Day, Benjamin, 1838-1916, American printer; son of Benjamin Henry Day. While working in New York City, Day invented a process, utilizing celluloid sheets, for shading plates in the color printing of maps and illustrations. It is known as the Ben Day, or Benday, process. The term "Ben Day" is used as a noun, verb, and adjective.
Day, Benjamin Henry, 1810-89, American journalist. He learned the printer's trade in the office of the Springfield (Mass.) Republican and opened a printing office in New York City. Lack of work during a financial depression led him to begin publishing (1833) the New York Sun. The first edition consisted of four small pages; he wrote the paper and set the type without assistance. The price of the paper was 1¢; much less than other New York dailies at the time. The Sun was the first paper in the city to employ newsboys. By 1835, Day claimed a circulation of 19,360, the largest in the world, and in 1838 sold the Sun to his brother-in-law, Moses Yale Beach, for $40,000. In 1842, Day founded the monthly Brother Jonathan, which later became the first illustrated weekly in the United States.
Day, Clarence Shepard, 1874-1935, American essayist, b. New York City, grad. Yale, 1896. His biographical sketches of his parents, God and My Father (1932), Life with Father (1935), and Life with Mother (1937), won him popular recognition; incidents from these three books were used by Howard Lindsay and Russel Crouse for the play Life with Father (1939), which was one of the longest-running plays in Broadway history. Day's other works include essays, This Simian World (1920), and a collection of light verse and drawings, Scenes from the Mesozoic (1935).
Day, John, 1522-84, English printer. At his London shop Day designed and made type for himself, but not for sale. His types included musical notes and the first Anglo-Saxon type. He printed the first English book of church music (1560) and the first English edition of John Foxe's Book of Martyrs (1563), though not under that title (see Foxe, John). His edition of Euclid was the first English translation of that work. Day's printer's mark was a rising sun, a sleeper awakening, and the motto, "Arise, for it is Day."
Day, John, 1574?-1640?, English dramatist. Educated at Cambridge, he was one of Philip Henslowe's group of playwrights, collaborating with Thomas Dekker, Henry Chettle, and others. The allegorical masque The Parliament of Bees, which was written c.1607 (pub. 1641) is his only important work. His other plays include The Isle of Gulls (1606) and The Travels of Three English Brothers (1607).
Day, Stockwell, 1950-, Canadian political leader, b. Barrie, Ontario. He grew up in Montreal, attended (1970-71) the Univ. of Victoria, and held such jobs as auctioneer, deckhand, lumberjack, contractor, Christian educator, and evangelical lay pastor before entering politics. In 1986, Day was elected to the Alberta legislature, and subsequently served in several provincial offices, becoming Alberta's tax-cutting treasurer in 1997 and winning notice a proponent of a flat income tax, smaller government, and increased provincial sovereignty. In 2000 the youthful and vigorous Day defeated Preston Manning to become head of the Canadian Alliance and leader of the opposition; he was subsequently elected to the Canadian parliament. Day and the Alliance failed, however, to defeat the Liberals in elections called by Prime Minister Jean Chrétien for Nov., 2000. He resigned as party leader in Dec., 2001, and was replaced by Stephen Harper the following year. Day became minister for public safety in the Conservative government in 2006.
Day, Thomas, 1748-89, English social reformer and author. He supported the American Revolution and the abolition of slavery and was interested in improving the lot of the small farmer. His moralistic History of Sandford and Merton (3 vol., 1783-89) contrasts the "natural" education of the virtuous Sandford with the conventional one of the objectionable Merton. In Lichfield he was a member of the literary group centering about Anna Seward.
Day, William Rufus, 1849-1923, American statesman and Associate Justice of the U.S. Supreme Court (1903-22), b. Ravenna, Ohio. Admitted (1872) to the bar, Day practiced law in Ohio and served (1886-90) as judge of the court of common pleas. He became (1897) assistant to the Secretary of State and then (Apr., 1898) Secretary of State in the month when war was declared against Spain. He was successful in converting France and Germany from an attitude of seeming hostility to definite neutrality. Made chairman (Sept., 1898) of the U.S. commission to arrange peace after the Spanish-American War, he insisted upon purchase of the Philippines rather than claiming these islands by right of conquest. The treaty therefore provided for the payment of $20 million. Day became (1899) a judge of the U.S. Circuit Court of Appeals, and in 1903 President Theodore Roosevelt appointed him to the Supreme Court, where he generally voted for the dissolution of trusts and the preservation of states' rights.
day, period of time for the earth to rotate once on its axis. The ordinary day, or solar day, is measured relative to the sun, being the time between successive passages of the sun over a stationary observer's celestial meridian. The length of a solar day varies during the course of a year, so for purposes of time measurement an average, or mean, solar day is used (see solar time), equal to exactly 24 hr. The sidereal day, used by astronomers, is measured relative to the fixed stars rather than the sun (see sidereal time); it is about 4 min shorter than the mean solar day. The term day is also used to refer to that part of each 24-hr period during which the sun's direct rays are not blocked by the earth, this period of daylight hours extending from sunrise to sunset; the remaining portion of the 24 hr is called night. If the plane of the earth's orbit about the sun coincided with the plane of the equator, day and night would each be 12 hr long everywhere on the earth all year long. However, because of the tilt of the earth's axis of rotation, the times of sunrise and sunset vary from day to day, with the result that in the Northern Hemisphere there are long days and short nights in the summer and short days and long nights in the winter. See equinox; solstice.

A day (symbol: d) is a unit of time equivalent to 24 hours and the duration of a single rotation of planet Earth with respect to the sun. It is not an SI unit but it is accepted for use with SI. It is the fundamental unit of time in the IAU system of units. The term comes from the Old English dæg. The word is also used to mean daytime, the period of daylight experienced once per day and alternating with night.


The day has several definitions.

International System of Units (SI)

A day contains 86,400 SI seconds. Each second is currently 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 caesium-133 atom.


A day of exactly 86,400 SI seconds is the fundamental unit of time in astronomy.

All planets have a rotation period relative to the fixed stars, commonly called their sidereal rotation periods.. For Earth this is 23hours and 56 minutes and 50 seconds. Earth also has a misnamed sidereal day, its rotation relative to its own precessing or moving mean vernal equinox, which is 8.4 ms smaller than its rotation period relative to the fixed stars. The rotation periods of the gas giants, which have no solid surface, are usually assumed to be the rotation of the magnetic fields residing in their deep interiors unless stated otherwise. Jupiter and Saturn also have specified equatorial and high latitude rotation periods for their cloud tops.


The word refers to various relatedly defined ideas, including the following:

  • The period of light when the Sun is above the local horizon (i.e., the period from sunrise to sunset), opposed to night. See Daytime (astronomy).
  • The full day covering a dark and a light period, beginning from the beginning of the dark period or from a point near the middle of the dark period.
  • A full dark and light period, sometimes called a nychthemeron in English, from the Greek for night-day.
  • The period from 06:00 to 18:00 or 21:00 or some other fixed clock period overlapping or set off from other periods such as "morning", "evening", or "night".
  • The mostly regular interval of one awaking, usually in the morning (personal day).


The word day is used for several different units of time based on the rotation of the Earth around its axis. The most important one follows the apparent motion of the Sun across the sky (solar day; see solar time). The reason for this apparent motion is the rotation of the Earth around its axis, as well as the revolution of the Earth in its orbit around the Sun.

A day, as opposed to night, is commonly defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles. Two effects make days on average longer than nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. The difference in time depends on the angle at which the Sun rises and sets (itself a function of latitude), but amounts to almost seven minutes at least.

Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example) The exact moment of, and the interval between, two sunrises or two sunsets depends on the geographical position (longitude as well as latitude), and the time of year. This is the time as indicated by ancient hemispherical sundials.

A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of such a day is nearly constant (24 hours ± 30 seconds). This is the time as indicated by modern sundials.

A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial equator; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth moves along its orbit around the Sun (due to both its velocity and its axial tilt).

The Earth's day has increased in length over time. The original length of one day, when the Earth was new about 4.5 billion years ago, was about six hours as determined by computer simulation. It was 21.9 hours 620 million years ago as recorded by rhythmites (alternating layers in sandstone). This phenomenon is due to tides raised by the Moon which slow Earth's rotation. Because of the way the second is defined, the mean length of a day is now about 86,400.002 seconds, and is increasing by about 1.7 milliseconds per century (an average over the last 2700 years). See tidal acceleration for details.

Leap seconds

The actual mean period of rotation of the earth with respect to the sun is slightly longer than the SI day of 86,400 seconds. It is more nearly 86,400.002 seconds. This additional time accumulates to about 0.7 s per year or about seven seconds every ten years, necessitating the addition of an extra second to the civil clock occasionally to retard it and keep it more closely synchronized to the apparent movement of the sun. By the middle of this century the amount of time to be added to the clock will increase to one second every year. This additional second is called a leap second. A civil clock day is typically 86,400 SI seconds long, but will be 86,401 s or 86,399 s long in the event of a leap second.

Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service which measures the Earth's rotation and determines whether a leap second is necessary. Leap seconds occur only at the end of a UTC month, and have only ever been inserted at the end of June 30 or December 31.


In astronomy, the sidereal day is also used; it is about 3 minutes 56 seconds shorter than the solar day, and close to the actual rotation period of the Earth, as opposed to the Sun's apparent motion. In fact, the Earth spins 366 times about its axis during a 365-day year, because the Earth's revolution about the Sun removes one apparent turn of the Sun about the Earth.

Boundaries of the day

For most diurnal animals, including Homo sapiens, the day naturally begins at dawn and ends at sunset. Humans, with their cultural norms and scientific knowledge, have supplanted Nature with several different conceptions of the day's boundaries. The Jewish day begins at either sunset or at nightfall (when three second-magnitude stars appear). Medieval Europe followed this tradition, known as Florentine reckoning: in this system, a reference like "two hours into the day" meant two hours after sunset and thus times during the evening need to be shifted back one calendar day in modern reckoning. Days such as Christmas Eve, Halloween, and the Eve of Saint Agnes are the remnants of the older pattern when holidays began the evening before. Present common convention is for the civil day to begin at midnight, that is 00:00 (inclusive), and last a full twenty-four hours until 24:00 (exclusive).

In ancient Egypt, the day was reckoned from sunrise to sunrise. Muslims fast from daybreak to sunset each day of the month of Ramadan. The "Damascus Document", copies of which were also found among the Dead Sea Scrolls, states regarding Sabbath observance that "No one is to do any work on Friday from the moment that the sun's disk stands distant from the horizon by the length of its own diameter," presumably indicating that the monastic community responsible for producing this work counted the day as ending shortly before the sun had begun to set. The Baha'i day begins and ends at sunset, for a calendar that has 19 days, in each of 19 months. The vernal equinox begins and ends the Baha'i year. Days in excess of 361 are called inter-calendar days.

In the United States, nights are named after the previous day, e.g. "Friday night" usually means the entire night between Friday and Saturday. This is the opposite of the Jewish pattern. Events starting at midnight are often announced as occurring the day before. TV-guides tend to list nightly programs at the previous day, although programming a VCR requires the strict logic of starting the new day at 00:00 (to further confuse the issue, VCRs set to the 12-hour clock notation will label this "12:00 AM"). Expressions like "today", "yesterday" and "tomorrow" become ambiguous during the night.

Validity of tickets, passes, etc., for a day or a number of days may end at midnight, or closing time, when that is earlier. However, if a service (e.g. public transport) operates from e.g. 6:00 to 1:00 the next day (which may be noted as 25:00), the last hour may well count as being part of the previous day (also for the arrangement of the timetable). For services depending on the day ("closed on Sundays", "does not run on Fridays", etc.) there is a risk of ambiguity. As an example, for the Dutch Railways, a day ticket is valid 28 hours, from 0:00 to 28:00 (i.e. 4:00 the next day). To give another example, the validity of a pass on London Regional Transport services is until the end of the "transport day" -- that is to say, until 4:30 am on the day after the "expiry" date stamped on the pass.


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