. ▁In ▁this ▁context ▁it ▁is ▁referred ▁to ▁as ▁sec ular ▁motion . ▁Sol ar ▁System ▁e ph emer ides ▁are ▁essential ▁for ▁the ▁navigation ▁of ▁space craft ▁and ▁for ▁all ▁kinds ▁of ▁space ▁observations ▁of ▁the ▁plan ets , ▁their ▁natural ▁sat ell ites , ▁stars ▁and ▁galaxies . ▁▁ ▁Most ▁of ▁the ▁known ▁perturb ations ▁to ▁motion ▁in ▁stable , ▁regular , ▁and ▁well - det erm ined ▁dynam ical ▁systems ▁tend ▁to ▁be ▁periodic ▁at ▁some ▁level , ▁but ▁in ▁many - body ▁systems , ▁cha otic ▁dynamics ▁result ▁in ▁some ▁effects ▁which ▁are ▁one - way ▁( for ▁example , ▁planet ary ▁migration ). ▁ ▁In ▁the ▁Sol ar ▁System ▁ ▁Sec ular ▁phen omena ▁create ▁variations ▁in ▁the ▁or bits ▁of ▁the ▁Moon ▁and ▁the ▁plan ets . ▁The ▁solar ▁emission ▁spectrum ▁and ▁the ▁solar ▁wind ▁are ▁under going ▁sec ular ▁tr ends ▁due ▁to ▁ ▁migration ▁through ▁the ▁gal actic ▁plane , ▁leading ▁to ▁effects ▁that ▁may ▁impact ▁on ▁climate ▁and ▁cause ▁ext inction ▁events . ▁ ▁Moon ▁ ▁The ▁sec ular ▁acceleration ▁of ▁the ▁Moon ▁depends ▁on ▁tid al ▁forces . ▁It ▁was ▁discovered ▁early ▁but ▁it ▁was ▁some ▁time ▁before ▁it ▁was ▁correctly ▁explained . ▁ ▁Earth ▁ ▁Depending ▁on ▁what ▁time ▁frames ▁are ▁considered , ▁perturb ations ▁can ▁appear ▁sec ular ▁even ▁if ▁they ▁are ▁actually ▁periodic . ▁An ▁example ▁of ▁this ▁is ▁the ▁pre cess ion ▁of ▁the ▁Earth ' s ▁axis ▁considered ▁over ▁the ▁time ▁frame ▁of ▁a ▁few ▁hundred ▁or ▁thousand