----------------------------------- viorel 19 Mar 2009 15:19 Echinoctiu si durata zilei ----------------------------------- Echinoctiul de primavara va avea loc pe data de 20 martie. Din cate stiu eu, la echinoctiu, ziua este egala cu noaptea. Totusi, azi 19 martie 2009, in Romania soarele rasare la ora 6:37 si apune la 18:43, deci ziua este deja mai lunga decat noaptea. Poate cineva sa-mi explice aceasta contradictie? Multumesc ----------------------------------- Cristian Monea 19 Mar 2009 16:54 ----------------------------------- "On a day of an Equinox, the centre of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, night and day being of roughly the same length. The word Equinox derives from the Latin words aequus (equal) and nox (night);in reality, the day is longer than the night at an Equinox. Commonly, the day is defined as the period when sunlight reaches the ground in the absence of local obstacles. From the Earth, the Sun appears as a disc rather than a single point of light, so when the centre of the Sun is below the horizon, its upper edge is visible. Furthermore, the atmosphere refracts light, so even when the upper limb of the Sun is below the horizon, its rays reach over the horizon to the ground. In sunrise/sunset tables, the assumed semidiameter (apparent radius) of the Sun is 16 minutes of arc and the atmospheric refraction is assumed to be 34 minutes of arc. Their combination means that when the upper limb of Sun is on the visible horizon, its centre is 50 minutes of arc below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer. These cumulative effects make the day about 14 minutes longer than the night at the Equator and longer still towards the Poles.The real equality of day and night only happens in places far enough from the equator to have a seasonal difference in day length of at least 7 minutes, actually occurring a few days towards the winter side of each Equinox. The date at which the time between sunset and sunrise crosses 12 hours, is known as the equilux. Because sunset and sunrise times vary with an observer's geographic location (longitude and latitude), the equilux likewise depends on location and does not exist for locations sufficiently close to the equator. The Equinox, however, is a precise moment in time which is common to all observers on Earth." Am citat de pe www.wikipedia.com :D http://en.wikipedia.org/wiki/Equinox ----------------------------------- Doru Dragan 19 Mar 2009 20:40 ----------------------------------- Aceasta contradictie poate fi explicata daca precizezi de unde ai luat timpii aia :) ----------------------------------- marius78 19 Mar 2009 21:59 ----------------------------------- Se produce cand Soarele ,in miscarea sa aparenta anuala ,trece prin punctul de intersectie a eclipticii cu ecuatorul ceresc ,ziua fiind egala cu noaptea in orice loc de pe Pamant.La echinoctiul de primavara ,in jurul datei de 21 martie .Soarele traverseaza ecuatorul trecand din emisfera sudica in cea nordica .La echinoctiul de toamna Soarele trece din emisfera nordica in cea sudica (in jurul datei de 23 sept). De la un an la altul (EC),nu se produce la aceeasi data datorita faptului ca anul calendaristic nu este egal cu cel tropic. Cele doua puncte de pe ecliptica in care se afla Soarele in momentul (EC),se numesc puncte echinoctiale ,fiind denumite punctul ''vernal'' si respectiv punctul ''autumnal''. Din cauza fenomenelor de precesie si nutatie ,punctele echinoctiale se deplaseaza pe ecliptica.Intrucat punctul vernal serveste la definirea origini sistemelor de coordonate ecuatoriale si ecliptice. ----------------------------------- Andi 19 Mar 2009 22:25 ----------------------------------- Echinocțiu, echinopțiu, oare cum este bine? Păi, trăim pe fundul oceanului aerian, deci avem refracție de care trebuie să ținem cont, nu? ----------------------------------- MariusP 19 Mar 2009 23:40 ----------------------------------- Pe langa refractia de care au amintit dxdx347 in citatul de pe Wikipedia si Andi, mai este o cauza a cresterii duratei zilei: deplasarea aparenta a Soarelui spre est, timp de circa 12 ore, de la rasarit pana la apus. Care ar avea o valoare de circa 0,5 grade sau aproximativ 2 minute de timp. Adaugate la cele doua refractii de la rasarit si apus, cred c-ar corespunde binisor cu diferenta dintre durata zilei semnalata de Viorel si 12 ore. ----------------------------------- Cristian Monea 21 Mar 2009 14:06 ----------------------------------- Totul este explicat clar în articolul acesta: http://www.jurnalcosmic.ro/?p=95 .