To convert hours to sidereal days, multiply by 0.04178074624. That factor is the whole relationship: both units measure time, so the conversion is a proportion with no offset and no substance-specific constant.
One rotation of Earth relative to the stars: 23 h 56 min 4.09 s, about four minutes short of a solar day. Sixty minutes; the standard unit of work and travel scheduling.
Months and years are not fixed. This page uses the average Gregorian month of 30.44 days and the Julian year of 365.25 days, which is right for durations and wrong for calendars: three months from 31 January is not 91.31 days in any useful sense. Use a date calculator for calendar arithmetic. Two tables follow: rounded steps for quick lookup, and a set of real reference quantities converted into sidereal days, which is usually more useful than the bare number.
The formula
hours- The amount you are converting, in hours
0.0417807462379- The exact number of sidereal days in one hour
sidereal days- The converted result
How it works, step by step
- Enter the value in hours.
- It is multiplied by 0.0417807462379, the exact factor between the two units.
- The answer is shown in sidereal days, with the reverse conversion beneath it.
- Check the reference table if you want a figure of comparable size.
Worked examples
A pregnancy: 6720 h in sidereal days
Take a pregnancy, roughly 6720 h. The conversion is a single multiplication — 6720 × 0.0417807 — giving 280.767 sd. Because the relationship is proportional, doubling the input doubles the answer to 561.533 sd.
A pop song: 0.0583 h in sidereal days
A pop song comes to about 0.0583 h. Multiplying by the factor: 0.0583 × 0.0417807 = 0.00243582 sd. Dividing the answer back by 0.0417807 returns the figure you started with, which is the quickest way to check you have the factor the right way up.
How to read your score
Frequently asked questions
How many sidereal days are in one hour?
Exactly 0.0417807462379. Every figure on this page is that one number multiplied by your input, so the only rounding is at the display step.
What is the formula to convert hours to sidereal days?
sidereal days = hours × 0.0417807462379. Reversed, hours = sidereal days ÷ 0.0417807462379, which is the same as multiplying by 23.9344695833.
Which is the bigger unit, the hour or the sidereal day?
The sidereal day. One hour is only 0.0417807 of a sidereal day, so it takes 23.9345 hours to make one sidereal day.
What does this quantity actually measure?
Both units measure time. That is why a single factor is enough: there is no temperature offset and no material property involved, unlike conversions that cross between different quantities.
What is a hour?
Sixty minutes; the standard unit of work and travel scheduling.
What is a sidereal day?
One rotation of Earth relative to the stars: 23 h 56 min 4.09 s, about four minutes short of a solar day.
Why does my answer differ from another converter?
Almost always because the other tool rounded the factor before multiplying. This page keeps full double precision through the calculation and rounds only for display, and it prints the factor it used so you can see where any difference came from.
Is there a quick way to do h to sd in my head?
Plenty of people do this in their head: divide by 24 instead of using the exact factor 0.04178074624. At 100 that shortcut gives 4.1666667 sidereal days, off by 0.01141 sidereal days or about 0.27%. Fine for a rough estimate, not fine for anything you are cutting to size or dosing.
Hours to sidereal days reference tables
| Hours | Sidereal Days |
|---|---|
| 0.5 h | 0.0208904 sd |
| 1 h | 0.0417807 sd |
| 2 h | 0.0835615 sd |
| 3 h | 0.125342 sd |
| 5 h | 0.208904 sd |
| 10 h | 0.417807 sd |
| 20 h | 0.835615 sd |
| 25 h | 1.04452 sd |
| 50 h | 2.08904 sd |
| 75 h | 3.13356 sd |
| 100 h | 4.17807 sd |
| 250 h | 10.4452 sd |
| 500 h | 20.8904 sd |
| 1000 h | 41.7807 sd |
| 5000 h | 208.904 sd |
Exact factor: 1 hour = 0.0417807462379 sidereal days.
| Quantity | h | sd |
|---|---|---|
| A camera flash | 2.778e-07 h | 1.161e-08 sd |
| A heartbeat | 0.0002381 h | 9.946e-06 sd |
| A pop song | 0.05833 h | 0.002437 sd |
| A working day | 8 h | 0.3342 sd |
| A pregnancy | 6720 h | 280.8 sd |
| A mortgage term | 2.104e+05 h | 8790 sd |
Published reference values, converted with the same factor this page uses.