specific months:
- 28 days (February, non-leap) = 4.00 weeks
- 29 days (February, leap) = 4.14 weeks
- 30 days (April, June, September, November) = 4.29 weeks
- 31 days (January, March, May, July, August, October, December) = 4.43 weeks
Implications:
- "4 weeks pregnant" ≠ "1 month pregnant" (4 weeks = 28 days, 1 month avg = 30.44 days)
- "Save $100/week" = $435/month (not $400)
6. How many months are in a year?
12 months in all major calendar systems (Gregorian, Julian, Hebrew, Chinese, Hindu).
Why 12 months?
- Lunar approximation: 12 lunar cycles (~354 days) close to solar year (365 days)
- Convenient division: 12 has many factors (1, 2, 3, 4, 6, 12), making quarters (3 months), half-years (6 months) easy
- Historical precedent: Babylonian, Roman calendars used 12 months
Alternative proposals (failed):
- French Republican Calendar (1793-1805): 12 months × 30 days + 5 epagomenal days (abandoned after Napoleon)
- International Fixed Calendar (proposed 1930s): 13 months × 28 days + 1 extra day (never adopted, opposed by religious groups)
7. What is a leap year and how does it affect months?
Leap year: Year with 366 days (not 365), adding 1 extra day to February (29 days instead of 28).
Leap year rule (Gregorian calendar):
- Year divisible by 4 = leap year (e.g., 2024)
- EXCEPT century years (1700, 1800, 1900) = NOT leap year
- EXCEPT century years divisible by 400 (1600, 2000, 2400) = leap year
Why leap years? Solar year = 365.2422 days (not exactly 365), so calendar gains ~0.2422 days per year = ~1 day every 4 years. Adding leap day keeps calendar aligned with seasons.
Impact on months:
- Only February affected (28 → 29 days)
- Leap year: 366 days = 52 weeks + 2 days (52.29 weeks)
- Non-leap year: 365 days = 52 weeks + 1 day (52.14 weeks)
Next leap years: 2024, 2028, 2032, 2036, 2040
8. What is the origin of month names?
Month names (Gregorian calendar, from Latin):
| Month | Origin | Meaning | |-----------|-----------|-------------| | January | Janus (Roman god) | God of beginnings, doorways (two faces looking forward/backward) | | February | Februa (Roman purification festival) | Purification ritual held mid-February | | March | Mars (Roman god) | God of war (originally first month of Roman year) | | April | Aprilis (Latin) | "To open" (buds opening in spring) or Aphrodite (Greek goddess) | | May | Maia (Roman goddess) | Goddess of growth, spring | | June | Juno (Roman goddess) | Goddess of marriage, queen of gods | | July | Julius Caesar | Roman dictator (month of his birth), originally Quintilis ("fifth") | | August | Augustus Caesar | First Roman emperor, originally Sextilis ("sixth") | | September | Septem (Latin) | "Seven" (originally 7th month before January/February added) | | October | Octo (Latin) | "Eight" (originally 8th month) | | November | Novem (Latin) | "Nine" (originally 9th month) | | December | Decem (Latin) | "Ten" (originally 10th month) |
Historical shift: September-December originally matched their numeric names (7th-10th months) when Roman year started in March. Adding January/February shifted them to 9th-12th positions.
9. Why is February the shortest month?
Roman superstition and politics:
- Roman numerology: Romans considered even numbers unlucky, so most months had 29 or 31 days (odd numbers)
- February = unlucky month: Month of purification rituals (Februa), associated with death/underworld, so Romans kept it short
- Julius Caesar's reform (46 BCE): Distributed days to create 365.25-day year, February remained shortest at 28 days
- Augustus's adjustment (8 BCE): Legend says Augustus took 1 day from February (29 → 28) to make August 31 days (matching July), but historians dispute this—likely just continued existing pattern
Result: February = 28 days (29 in leap years), shortest month by 1-3 days.
10. What are the financial quarters?
Financial quarters (Q1, Q2, Q3, Q4): 3-month periods dividing the fiscal year for business reporting.
Calendar year quarters:
- Q1 = January, February, March (90/91 days)
- Q2 = April, May, June (91 days)
- Q3 = July, August, September (92 days)
- Q4 = October, November, December (92 days)
Fiscal year variations: Many companies/governments use different fiscal years:
- US federal government: Oct 1 - Sep 30 (Q1 = Oct-Dec)
- UK government: Apr 1 - Mar 31 (Q1 = Apr-Jun)
- Japan/India: Apr 1 - Mar 31
- Australia: Jul 1 - Jun 30
Why quarters? Balance between frequent reporting (not too infrequent like annual) and manageable workload (not too frequent like monthly for major reporting).
11. How do I calculate age in months?
Formula: (Current year - Birth year) × 12 + (Current month - Birth month)
Example 1: Born March 15, 2020, today is June 15, 2024
- (2024 - 2020) × 12 + (6 - 3) = 4 × 12 + 3 = 51 months old
Example 2: Born November 20, 2022, today is January 10, 2024
- (2024 - 2022) × 12 + (1 - 11) = 2 × 12 - 10 = 14 months old
Precision note: Calculation above assumes same day of month. For exact age:
- If current day ≥ birth day: Use formula above
- If current day < birth day: Subtract 1 month (haven't reached full month yet)
When to use months for age:
- 0-23 months: Infant/toddler development changes rapidly monthly
- 24+ months: Typically switch to years ("2 years old" not "24 months old")
12. What's the difference between bi-monthly and semi-monthly?
Confusing terminology:
Bi-monthly = Ambiguous (avoid using)
- Meaning 1: Every 2 months (6 times per year)
- Meaning 2: Twice per month (24 times per year)
Semi-monthly = Twice per month (24 times per year)
- Example: Paycheck on 1st and 15th of each month
- 12 months × 2 = 24 pay periods per year
Bi-weekly = Every 2 weeks (26 times per year, not 24)
- Example: Paycheck every other Friday
- 52 weeks ÷ 2 = 26 pay periods per year
Recommendation: Avoid "bi-monthly" (ambiguous). Use "every 2 months" (6×/year) or "twice per month"/"semi-monthly" (24×/year).
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: Month to Sidereal Day
| Month (mo) | Sidereal Day (sidereal day) |
|---|---|
| 0.5 | 15.26 |
| 1 | 30.52 |
| 1.5 | 45.78 |
| 2 | 61.04 |
| 5 | 152.601 |
| 10 | 305.202 |
| 25 | 763.005 |
| 50 | 1,526.01 |
| 100 | 3,052.021 |
| 250 | 7,630.052 |
| 500 | 15,260.104 |
| 1,000 | 30,520.208 |
People Also Ask
How do I convert Month to Sidereal Day?
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Can I convert Sidereal Day back to Month?
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Learn more →What are common uses for Month and Sidereal Day?
Month and Sidereal Day 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.
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Last verified: December 3, 2025