When does the day begin?

Day. Rhythmic change of day and night. .. The unique beauty of these phenomena is equally impressive for a person both in childhood and in old age. Can not it worry, for example, the summer sunrise? Gradually, the night gloom melts, the stars slowly die out, the east glows with a glow, nature awakens from sleep. And suddenly in every drop of morning dew the sunrise is displayed, bringing to all living light, warmth, joy of life.

Gradually climbing up, the Sun reaches its highest position in the sky( the moment of the upper culmination), then slowly descends to a few hours again to hide behind the horizon. After 30-40 minutes after sunset, when the evening twilight ends, the first stars appear in the sky. And again everything falls asleep. This is the correct alternation of day and night, which is a reflection of the Earth's rotation around its axis, and gave people a natural unit of time - a day.

So, the day is the time interval between two consecutive same-named climaxes of the Sun. For the beginning of a true solar day take the moment of the lower culmination of the center of the disk of the Sun( midnight).In accordance with the tradition that came to us from ancient Egypt and Babylonia, the day is divided into 24 hours, every hour - for 60 minutes, every minute - for 60 seconds. Time T, measured from the lower culmination of the center of the solar disk, is called true solar time.

But our Earth is a ball! Therefore, at the moment when, say, a resident of Kiev claims that the Sun is in the lower climax, and quietly goes to bed, in Yuzhno-Sakhalinsk it's morning and people are rushing to work. And this means that every point on the globe, the geographical longitude of which I, corresponds to its own( local) time, which will be the same only for points located on the same geographic meridian. The geographic longitude of observer A, measured usually in degrees, minutes and seconds of the arc, when translated into hour units, is based on the calculation that 15 ° = 1 h , 15 = lm, 15 "= Is, 1 ° = 4t, 1° m

For example, the geographical longitude of Kharkov is X = 36 ° 15 = 2h25m

Average solar time The measurements show that the duration of a true solar day is not the same throughout the year, they have the greatest length on December 23, the smallest 16September, and the difference in their duration on the days indicated is 51 secondsThis is due to two reasons:

1) the uneven movement of the Earth around the sun in an elliptical orbit,

2) the inclination of the axis of the daily rotation of the Earth to the plane of the ecliptic

It is obvious that one can not use such an unstable unit as a true day when measuring time. In astronomy, the notion of the average sun was introduced: it is a fictitious point, which throughout the year moves evenly along the celestial equator. The time interval between the two eponymous successive culminations of the middle sun is called the average solar day. The time measured from the lower culmination of the average sun is called the mean solar time. It is the average solar time and our hours that we use in all our practical activities.

The difference between the average solar time of 1 m( average local time of the observer) and the true solar time T, measured on the same meridian, is called the time equation( Fig.):

Fig. Change the equation of time throughout the year. The right scale indicates the local mean solar time at the time when the center of the solar disk is in the upper culmination of the

The time equation η can be found at the beginning of every day in any astronomical calendar.

World Time, Daylight Saving Time. At the end of the last century, the globe was broken every 15 ° by geographic longitude into 24 time zones so that inside each belt having the number N( N varies from 0 to 23), the clock indicated the same time zone - Tn -the average solar time of a geographic meridian passing through the middle of this belt. When moving from the waist belt in the direction from west to east, the time at the border of the belt jumps by exactly one hour. As a zero belt, located( in longitude) in the band ± 7 °, 5 from the Greenwich meridian. The average solar time of this belt, denoted To or UT, is called Greenwich or worldwide. Almost the entire European part of the USSR is in the second time zone( N = 2), whose central meridian, corresponding to the eastern geographical longitude of 30 °, passes near Leningrad and Kiev. It is obvious that the culmination of the Sun on the central meridian of the 2nd belt occurs 2 hours earlier than on the Greenwich meridian, 2 hours earlier here begin and day. In many countries of the world in the summer months of the year, the transition to the time of a neighboring, eastern-time time zone is practiced. In the USSR, the hands of the clock, translated by Decree of the Council of People's Commissars on June 16, 1930, one hour ahead, and today( "until the abolition") count the maternity time Td instead of the belt. The connection of this time with the local Tm, local Tm and the world T0 is determined by the ratios.

So, to the question: "By what time do you live?" A resident of Kharkov( N = 2) will answer: "According to the mint time of the second time zone".

Since 1981, summer time has also been introduced in our country: at night on the last Sunday in March the hands of the clock are moved one hour ahead of the mint time, and at night on the last Sunday of September they return back.

Starry day. Above it was said about the rotation of the Earth around its axis relative to the Sun. It turned out to be convenient and even necessary to introduce one more unit of time - a star day as a time interval between two consecutive same-named culminations of the same star. Since, while rotating around the axis, the Earth is still moving in its orbit, the starry days are shorter than the solar ones for almost four minutes. In the year of the star day, exactly one more than the solar ones.

The beginning of the star day is the moment of the upper climax of the vernal equinox point. Hence the sidereal time s is the time elapsed since the top culmination of the vernal equinox point. It is measured by the hour angle of the point of the vernal equinox t¡ . It can be determined by the formula s = a + t, where a and t are the right ascension and the hour angle of any star. It follows that the sidereal time s is equal to the direct ascension of the luminary being

Tp, the local Tm and the world T0 is determined by the relations

Why is it necessary to know about this? The equation of time says that the true Sun in its movement on the celestial sphere "outruns" the average, the sun, then "lags" behind it, and if we measure time in the middle sun, then the shadows from all objects are discarded because of theirillumination by the true Sun! Suppose that someone decided to build a building facing south. The desired direction will be indicated by the noonday line: at the time of the upper culmination of the Sun, when it crosses the celestial meridian "passes over the point of the south," the shadows from the vertical objects fall along the midday line towards the north. . Therefore, to solve the problem,and at this point in time, drive the pegs along the shadow cast away.

But to install "by eye" when the center of the Sun's disk crosses the celestial meridian, it is impossible, this moment should be calculated in advance. To do this, the observer must know his geographic, longitude λ.

It is obvious that an archaeologist who is interested in the orientation of ancient structures( pyramids, temples, etc.) solves the reverse problem - the direction of one of the main walls of the building already exists, it is necessary to establish whether it coincides with the direction of the midday line. ..

We use stellar time to determineWhat parts of the starry sky( constellations) will be visible above the horizon at one or another time of day and year. As already noted, at each particular moment in time, in the upper culmination are those stars for which a = s. Calculating the stellar time s, and determine the conditions for the visibility of stars and constellations.

Through the Bering Strait and along the waters of the Pacific Ocean, approximately along the meridian 180 ° from the north pole to the south there is a line of date change. Hence, a new day, a new day and a new year begin on our planet.