Grinički meridijan
ISTORIJAT: Na internacionalnoj konferenciji u Vašingtonu 13. oktobra 1884. godine je određen početni meridijan za računanje vremena. Povod za održavanje pomenute konferencije bio je sve veći problem određivanja vremena u svetu i potreba za standardizacijom vremena. Samo na teritoriji SAD 1833. godine bilo je u upotrebi oko 300 različitih lokalnih vremena (tada je tačno vreme počelo da se prenosi telegrafom). Sve veća povezanost različitih delova sveta primorala je velike države da se dogovore oko standarda. Na konferenciji je učestvovalo 25 država i svaka od njih je vreme računala na različite načine. Griničko vreme (GMT) je tada već bilo u upotrebi u Velikoj Britaniji. Francuzi su se zalagali za ideju da početni meridijan prolazi kroz Kraljevsku opservatoriju u Parizu. Konferencija je trajala skoro deset dana i odlučeno je da početni nulti meridijan bude onaj koji prolazi kroz Griničku opservatoriju, blizu Londona. Ovaj Grinički meridijan izabran je kao početna tačka za određivanje časovnih zona i usaglašavanje vremena na celoj planeti Zemlji. Ubrzo, posle prihvatanja standarda od strane ovih 25 država, primenile su ga i ostale države sveta.
Podnevci (meridijani)
Podnevak ili meridijan je luk koji spaja Severni i Južni pol prostirući se pravcem sever-jug. Ovaj luk nema oblik polovine kružnice, zato što je Zemlja blago spljoštena na polovima i njen poprečni presek na pravcu sever-jug je u stvari elipsa. Podnevci su dobili naziv po tome što je Sunce tačno iznad ovog podnevka u podne. Za razliku od Ekvatora, koji se prirodno izdvaja među uporednicima, podnevci su svi jednaki, tako da je dogovorno izabran podnevak koji prolazi kroz opservatoriju u Griniču kao nulti. Grinički podnevak se koristi kao početni (Prime meridian) u određivanju geografske dužine i njegova geografska dužina je 0°. Ostali podnevci se prostiru istočno i zapadno od Griničkog podnevka. Za podnevke je karakteristično da svaki od njih zaklapa ugao od devedeset stepeni sa uporednicima na mestu ukrštanja. Podnevak koji se nalazi nasuprot Griničkom na 180° je upotrebljen za datumsku granicu, jer najvećim delom prolazi preko okeana. Datumska granica većim delom prati ovaj meridijan, osim u naseljenim područjima, gde je ona pomerena iz praktičnih razloga. Prilikom prelaska preko datumske granice datum se pomera, pa tako ko pređe sa istoka na zapad pomera kalendar za jedan dan unazad, a ko prelazi sa zapada na istok vraća kalendar za jedan dan.

Danas se vreme u celom svetu određuje prema časovnim zonama koje počinju od Griniča. Svet je podeljen na 24 časovne zone, širine 15o. Vreme u zonama koje se nalaze istočno od Griniča obeležava se pozitivno, a one koje su zapadno, negativno. Tako je na primer, Srbija u zoni GMT+1, Bugarska GMT+2, dok je Rio de Žaneiro (Brazil) GMT-3, a Aljaska GMT-9 itd.
ŠTA JE ZAPRAVO GRINIČKI MERIDIJAN?
Grinički meridijan (slika 2. i 3) je linija geografske dužine koja prolazi kroz Grinič u Engleskoj. On ima ugaonu meru od 0°. Ovaj meridijan i suprotni 180. meridijan, kroz koji prolazi Međunarodna datumska granica, deli istočnu i zapadnu polovinu. Za razliku od linija geografske širine, koje su definisane prema osi Zemlje (da su polovi 90°i ekvator 0°), grinički meridijan je proizvoljan i stav je međunarodnog dogovora. Polazeći od severne strane zemaljske kugle Grinički meridijan prolazi kroz osam zemalja:
 Velika Britanija (primedba: London je u istočnoj i u zapadnoj polovini, zbog blizine Griničkog meridijana)
 Francuska
 Španija
 Alžir
 Mali
 Burkina Faso
 Togo
 Gana



The Greenwich Meridian

HISTORY: At international conference in Washington on October 13, 1884 prime meridian for measuring time was established. The reason for holding the aforementioned conference was the growing concern about determining time in the world and the need for time standardization. In 1833, on the territory of the USA alone, about 300 different local times were in use (at the time, correct time began to be transmitted by telegraph). The increasing connections between different parts of the world compelled large states to agree on the standard. Twenty-five states attended the conference and each of them measured time differently. Greenwich Mean Time (GMT) was at the time already in use in Great Britain. The French advocated the idea that the prime meridian should pass through the Paris Observatory. The conference lasted for almost ten days and it was decided that the prime meridian should be the one that passes through Greenwich Observatory, near London. This, Greenwich Meridian was chosen as the starting point for determining time zones and synchronizing the time on the entire planet. Soon after accepting the standard by these 25 states, other states applied it as well.

Lines of longitude (meridians)

Line of longitude or meridian is an arc that connects the North and the South Pole in the north and south direction. This arc does not have the shape of a semicircle because the Earth is slightly flattened on the Poles and its cross section in the north-south direction is, in fact, elliptical.  Meridians are called like that because the sun is right above a given meridian at noon. In contrast to the Equator which naturally stands out among the parallels, meridians are all equal so that the meridian that passes through the Greenwich Observatory was chosen by agreement as the zero meridian. The Greenwich Meridian is used as Prime Meridian in determining longitude and its longitude is 0°. The other meridians stretch to the east and to the west of the Greenwich Meridian. It is characteristic of the meridians to form a 90° angle with parallels at the intersection. The meridian that is located opposite the Greenwich one at 180° is used as date line, since it largely passes through the ocean. The date line follows this meridian to a great extent except in populated areas where it is shifted for practical reasons. The date gets shifted in the process of crossing the date line, so that a person crossing the line from the east to the west should advance the calendar date by one day, whereas the person crossing the line from the west to the east should subtract one day.

Nowadays, time in the entire world is determined according to the time zones that start from Greenwich. The world is divided into 24 time zones, 15° wide. The time in those zones that are located to the east of Greenwich is marked positive and in those zones to the west of Greenwich as negative. So, for example, Serbia is in the zone GMT +1, Bulgaria GMT +2 whereas Rio de Janeiro (Brazil) GMT -3 and Alaska GMT -9, etc.

WHAT IS ACTUALLY THE GREENWICH MERIDIAN?

The Greenwich Meridian (images 2 and 3) is the line of longitude that passes through Greenwich in England. This meridian has longitudinal position of 0°. This meridian and its opposite, 180th meridian, the one the International Date Line passes through, separate the Eastern and Western Hemisphere.  In contrast to the lines of latitude that are defined according to the Earth’s rotational axis (the Poles 90° and the Equator 0°), the Greenwich Meridian is arbitrary and it represents an international agreement. Starting from the north of the Earth, the Greenwich Meridian passes through eight countries.

  • Great Britain (note: London is located in both Eastern and Western Hemisphere because of the vicinity of the Greenwich Meridian)
  • France
  • Spain
  • Algeria
  • Mali
  • Burkina Faso
  • Togo
  • Ghana