Months to Sidereal Days (mo to d (sid))
1 month equals 30.52020842 sidereal days (rounded).
How to convert months to sidereal days
To convert months to sidereal days, multiply the number of months by 30.52020842. The factor is rounded to 10 significant digits. To go the other way, multiply sidereal days by 0.03276517599 (rounded).
sidereal days = months × 30.52020842
Example: 10 months = 305.2020842 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
Month (mo)
One twelfth of an average Gregorian year: 30.436875 days. Calendar months actually range from 28 to 31 days.
Month and Moon are cognates, and the unit began as the Moon's cycle of phases, about 29.53 days from one new moon to the next. The trouble is that 12 of these fall short of a solar year, so calendars have had to choose. The Islamic calendar stays purely lunar, so its year is about 11 days shorter than the solar year, while the Hebrew calendar is lunisolar, using a 19-year cycle with an extra month. Even the start of a lunar month is still debated between observing the crescent and calculating it.
Gregorian months are a compromise that drifted far from the Moon. Their lengths run from 28 to 31 days, and this site uses their mean of 30.436875 days, one twelfth of the average year. The names also keep Roman history: September to December preserve the Latin numerals for seven to ten, since they were originally the seventh to tenth months, and Quintilis and Sextilis were renamed for Julius Caesar and Augustus. When a contract says month, check whether it means a calendar month or thirty days.
1 month = 2,629,746 seconds (exactly).
Sources: Wikipedia: Month; Wikipedia: Lunar calendar
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 months (reverse)
- Months to seconds
- Months to minutes
- Months to hours
- Months to days
- Months to weeks
- Months to years
- Months to milliseconds
- Months to microseconds
- Months to nanoseconds
- Months to decades
- Months to centuries
- Months to millennia
- Seconds to sidereal days
- Minutes to sidereal days
- Hours to sidereal days
- Days to sidereal days
- Weeks to sidereal days
- Years to sidereal days
See all time units.