ic ▁sec ular ▁variation ▁refers ▁to ▁changes ▁in ▁the ▁Earth ' s ▁magnetic ▁field . ▁The ▁field ▁has ▁variations ▁on ▁time ▁scales ▁from ▁mill iseconds ▁to ▁millions ▁of ▁years , ▁but ▁rapid ▁changes ▁mostly ▁come ▁from ▁curr ents ▁in ▁the ▁ion os phere ▁and ▁magnet os phere . ▁The ▁sec ular ▁variation ▁is ▁the ▁changes ▁over ▁periods ▁of ▁a ▁year ▁or ▁more , ▁reflect ing ▁changes ▁in ▁the ▁Earth ' s ▁core . ▁Ph en omena ▁associated ▁with ▁this ▁sec ular ▁variation ▁include ▁geom agnet ic ▁j erk , ▁west ward ▁dr ift ▁and ▁geom agnet ic ▁revers als . ▁ ▁Bi ological ▁anth rop ology ▁A ▁sec ular ▁tr end ▁has ▁been ▁observed ▁in ▁the ▁age ▁of ▁on set ▁of ▁pub ert y ▁( men ar che / first ▁men stru ation ▁and ▁beginning ▁of ▁breast ▁development ) ▁of ▁girls ▁around ▁the ▁world ▁beginning ▁roughly ▁ 4 ▁months ▁earlier ▁each ▁de cade . ▁This ▁is ▁largely ▁believed ▁to ▁be ▁caused ▁by ▁nut r itional ▁changes ▁in ▁children ▁over ▁time . ▁ ▁References ▁ ▁Category : Ge om agnet ism ▁Category : Time ▁series <0x0A> </s> ▁Den ise ▁N icole ▁White ▁( born ▁May ▁ 2 2 , ▁ 1 9 7 5 ), ▁known ▁profession ally ▁ ▁as ▁A very S un sh ine , ▁is ▁an ▁American ▁singer , ▁song writer ▁and ▁pian ist . ▁ ▁Early ▁life ▁and ▁education ▁ ▁White ▁was ▁born ▁in ▁Ch ester , ▁Pennsylvania , ▁to ▁Ruth ▁Ele an or ▁White ▁and ▁Ir ving ▁Cy ril