Determining the time of dawn and darkness, sunny day, standard time, maternity time, longitude of the day, civil twilight.

A sunny day is the time between two successive upper climaxes of the Sun. The climax is the passage of the Sun through the meridian of the observation point. The duration of true sunny days is inconsistent due to the uneven speed of the sun moving along the ecliptic and the inclination of the ecliptic to the celestial equator. 

Determining the time of dawn and darkness, sunny days, zone and maternity hours, longitude of the day, civil twilight.

The basis of the civil time account is the average sunny day. The time interval between two successive upper climaxes of the middle (imaginary) Sun, which would move precisely along the celestial equator with uniform speed. Average solar time is counted from midnight. The Moment of the Lower Culmination of the Middle Sun.

Standard time.

Standard Time is a time tracking system based on dividing the Earth in the meridional direction into 24 time zones. From zero to twenty third. For any point within the time zone, the same time is accepted – the average solar time of the middle meridian of the belt. Standard time in everyday life is called local. In astronomy, local time is considered the time for each point, depending on its longitude.

Belt and average solar times for points located west and east of the middle meridian will be different. The discrepancies reach the greatest magnitude at the borders of the belt. Time in neighboring zones differs by exactly 1 hour.

The boundaries of time zones on the seas and oceans, as well as in sparsely populated areas, are meridians that are 7.5 degrees east and west of the middle meridian of the belt. In habitable areas, time zone borders extend along state and administrative borders and natural boundaries (rivers, mountain ranges, etc.) located near the boundary meridians of the zone.

Daylight saving time.

Daylight saving time is the standard time increased by 1 hour. Adopted in the USSR by Decree of the Council of People’s Commissars of June 16, 1930. The clock hands in all zones were moved forward an hour. Some countries also have maternity hours that are 1 hour different from the standard time..

Longitude of the day.

Longitude of the day is the period of time from sunrise to sunset. Depends on the date (month and day) and latitude of the place. To determine the longitude of the day, special tables are published indicating the exact time of sunrise and sunset for various settlements.

Civil twilight.

Civil twilight – the period of daylight before sunrise and after sunset, when natural light allows you to perform orientation, target designation, firing and other actions in the same manner as during the day. The beginning (end) of civil twilight is considered the moment when the Sun goes down the horizon line at 6 degrees.

The duration of morning and evening twilight depends mainly on the latitude of the place and date, as well as on meteorological conditions and the nature of the terrain. With dense and low cloud cover, as well as in the forest and deep gorges, the duration of twilight is reduced. In mid-latitudes, the duration of both morning and evening twilight is approximately 40 minutes.

Determining the time of dawn and darkness by latitude and date.

The time of dawn and darkness is determined by the latitude of the place and date (month and day) using a graph. For latitude, you can take the digitization in degrees of the south or north side of the frame of the topographic map sheet on which the area of ​​action is located.

The time of dawn is set as follows. On the top of the graph, where the months and dates are signed, a point is found by the corresponding date. From this point, draw mentally or along a ruler a straight line parallel to vertical lines, until it intersects with a curve corresponding to the latitude of the place. If the digitization of the curve given on the graph does not coincide with the latitude of the place, then interpolate.

Graph for determining the time of dawn and darkness.

A horizontal line is drawn from the point of intersection of the date line with the curved line of latitude to the nearest side of the graph. And they determine the time of dawn. Hours by signature, minutes by eye. On the graph in the upper left part the dashed line shows the vertical line corresponding to the date of June 5th. The circle marks the point of intersection of the date line and the curve for latitude 55 degrees, from which a horizontal line is also drawn with a dotted line to the left side of the graph and a 3 hours 20 minutes count is obtained..

This is the time of dawn on June 5 at a latitude of 55 degrees. Similarly, at the bottom of the graph, the time of darkness is determined. The graph in the left corner shows the dashed line the reference time of darkness (22 hours 40 minutes) for a point with a latitude of 55 degrees on June 5.

According to the schedule, the average solar (maternity) time is determined. To switch to the local time, you must enter the correction in the received data. The absolute value of this correction is equal to the difference between the longitudes of the average meridian of the time zone and the meridian of this item in the hourly measure. If the point is located east of the middle meridian, the amendment is deducted. And if to the west – it is added. The longitude of a point is determined by the map, and the belt number and longitude of the average time zone meridian are determined by the table.

Example.

The longitude of the point is 35 degrees, the time of dawn, taken from the schedule, 3 hours 20 minutes. The amendment is defined as follows. The time zone number is 2, the longitude of the middle meridian of the belt is 30 degrees, the difference in longitudes of the middle meridian and the meridian of this item is 5 degrees (35-30). Correction in an hourly measure will be equal to 20 minutes (1 hour corresponds to 15 degrees). Thus, the estimated time of dawn is 3 hours (3 hours 20 minutes – 20 minutes).

Based on materials from the Handbook of Military Topography.
A. M. Govorukhin, A. M. Kuprin, A. N. Kovalenko, M. V. Gamezo.

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