units of time


A day is the time it takes Earth to spin once around on its axis relative to some external reference. The two main references are the Sun, which leads to the solar day, and the stars, which leads to the sidereal day.


The apparent solar day is the interval between two consecutive upper culminations (or upper transits) of the Sun, i.e., the period between one passage of the Sun at maximum altitude across the observer's meridian and the next. The apparent solar day varies with the time of year because the Sun moves in the ecliptic instead of along the celestial equator, and also because the Sun moves along the ecliptic at a variable rate (due to the varying distance of Earth from Sun during the year). The mean solar day is the average of the apparent solar day over a whole year or – what amounts to the same thing – the length of day reckoned according to the mean sun.


The equinoctial sidereal day is the interval between two successive meridian transits of the vernal equinox (equal to 23h 56m 4.091s). Because of precession, the sidereal day is about 0.0084 second shorter the true sidereal day, which is the period of Earth's rotation relative to a fixed direction, i.e., the interval between two successive upper transits of a star from a fixed point on Earth's surface.



An hour is one twenty-fourth part of a day, equal to 60 minutes or 3,600 seconds. In one hour, the Earth rotates through 15°. The time of day at any point on Earth is expressed as the number of hours and minutes that have elapsed since midnight for the time zone in which the point is situated, the time zones being fixed intervals behind or ahead of Greenwich Mean Time.


The hour is also the unit of right ascension, equivalent to 15 degrees of arc.



A month is a period of time connected with the motion of the Moon around Earth. The familiar calendar (or civil) month is an artificial unit consisting of a whole number of days. For astronomical purposes there are several other types of month.


Synodic month or lunar month
   The average interval from one new Moon to the next – 29.53059 days


Anomalistic month
   The interval in which the Moon passes from perigee to perigee – 27.55464 days


Sidereal month
   The interval in which the Moon passes from a fixed position with respect to the stars back to the
   same position – 27.32166 days


Draconic month or nodal month
   The interval between successive passages of the Moon through its ascending node


Tropical month
   The interval between successive passages of the Moon through the vernal equinox – a mean
   of 27.21222 days



The second is the base unit of time (symbol: s) in the SI system. It is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.


A second is also a unit of angle (symbol: ") equal to 1/3600 of a degree or 1/60 of a minute.



A year is the time taken for Earth, or more generally any planet, to go once around the Sun. In astronomy, there are different kinds of year, each distinguished by the reference point used to measure the period of revolution.


Different types of year
year reference point(s) length (days)
sidereal background stars 365.25636
tropical equinoxes 365.24219
anomalistic apsides 365.25964
eclipse Moon's node 346.62003


anomalistic year

An anomalistic year is the interval between successive passages of Earth through perihelion or aphelion.


Besselian year

A Besselian year is the basic unit of the Besselian epoch, defined as the time taken for the right ascension of the mean sun to increase by 24 hours starting from when the mean sun's longitude is 280° (chosen because it corresponds roughly to 1 January). It is virtually the same as the tropical year.


eclipse year

An eclipse year is the time between successive returns of the Sun to the same node of the Moon's orbit. This period is keyed to the regular recurrence of both solar and lunar eclipses, which can only take place when the Sun and Moon are close to the node. Nineteen eclipse years are 6585.78 days which is almost exactly the same as the ancient Saros cycle of 6585.32 days – the period that separates eclipses in a given series.


leap year

In the Gregorian calendar, a leap year is a year lasting 366 days rather than 365, with 29 February (leap day) added as the extra day; this occurs in years whose last two digits are evenly divisible by 4; e.g., 1996. A leap year is also a year with an extra day in any other calendar.


lunar year

A lunar year is made of 12 lunations or synodic months (354.3672 days). A civil year has an exact number of days, which is determined by the calendar being used.


sidereal year

A sidereal year is the time taken by Earth to make one complete circuit round the celestial sphere as seen from the Sun. (Or, equivalently, the time for the Sun to make one complete trip against the background stars as seen from the center of Earth.)


solar or tropical year

A tropical year (or solar year) is the interval between successive vernal equinoxes. Because the equinoxes have an annual retrograde motion (due to precession), of 50.26" relative to the stars, the tropical year is about 20 minutes shorter than the sidereal year.