# day

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## day

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.

## day

The period of the Earth's rotation on its axis, equal to 86 400 seconds (24 hours) unless otherwise specified.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Day

(1) The same as sutki (24 hours).

(2) The light portion of the 24-hour period, between the rising and setting of the upper edge of the sun. Its length depends on geographical latitude and varies with the declination of the sun. At the equator the length of the day is roughly constant and is equal to approximately 12 hrs. At the poles the day lasts half a year. At latitudes from 0° to + 66°37’ the length of the day is at a maximum at the summer solstice (June 22) and at a minimum at the winter solstice (December 22). On the other hand, at latitudes from 0° to -66°37’ the day is shortest on June 22 and longest on December 22. Beyond the polar circles (latitude greater than +66°37’ or less than -66°37’), the day lasts more than 24 hours (polar day), depending on the time of year (in the summer at the north pole and in the winter at the south pole). The length T of a day may be calculated from the formula cos t = -[sin(R + ρ) - sin δsin φ ]/(cos δ cos φ) T = 2t, where δ is the sun’s declination, φ the geographical latitude, R the sun’s angular radius (16’), and ρ the refraction at the horizon (34’). The length of the day between latitudes +34° and +64° on the 1st, 11th and 21st of each month is given in Table 1. The values are accurate to 2-3 minutes over the next 50. years.

E. A. IUROV

Table 1. Length of day at various latitudes of the northern hemisphere
Local latitude (north)
Date34°
hr min
40°
hr min
46°
hr min
52°
hr min
58°
hr min
64°
hr min
January
1...............9 549 238 437 516 364 27
11...............10 049 348 568 086 585 05
21...............10 169 499 158 327 315 59
February
1...............10 3410 109 429 068 187 06
11...............10 5110 3310 109 429 048 10
21...............11 1010 5810 4010 219 539 16
March
1...............11 2711 1811 0610 5210 3310 08
11...............11 4811 4411 3911 3211 2411 12
21...............12 1012 1212 1212 1212 1512 18
April
1...............12 3212 3812 4812 5713 1013 28
11...............12 5313 0513 1913 3814 0214 34
21...............13 1313 3013 5114 1614 5015 40
May
1...............13 3213 5414 2114 5215 3716 46
11...............13 5014 1514 4715 2816 2317 52
2114 0414 3415 1015 5817 0418 56
June
1...............14 1614 4815 3016 2417 4220 00
11...............14 2314 5815 4216 3818 0420 44
2114 2615 0115 4616 4518 1121 00
July
1...............14 2314 5815 4216 4018 0320 45
11...............14 1614 4915 3016 2417 4220 04
21...............14 0614 3615 1416 0217 1019 07
August
1...............13 5014 1614 4815 2916 2917 58
11...............13 3213 5614 2014 5615 4316 53
21...............13 1413 3213 5314 2014 5515 48
September
1...............12 5313 0513 2013 3814 0314 36
11...............12 3212 4012 4813 0013 1313 32
21...............12 1212 1412 1712 2012 2312 28
October
1...............11 5011 4711 4411 4011 3411 24
11...............11 2911 2011 1111 0010 4310 20
21...............11 0910 5610 4010 209 549 15
November
1...............10 4910 2910 069 399 018 05
11...............10 3110 079 409 028 147 02
21...............10 169 489 158 327 336 01
December
1...............10 049 348 568 086 595 06
11...............9 569 238 437 516 364 28
21...............9 549 208 387 456 284 12

## Day

(holiday), a calendar date devoted to a historical event, labor holiday, international solidarity, and so on. Special days that have been established in the USSR are shown in Table 1.

The dates of observance of certain special days are associated with historical events—for example, the date established for Radio Day is associated with the 50th anniversary of the invention of radio by A. S. Popov (May 7, 1895). Miners’

Table 1. Special days of the USSR
Date of establishmentDate of observance
All-Union Farm Workers’ Day...............Aug. 26, 1966second Sunday in October
All-Union Petroleum and Gas Workers’ Day0...............Aug. 28, 1965first Sunday in September
All-Union Physical Education Day...............June 16, 1939second Saturday in August
All-Union Railroad Workers’ Day...............July 28, 1936first Sunday in August
Builders’ Day...............Sept. 6, 1955second Sunday in August
Chemists’ Day...............Dec. 10, 1965last Sunday in May
Cosmonautics Day; World Aviation and Cosmonautics Day...............Apr. 9, 1962April 12
Electric Power Workers’ Day...............May 23, 1966December 22
Fishermen’s Day...............May 3, 1965second Sunday in July
Food Industry Workers’ Day...............Aug. 30, 1966third Sunday in October
Forestry Workers’ Day...............Aug. 13, 1966third Sunday in September
Frontier Corps Day...............May 15, 1958May 28
Geologists’ Day...............Mar. 31,1966first Sunday in April
Komsomol Foundation Day...............1918October 29
Light Industry Workers’ Day...............May 30, 1966second Sunday in June
Machine Builders’ Day...............Aug. 15, 1966last Sunday in September
Medical Workers’ Day...............Dec. 10, 1965third Sunday in June
Metallurgists’ Day...............Sept. 28, 1957third Sunday in July
Miners’ Day...............Sept. 10, 1947last Sunday in August
Paris Commune Day...............Feb. 20, 1872March 18
Press Day...............May5
Rocket and Artillery Forces’ Day (before 1964, Artillery Day; established Oct. 21, 1944)...............Nov. 17, 1964November 19
Soviet Army and Navy Day...............February 23
Soviet Militia Day...............Sept. 26, 1962November 10
Soviet Youth Day...............Feb. 7, 1958last Sunday in June
Tank Forces’ Day...............July 11, 1946second Sunday in September
Teachers’ Day...............Sept. 29, 1965first Sunday in October
Trade Workers ’ Day...............June 29, 1966fourth Sunday in July
USSR Air Force Day (Aviation Day)...............Apr. 28, 1933third Sunday in August
USSR Constitution Day...............Dec. 5, 1936December 5
USSR Navy Day...............July 22, 1939first Sunday after July 22
Victory Holiday...............May 8, 1945May 9
V. I. Lenin Memorial Day...............April 22 (V. I. Lenin’s birthday)
V. I. Lenin Pioneer Organization Founding Day...............1922May 19

Day was first observed in connection with the night of Aug. 30-31, 1935, when the miner A. G. Stakhanov set a record that served as the beginning of the Stakhanovite movement; Electric Power Workers’ Day was established in honor of the opening day in 1920 of the All-Russian Congress of Soviets, which adopted GOELRO (State Commission for the Electrification of Russia).

Since 1955 it has become a tradition to hold a Poetry Day: each year during the autumn in major cities there are meetings of poets, critics, translators, actors, and readers. A collection entitled Poetry Day is usually published in observance of this day. Since 1957 a Film Day has been observed in Moscow and many other cities. This day usually is timed to coincide with the anniversary of Lenin’s decree on the nationalization of the motion-picture industry (Aug. 27, 1919).

Table 2. International special days
Dafe of establishmentDate of observance
African Liberation Day...............1963May 25
International Chess Day...............July 20, 1966July 20
International Children’s Protection Day...............1949June 1
International Cooperation Day...............1923first Saturday in July
International Journalists’ Solidarity Day...............1958September 8
International Students’ Day...............1950November 17
International Theater Day...............June 10, 1961March 27
International Workers’ Solidarity Day—May Day...............May 1 and 2
International Youth Solidarity Day...............1957April 24
March 8 International Women’s Day...............1910March 8
United Nations Day...............Oct. 31, 1947October 24
World Sister Cities Day...............1963last Sunday in April
World Health Day...............1948April 7
World Youth Day...............November 1945November 10

Birds’ Day, a spring holiday for schoolchildren, has been observed since 1926 (March 24 or on one of the first Sundays in April). Mass construction of wooden birdhouses is carried out in preparation for the arrival of the birds. Harvest Day is observed in many regions of the country.

The USSR and several foreign countries observe worldwide and international special days, which are given in Table 2.

## Day

a unit of time equal to 24 hours. A distinction is made between the sidereal day and the solar day. The sidereal day is the period of rotation of the earth relative to the vernal equinox. The solar day is the period of rotation of the earth relative to the sun.

The sidereal day is equal to the interval of time between two successive upper (or lower) transits of the vernal equinox. The moment at which this transit occurs is regarded as the beginning of the sidereal day—that is, as zero hour (hr) of sidereal time. A distinction is made between the apparent, or true, sidereal day and the mean sidereal day, depending on whether the true or mean vernal equinox is used: the motion of the true equinox is affected by precession and nutation, and the motion of the mean equinox is affected only by precession. As a result of the preces-sional motion of the vernal equinox, the mean sidereal day is 0.0084 sec shorter than the actual period of the earth’s rotation. The duration of the apparent sidereal day is not constant and varies continuously owing to nutation.

It is inconvenient to use sidereal days for the measurement of time, since they do not coincide with the alternation of day and night. Consequently, solar days are generally employed. The solar day is equal to the time interval between two successive upper or lower transits of the sun—that is, between two successive noons or midnights. As a result of the ellipticity of the earth’s orbit and the obliquity of the ecliptic, the time between two successive transits of the apparent, or true, sun—that is, the duration of the apparent, or true, solar day—is not constant. During the course of a year, the length of the apparent solar day varies from 24 hr 3 min 36 sec of sidereal time, in mid-September, to 24 hr 4 min 27 sec of sidereal time, in late December. To eliminate this variation, the mean length of the solar day over a year is used. Such a day is called the mean solar day and is equal to 24 hr 3 min 56.55536 sec of sidereal time. In civil time, the beginning of the mean solar day is the mean midnight, that is, the moment of the lower transit of the fictitious point on the celestial sphere known as the mean sun.

Like the mean solar day, the sidereal day is divided into hours, minutes, and seconds. The relation between the sidereal and solar units is as follows: one day, minute, or second of sidereal time is equal to 0.9972696 of the corresponding unit of mean solar time. The circumstance that the year does not contain a whole number of mean solar days is the principal difficulty encountered in the construction of the calendar.

### REFERENCE

Spravochnoe rukovodstvo po nebesnoi mekhanike i astrodinamike. Edited by G. N. Duboshin. Moscow, 1971.

## What does it mean when you dream about daylight?

Daytime as opposed to nighttime represents the conscious mind as opposed to the unconscious. Seeing what we are doing. Can refer to one’s mood: a sunny day is bright and cheerful whereas an overcast day is different. The word “day” plays a role in many idioms, from “bad hair day” to “make my day.”

## day

[]
(astronomy)
One of various units of time equal to the period of rotation of the earth with respect to one or another direction in space; specific examples are the mean solar day and the sidereal day.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

## day

One division in a window, as in a large church window.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

## day

The period taken by the earth to make one revolution around its own axis. A day is measured by successive transits of a reference point on the celestial sphere over the meridian, and each type takes its name from the reference used. Thus, for a solar day, the reference is the sun; for a mean solar day, the mean sun; and for an apparent solar day, the apparent sun. For a lunar day, the reference is the moon; for sidereal day, the vernal equinox; and for a constituent day, an astre fictif, or fictitious star. The expression lunar day refers also to the duration of one rotation of the moon with respect to the sun. A mean solar day is equal to 8.64 × 104 s, and a sidereal day is approximately equal to 8.616 × 104 s. A Julian day is from noon, universal time, on the given date to the noon of the following date.