Day

Day picture of the Bay of Naples, Italy

A day is roughly the period during which the Earth completes one rotation around its axis, which takes about 24 hours. A solar day is the time between when the Sun it reaches its highest point in the sky twice in succession. Days on other planets are defined similarly and vary in length due to different rotation periods, with Mars' being slightly longer and sometimes called the sun.

The unit of measure "day" (symbol d) is defined as 86400 SI seconds. The second is designated as the SI base unit of time. Previously, it was defined based on the orbital movement of the Earth in the year 1900, but since 1967 the second and, therefore, the day are defined by atomic electronic transition. A civil day usually has 24 hours, plus or minus one possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus one hour in places that switch to or from daylight saving time. The day can be defined as each of the periods of twenty-four hours, counted from one midnight to the next, into which a week, a month or a year is divided, and which correspond to one rotation of the earth on its axis. However, its use depends on its context; for example, when people say "day and night", "day" will have a different meaning: the light interval between two successive nights, the time between sunrise and sunset; the light time between one night and the next. To clarify the meaning of "day" in that sense, the word "day" instead, although the context and wording often make the meaning clear. The word day can also refer to a day of the week or a date on the calendar, as in the answer to the question "On what day?".

The biologically determined life patterns (circadian rhythms) of humans and many other species are related to Earth's solar day and day-night cycle.

Introduction

Dagr, the Nordic god of the day, rides his horse in this century paintingXIX of Peter Nicolai Arbo.

Apparent and mean solar day

Various definitions of this universal human concept are used depending on context, necessity, and convenience. In addition to the 24-hour day (86400 seconds), the word day is used for several different lengths of time based on the Earth's rotation around its axis. An important one is the solar day, defined as the time it takes for the Sun to return to its culmination point (its highest point in the sky). Because celestial orbits are not perfectly circular, and therefore objects travel at different speeds at different positions in their orbit, a solar day is not the same length throughout the orbital year. Since the Earth moves along an eccentric orbit around the Sun while the Earth rotates on a tilted axis, this period can be up to 7.9 seconds more (or less) than 24 hours. In recent decades, the average length of a solar day on Earth has been about 86,400,002 seconds (24,000,000 6 hours) and there are currently about 365,242,199 solar days in an average tropical year.

According to ancient custom, a new day begins with sunrise or sunset on the local horizon (the Italian reckoning, for example, is 24 hours from sunset, old style). The exact moment and the interval between two sunrises or sunsets depends on the geographical position (longitude and latitude) and the time of the year (as indicated by ancient hemispherical sundials).

A more constant day can be defined by the passage of the Sun through the local meridian, which occurs at local noon (upper culmination) or midnight (lower culmination). The exact moment depends on geographic longitude and, to a lesser extent, on the time of year. The duration of said day is almost constant (24 hours ± 30 seconds). This is the time indicated by modern sundials.

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

Star Day

A day, understood as the length of time it takes for the Earth to make a complete rotation with respect to the celestial background or a distant star (which is assumed to be fixed), is called stellar day. This rotation period is about 4 minutes less than 24 hours (23 hours 56 minutes and 4.09 seconds) and there are about 365.2422 stellar days in a mean tropical year (one stellar day more than the number of solar days). Other planets and moons have stellar and solar days of different lengths than Earth's.

In addition to a stellar day on Earth, other bodies in the Solar System have daytimes, the lengths of which are:

• name: duration of the day in hours

• Pluto: 153.3
• Neptune:16.1
• Uranus: 17.2
• Saturn: 10.7
• Jupiter: 9.9
• Ceres: 9 - 9.1 hours
• Mars: 24.7
• Earth Moon:708.7
• Venus: 2802.0
• Mercury: 4222.6

Day-Night

A day, in the sense of daytime as distinguished from night, is commonly defined as the period during which sunlight strikes the ground directly, assuming no local obstacles. The duration of daytime is, on average, slightly more than half of the 24-hour day. There are two effects that make the day, on average, longer than the nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc. Also, 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 arc minutes. Thus, the first light reaches the ground when the center of the Sun is still below the horizon by about 50 minutes of arc. Thus, the day lasts on average about 7 minutes longer than 12 hours.

Etymology

The term in Spanish comes from the Latin dies, and this one from the Proto-Indo-European *dyḗm, accusative of *dyḗws (sky, day).

International System of Units (SI)

A day, symbol d, defined as 86400 seconds, is not an SI unit, but its use with the SI is accepted. The second is the base unit of time in SI units.

In 1967-68, during the 13th CGPM (Resolution 1), the International Bureau of Weights and Measures (BIPM) redefined a second as

... the duration of 9 192 631 770 radiation periods corresponding to the transition between the two hyperphin levels of the basic state of the isotope 133 of the cesium atom.

This makes the day based on the SI exactly 794,243,384,928,000 of those periods long.

There are 365.25 days in a Julian year.

Longitudinal change

Mean solar day

It is used for all daily affairs. It is defined as the period that the Sun uses to culminate two consecutive times in the meridian of the observer. The days have a variable duration, depending on the time of year in which we are. This variability is caused by the eccentricity of the Earth's orbit and the obliquity of the ecliptic. For practical purposes, the concept of mean solar day is used, which lasts 24 hours or, what is the same, 86,400 seconds.

With the same reference, the Sun, we have the tropical year or tropical year, the period that the Earth takes in its annual movement. In a tropical year the Earth makes 365.242189 revolutions around its axis with respect to the Sun and, therefore, a tropical year lasts 365.242189 average solar days.

Sidereal day

Also called sidereal day, it is the time between two successive transits of the mean equinox or, equivalently, it is the time between two successive culminations of a star in the local meridian. For a determined observer, the sidereal day begins when the Aries point crosses its meridian.

In a tropical year the Earth makes 365.242189 turns around its axis with respect to the Sun, but with respect to the stars it makes one more turn: 366.242189. A good enough approximation of the value of the sidereal day can be obtained:

• 1 tropic year = 365,242189 days = 8.765,8125 hours
• 1 day straight = (8,765,8125 h/366,242189) = 23,9345 hours

The sidereal day turns out to be less than 24 hours: 23 h 56 min 4 s, approximately.

In observational astronomy, sidereal time is used. Suppose today we align a star and note the time. Tomorrow the star will reach the same alignment about 3 min 55.9 s sooner.

On the other hand, we must distinguish between the period of rotation of the Earth with respect to the stars and the sidereal day itself. As the mean equinox is a moving point due to precession, the sidereal day is 0.0084 seconds shorter than the rotational period with respect to the stars.

Summarizing:

• Rotary period compared to the stars: 23 h 56 min 4.0989 s
• Sidereal day (average): 23 h 56 min 4,0916 s

Length of days on different planets

The duration of the sidereal day of the different planets is indicated in terrestrial days (and not the duration of the solar day). The indicated values are rounded to two decimal places.

Day as a concept opposed to night

Variation of the duration of the day, sunset and sunrise to 40° latitude (Madrid) during 2011

It is also known as day, in a generic way, to the period that goes from sunrise to sunset. Refraction in the Earth's atmosphere causes light to be seen even when the Sun has not yet risen: dawn, dawn, or morning twilight. Such diffusion lengthens the luminosity time.

Measured from noon, the ortho is characterized by an hour angle -H, where:

# (H)=− − So... (λ λ )↓ ↓ So... (D){displaystyle cos(H)=-tan(lambda)*tan(D),}

where λ λ {displaystyle lambda ,} is the latitude of the place and D the solar declination. The occasion occurs at a time angle H.

The hard day 2H{displaystyle 2,H,} and the night 24− − 2H{displaystyle 24-2,H,}.

The duration of the day and night changes throughout the year, being 12 hours (in all latitudes) at the equinoxes, more than 12 hours in spring and summer (reaching the longest day at the solstice corresponding summer, where the shortest night also occurs), and less than 12 hours in autumn and winter (reaching the shortest day and longest night on the corresponding winter solstice).

This effect is more accentuated the higher the latitude. At some time of the year there is permanent day or night in the polar regions —both in the Northern Hemisphere and in the Southern Hemisphere— characterized by being at a latitude that, in absolute value, is greater than λ = 90° -23°26′ = 66°34′. This is precisely the definition of polar circle.

Twilight, in Plaza Mayor de Madrid.

Days of the week

In the Gregorian calendar, a day is one seventh of a week. Each day of a week has a different name, consecutive and cyclical: Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday.

Equivalences of other time units in days

• One week is 7 days
• One month is 30/31 days (with the exception of February, which has 28 days and 29 years biscuit)
• One year is equivalent to 365 days (except the biased year that has 366 days)
• A century equals 36,525 days (75 years with 365 days + 25 years with 366 days)
• A millennium is equivalent to 365,250 days (742 years with 365 days + 258 years with 366 days, approximately)

This equivalence is based on the Julian calendar which establishes the average duration of a year as 365.25 days, since it assigns a leap year every 4 years. However, due to the gap with a tropical year (365.242189 days on average), to correct this problem from 1582 the Gregorian calendar was gradually adopted, which assigns a year as 365.2425 days on average, because it is considered as a leap year: that year divisible by 4, except the secular year (multiple of 100), in which case it must be divisible by 400.

The day in the International System of Units (SI)

The concept of day is not part of the International System of Units, SI, but its use is accepted. The definition of the day is made based on the unit of time of the International System, the second, and in this way one day equals 86,400 seconds. Since the definition of the second is the duration of 9,192,631,770 radiation periods corresponding to the transition between two levels of the hyperfine structure of the ground state of cesium 133, one day is equivalent to 794,243,384,928,000 periods.

A day on the time scale called Coordinated Universal Time (UTC) can include a positive or negative leap second, and therefore can be 86.399 or 86.401 seconds.

Start of the day

Moon and sun in solar eclipse of October 7, 2010

For most diurnal animals, the day naturally begins at sunrise and ends at sunset. Humans, with their cultural norms and scientific knowledge, have employed various day limits.

In Ancient Egypt and Mesopotamia, the day began to be counted at dawn. In Ancient Greece, the starting point was at dusk. In Ancient Rome, it began at midnight, as it does today. The Jewish day begins at sunset or nightfall (when three second-magnitude stars appear). Medieval Europe followed this tradition, known as Florentine adjustment: in this system, a reference such as "two hours in the day" meant "two hours after sunset" and therefore the time during the night had to be changed. one day back by the modern computing calendar. Thus, days like Christmas Eve, Halloween or the eve of Saint Agnes are what remain of the old traditions when religious festivals began on the eve. The common current convention referring to a calendar day begins at midnight, that is to say at 00:00 (inclusive) and lasts 24 hours, until 24:00 (exclusive).

The day from a legal point of view

Business day

The day can also be understood in terms of the Work day (economics): the work of a person in a day. This is where the terms working day, business day, hire people for a day or derivative concepts such as daily come from. (salary for a day's work). In general, a working day is eight hours a day, according to the regulations in force in many countries, although this depends on the labor legislation of each country.

Holiday

A public holiday is the opposite of a working day, that is, a day officially recognized by law for rest and leisure (except emergency services and professions with specific hours). For historical reasons, a fixed day a week is usually considered a holiday; This day coincides with Sunday (in countries with a Christian tradition), Friday (in the case of those with a Muslim tradition), or Saturday (in the case of those with a Jewish tradition). In general, the creation of the world is remembered according to the religions of the Bible, when God rested at the end after seven days.

Apart from the weekly breaks, the dates indicated in each country are also holidays, for example memories of historical events or battles and the like, or the day of the patron saint, or of other revered religious figures. These days, apart from allowing not to go to work, are usually celebrated with a specific party.

Holidays are recognized in labor law as paid, even if you don't work, as a right of workers. The holidays of each year (the official working calendar) are officially set by a government decree, but each sector or company can decide to grant additional holidays. Holidays should not be confused with vacations, a longer period of work rest

International days

International days are internationally recognized dates to commemorate an event or fight an issue. Many of them are sponsored by the United Nations and are the generalization of local anniversaries or demands of civil society. These days, awareness campaigns, meetings and agreements are usually held.