The year, month, and day are natural units of time derived from three different astronomical cycles caused by the relative motions of the Earth, Moon, and Sun.
Different techniques of measurement give different times for years, months, and days. Sidereal or stellar times are calculated according to the fixed positions of stars. An anomalistic year requires the Earth’s orbit to be measured in relation to the minimum or maximum distance from the sun, while tropical measurements refer to the apparent passage of the Sun and the actual passage of the Moon across the Earth’s equatorial plane.
A year is the period of one complete orbit of the Earth about the Sun. The sidereal year occurs once every 365 days, 6 hours, 9 minutes, and 10 seconds. The anomalistic year has a period of 365 days, 6 hours, 13 minutes, and 53 seconds, while the tropical year occurs once every 365 days, 5 hours, 48 minutes, and 45 seconds.
The regular and irregular fluctuations of the Earth’s orbit about the Sun amount to slight variations within a few milliseconds per year.
A month is the period of one complete orbit of the Moon about the Earth. The sidereal month occurs once every 27 days, 7 hours, 43 minutes, and 11 seconds, while a tropical month has a period which differs from the sidereal month by only 6 seconds.
The synodic month corresponds to the observation of the phases of the Moon, caused by the amount of sunlight which is reflected on the Moon’s surface. One full cycle of the Moon’s phases occurs every 29 days, 12 hours, 44 minutes, and 3 seconds.
A day is the period of one complete rotation of the earth about its axis, which results in regular periods of light and darkness on the Earth’s surface. However, the rotation of the Earth does not represent an exact unit of time because regular and irregular fluctuations of its motion cause a slow decrease in the rotational rate, due to tidal effects of the Moon on the Earth’s oceans, polar shifts, seasonal effects, and redistribution in the Earth’s structure.
The mean solar day is the average period of all the individual days which occur throughout the year. The mean solar day occurs once every 24 hours, whereas the sidereal day is shorter in duration than the mean solar day by about four minutes.
The mean solar day accounts for variations in the speed of the Earth’s rotation and revolution, due to the slight tilting and elliptical motion of the Earth about the Sun. The Earth’s rotational axis is tilted toward its orbital axis. In conjunction with the tilting, the Earth wobbles on its rotational axis, probably due to internal earthquake activity.
Universal time or so-called Earth time is the scale generated by the mean solar day which corrects for the tilted Earth traveling about the Sun in an elliptical orbit, the polar motion of the earth, and the regular slowing and speeding up of the earth in spring and fall. Earth time is based on daily observations of the stars.
Ephemeris time is Earth time which is determined by making predictions of certain astronomical events. Ephemeris time produces an exact time value, but requires making observations over a period of several years in order to establish a precise interval of a second.
Periodic seasonal variations on the Earth result from the inclination of the Earth’s axis to its orbital plane around the Sun. The inclination of the Earth to its orbital plane also produces variations in periods of light and darkness, which results in so-called solstices and equinoxes.
Solstices occur on two different days of the year, when the Sun appears to be overhead at midday at the maximum distance North and South of the Equator. Days are longest and nights are shortest during the summer solstice, while nights are longest and days are shortest during the winter solstice. Spring and autumn equinoxes occur when the periods of a single day and night are exactly equal, when the sun appears directly overhead at midday at the equator. Solstices occur at opposite times in the two different hemispheres.
On the longest day of the year, areas of the northern hemisphere have the maximum number of hours of daylight, while the southern hemisphere has the minimum number of hours. On the shortest day, the reverse is true.
The North and South Poles each have alternating 6-month periods of light and darkness.