Indian Standard Time (IST) kya hai?

Samay longitude pe kyun nirbhar hai

Prithvi apni dhuri par west se east ghoomti hai. Globe ke paas ek lamp rakho aur globe ko dheere-dheere east ki taraf ghumao: kuch jagahon par subah ki roshni hai, kuch par dopahar, aur kuch par shaam ya raat. Jab ek desh mein naashte ka samay hai, doosre mein dopahar ka khaana hai, aur teesre mein log gehri neend mein hain. Isliye kisi jagah ka longitude naapne se uske samay ka bhi pata chalta hai.

Prithvi lagbhag 24 ghante mein ek poora chakkar, 360°, lagaati hai. Yaani:

• 360° ÷ 24 = har ghante 15°, aur
• 60 minute ÷ 15 = har 1° longitude par 4 minute.

Local time

Greenwich se har 15° par meridians lagao. East ki taraf har meridian local time mein ek ghanta jodta hai; west ki taraf har meridian ek ghanta ghataata hai.

Agar Greenwich mein dopahar 12 baje hain, to 15°E par 1 pm, 30°E par 2 pm aur 45°E par 3 pm hai. West mein ulta hai: 15°W par 11 am, 30°W par 10 am aur 45°W par 9 am.

Do jagahon ki tulna ke liye unke longitude ke fark ko 4 minute se guna karo. East waali jagah aage hai.

Udaharan. Do dost shaam ko phone par baat karte hain, ek Tinsukia (Assam) mein aur doosra Porbandar (Gujarat) mein. Tinsukia waala kehta hai ki suraj doob gaya aur andhera ho gaya; Porbandar waale ke yahan abhi poori roshni hai. Unhe lagbhag 30° door maano, to local time ka fark 30 × 4 = 120 minute, yaani 2 ghante hai. Suraj door east mein Assam par bahut pehle pahunchta hai. (Extra: unke asli longitudes, lagbhag 95.4°E aur 69.6°E, kareeb 26° door hain: lagbhag 1 ghanta 43 minute ka fark.)

Standard time aur IST

Har meridian ka apna local time hai, to agar ek desh sab use kare to uljhan hogi: train, office aur phone calls ek samay par sahmat nahi ho paayenge. Isliye zyaadatar desh ek standard time apnaate hain, jo desh se guzarne waale ek meridian par aadhaarit hota hai.

India ka standard time, Indian Standard Time (IST), Greenwich ke local time se 5 ghante 30 minute, yaani 5.5 ghante, aage hai; Greenwich ke time ko Greenwich Mean Time (GMT) kehte hain. IST 82°30′E meridian ka local time hai, jo Uttar Pradesh mein Mirzapur ke paas se guzarta hai: 82.5 × 4 = 330 minute, yaani 5 ghante 30 minute.

India lagbhag 68°E se 97°E tak faila hai, kareeb 29° longitude, jo lagbhag 2 ghante ka local time hai. Jab har jagah ghadi dopahar 12 baje IST dikhaati hai, tab Gujarat ke door west mein suraj ke hisaab se lagbhag 11 baje hote hain, aur Arunachal Pradesh ke door east mein lagbhag 1 baja. Assam mein local time IST se aage hai, isliye wahan suraj jaldi ugta aur doobta hai.

Duniya ke time zones

Standard times ko time zones mein rakha jaata hai, jo mote taur par 15° ki pattiyon ke saath chalte hain. Par inke beech ki lines seedhi nahi hain: ve antarrashtriya seemaaon ke saath mudti hain taaki har desh apna standard time rakh sake. Time zone map par +5:30 (India) jaisa number GMT mein joda jaata hai, aur minus waala number GMT se ghataaya jaata hai.

Har desh ka ek hi standard time nahi hota. Russia, Canada aur USA ek ke liye bahut bade hain. USA mein chhe time zones hain aur Russia mein gyaarah, isliye Russia ko east se west paar karte hue ghadi 10 baar badalni padegi.

International Date Line

Prime Meridian ke saamne, lagbhag 180° longitude par, International Date Line hai. +12 aur −12 time zones isi par milte hain. Ise ship ya plane se paar karne par taareekh badalti hai:

• east ki taraf jaate hue ek din ghatao (Monday, Sunday ban jaata hai);
• west ki taraf jaate hue ek din jodo (Sunday, Monday ban jaata hai).

Ye line sirf ‘lagbhag’ 180° par hai kyunki ye kahin-kahin mudti hai taaki koi desh do alag dinon mein na bante. Latitude aur longitude ke liye latitude aur longitude kya hai dekho; har textbook sawaal ke liye Chapter 1 NCERT solutions dekho.