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.

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)
Date	34° hr min	40° hr min	46° hr min	52° hr min	58° hr min	64° hr min
January
1...............	9 54	9 23	8 43	7 51	6 36	4 27
11...............	10 04	9 34	8 56	8 08	6 58	5 05
21...............	10 16	9 49	9 15	8 32	7 31	5 59
February
1...............	10 34	10 10	9 42	9 06	8 18	7 06
11...............	10 51	10 33	10 10	9 42	9 04	8 10
21...............	11 10	10 58	10 40	10 21	9 53	9 16
March
1...............	11 27	11 18	11 06	10 52	10 33	10 08
11...............	11 48	11 44	11 39	11 32	11 24	11 12
21...............	12 10	12 12	12 12	12 12	12 15	12 18
April
1...............	12 32	12 38	12 48	12 57	13 10	13 28
11...............	12 53	13 05	13 19	13 38	14 02	14 34
21...............	13 13	13 30	13 51	14 16	14 50	15 40
May
1...............	13 32	13 54	14 21	14 52	15 37	16 46
11...............	13 50	14 15	14 47	15 28	16 23	17 52
21	14 04	14 34	15 10	15 58	17 04	18 56
June
1...............	14 16	14 48	15 30	16 24	17 42	20 00
11...............	14 23	14 58	15 42	16 38	18 04	20 44
21	14 26	15 01	15 46	16 45	18 11	21 00
July
1...............	14 23	14 58	15 42	16 40	18 03	20 45
11...............	14 16	14 49	15 30	16 24	17 42	20 04
21...............	14 06	14 36	15 14	16 02	17 10	19 07
August
1...............	13 50	14 16	14 48	15 29	16 29	17 58
11...............	13 32	13 56	14 20	14 56	15 43	16 53
21...............	13 14	13 32	13 53	14 20	14 55	15 48
September
1...............	12 53	13 05	13 20	13 38	14 03	14 36
11...............	12 32	12 40	12 48	13 00	13 13	13 32
21...............	12 12	12 14	12 17	12 20	12 23	12 28
October
1...............	11 50	11 47	11 44	11 40	11 34	11 24
11...............	11 29	11 20	11 11	11 00	10 43	10 20
21...............	11 09	10 56	10 40	10 20	9 54	9 15
November
1...............	10 49	10 29	10 06	9 39	9 01	8 05
11...............	10 31	10 07	9 40	9 02	8 14	7 02
21...............	10 16	9 48	9 15	8 32	7 33	6 01
December
1...............	10 04	9 34	8 56	8 08	6 59	5 06
11...............	9 56	9 23	8 43	7 51	6 36	4 28
21...............	9 54	9 20	8 38	7 45	6 28	4 12

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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 establishment	Date of observance
All-Union Farm Workers’ Day...............	Aug. 26, 1966	second Sunday in October
All-Union Petroleum and Gas Workers’ Day0...............	Aug. 28, 1965	first Sunday in September
All-Union Physical Education Day...............	June 16, 1939	second Saturday in August
All-Union Railroad Workers’ Day...............	July 28, 1936	first Sunday in August
Builders’ Day...............	Sept. 6, 1955	second Sunday in August
Chemists’ Day...............	Dec. 10, 1965	last Sunday in May
Cosmonautics Day; World Aviation and Cosmonautics Day...............	Apr. 9, 1962	April 12
Electric Power Workers’ Day...............	May 23, 1966	December 22
Fishermen’s Day...............	May 3, 1965	second Sunday in July
Food Industry Workers’ Day...............	Aug. 30, 1966	third Sunday in October
Forestry Workers’ Day...............	Aug. 13, 1966	third Sunday in September
Frontier Corps Day...............	May 15, 1958	May 28
Geologists’ Day...............	Mar. 31,1966	first Sunday in April
Komsomol Foundation Day...............	1918	October 29
Light Industry Workers’ Day...............	May 30, 1966	second Sunday in June
Machine Builders’ Day...............	Aug. 15, 1966	last Sunday in September
Medical Workers’ Day...............	Dec. 10, 1965	third Sunday in June
Metallurgists’ Day...............	Sept. 28, 1957	third Sunday in July
Miners’ Day...............	Sept. 10, 1947	last Sunday in August
Paris Commune Day...............	Feb. 20, 1872	March 18
Press Day...............	—	May5
Radio Day...............	May 2, 1945	May 7
Rocket and Artillery Forces’ Day (before 1964, Artillery Day; established Oct. 21, 1944)...............	Nov. 17, 1964	November 19
Soviet Army and Navy Day...............	—	February 23
Soviet Militia Day...............	Sept. 26, 1962	November 10
Soviet Youth Day...............	Feb. 7, 1958	last Sunday in June
Tank Forces’ Day...............	July 11, 1946	second Sunday in September
Teachers’ Day...............	Sept. 29, 1965	first Sunday in October
Trade Workers ’ Day...............	June 29, 1966	fourth Sunday in July
USSR Air Force Day (Aviation Day)...............	Apr. 28, 1933	third Sunday in August
USSR Constitution Day...............	Dec. 5, 1936	December 5
USSR Navy Day...............	July 22, 1939	first Sunday after July 22
Victory Holiday...............	May 8, 1945	May 9
V. I. Lenin Memorial Day...............	—	April 22 (V. I. Lenin’s birthday)
V. I. Lenin Pioneer Organization Founding Day...............	1922	May 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 establishment	Date of observance
African Liberation Day...............	1963	May 25
International Chess Day...............	July 20, 1966	July 20
International Children’s Protection Day...............	1949	June 1
International Cooperation Day...............	1923	first Saturday in July
International Journalists’ Solidarity Day...............	1958	September 8
International Students’ Day...............	1950	November 17
International Theater Day...............	June 10, 1961	March 27
International Workers’ Solidarity Day—May Day...............	—	May 1 and 2
International Youth Solidarity Day...............	1957	April 24
March 8 International Women’s Day...............	1910	March 8
United Nations Day...............	Oct. 31, 1947	October 24
World Sister Cities Day...............	1963	last Sunday in April
World Health Day...............	1948	April 7
World Youth Day...............	November 1945	November 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.

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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

[dā]

(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.

day

One division in a window, as in a large church window.

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.

day

1. the period of time, the sidereal day during which the earth makes one complete revolution on its axis relative to a particular star. The mean sidereal day lasts 23 hours 56 minutes 4.1 seconds of the mean solar day

2. the period of time, the solar day during which the earth makes one complete revolution on its axis relative to the sun. The mean solar day is the average length of the apparent solar day and is some four minutes (3 minutes 56.5 seconds of sidereal time) longer than the sidereal day

3. the period of time taken by a specified planet to make one complete rotation on its axis

4. day of rest the Sabbath; Sunday