▁the ▁earth ▁or ▁per pend icular ▁to ▁it . ▁It ▁would ▁have ▁fully ▁conform ed ▁to ▁rules ▁prev ail ing ▁in ▁a ▁Gal ile an ▁in var iance ▁system ▁of ▁coordinates , ▁also ▁called ▁Newton ian ▁in ert ial ▁system , ▁which ▁apply ▁to ▁moving ▁material ▁particles . ▁▁ ▁In ▁the ▁ 1 8 8 7 ▁paper , ▁written ▁with ▁Edward ▁Mor ley , ▁Michel son ▁am ended ▁the ▁route ▁of ▁the ▁per pend icular ▁light ▁from ▁ab 1 ▁to ▁ab : ▁“ It ▁may ▁be ▁mentioned ▁here ▁that ▁the ▁error ▁was ▁pointed ▁out ▁to ▁the ▁author ▁of ▁the ▁former ▁paper ▁by ▁M . A . ▁Pot ier , ▁of ▁Paris , ▁in ▁the ▁winter ▁of ▁ 1 8 8 1 .” ▁When ▁the ▁instrument ▁on ▁the ▁moving ▁earth ▁is ▁observed ▁from ▁a ▁station ary ▁point ▁outside ▁earth , ▁the ▁mirror ▁at ▁b 1 ▁already ▁moved ▁to ▁b ▁while ▁light ▁was ▁travel ing ▁there ▁from ▁a . ▁“ This ▁meant ▁that ▁Michel son ▁had ▁over est im ated ▁by ▁a ▁factor ▁of ▁two ▁the ▁fr inge ▁sh ifts ▁originally ▁expected .” ▁The ▁mirror ▁at ▁c ▁also ▁moved ▁forward ▁at ▁the ▁same ▁time , ▁but ▁this ▁was ▁not ▁taken ▁into ▁account . ▁It ▁thus ▁seemed ▁that ▁light ▁tra ve led ▁different ▁distances ▁at ▁the ▁same ▁time , ▁which ▁led ▁George ▁Fitz G er ald ▁and ▁Hend rik ▁Lor ent z ▁( L or ent z ▁ ether ▁theory ) ▁to ▁post ulate ▁that ▁distances ▁short ened ▁and ▁time ▁dil ated ▁in ▁direction ▁of ▁motion