Hours to Sidereal Days (h to d (sid))
1 hour equals 0.04178074624 sidereal days (rounded).
How to convert hours to sidereal days
To convert hours to sidereal days, multiply the number of hours by 0.04178074624. The factor is rounded to 10 significant digits. To go the other way, multiply sidereal days by 23.93446958 (rounded).
sidereal days = hours × 0.04178074624
Example: 10 hours = 0.4178074624 sidereal days.
Months and years are not constant. This site uses the average Gregorian year of 365.2425 days, so one 'year' converts to 365.2425 days, not 365; use 'common year' for exactly 365 days.
Approximate value. Standards for the sidereal day varied by place and period, so this converter uses the most commonly cited value. Treat results as estimates, not exact figures.
About these units
Hour (h)
60 minutes or 3,600 seconds, one twenty-fourth of a mean solar day.
Egyptian astronomers gave us the twelve. Coffin lids from about 2150 BC show the night divided into exactly 12 intervals, tied to the rising of star groups called decans, and the day was later split into 12 hours with sundials. These were seasonal hours, a twelfth of the daylight, so a summer hour was longer than a winter one. The word itself comes from Greek hora, originally any span of time, through Latin and Old French into English in the 13th century.
Equal hours of 1/24 of a day were probably first proposed by Hipparchus, and mechanical clocks made them the practical choice, since a clock showing seasonal hours had to be readjusted every morning and evening. Today the hour is exactly 3,600 seconds and is a non-SI unit accepted for use with the SI. Civil time still bends it a little: UTC hours have lasted 3,601 seconds 27 times since 1972 to keep clocks within 0.9 seconds of the Earth's rotation. In practice we use it for working shifts, travel times and electricity bills measured in kilowatt-hours.
1 hour = 3,600 seconds (exactly).
Sources: Wikipedia: Hour; BIPM: SI Brochure, 9th edition
Sidereal Day (d (sid))
About 86,164.09 seconds (23 h 56 min 4 s), one rotation of the Earth relative to the stars.
Sidereal comes from the Latin word for star, and the unit measures how long Earth takes to turn once relative to the distant stars rather than to the Sun. The figure used here is about 86,164.09 seconds, or 23 hours 56 minutes 4.09 seconds. NASA's Earth fact sheet gives the same period as 23.9345 hours.
The sidereal day is about four minutes shorter than the mean solar day because Earth also moves roughly one degree along its orbit during each rotation, so it must turn slightly further before the Sun is back on the meridian. Over a year this adds up to one extra turn, giving about 366.24 sidereal days against 365.24 solar days.
For centuries astronomers measured it by timing stars as they crossed the meridian and checking observatory clocks against star catalogues. From the 1970s, radio methods such as very long baseline interferometry and pulsar timing proved more precise, and since 2003 the Earth Rotation Angle has replaced the older sidereal time conventions in formal use.
1 sidereal day = 86,164.0905 seconds (approximately).
Sources: Wikipedia: Sidereal time; NASA Earth Fact Sheet
Related conversions
- Sidereal days to hours (reverse)
- Hours to seconds
- Hours to minutes
- Hours to days
- Hours to weeks
- Hours to months
- Hours to years
- Hours to milliseconds
- Hours to microseconds
- Hours to nanoseconds
- Hours to decades
- Hours to centuries
- Hours to millennia
- Seconds to sidereal days
- Minutes to sidereal days
- Days to sidereal days
- Weeks to sidereal days
- Months to sidereal days
- Years to sidereal days
See all time units.