At certain points on Mercury's surface, an observer would be able to see the Sun peek up a little more than two-thirds of the way over the horizon, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four Earth days before perihelion, Mercury's angular orbital velocity equals its angular rotational velocity so that the Sun's apparent motion ceases; closer to perihelion, Mercury's angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde direction. Four Earth days after perihelion, the Sun's normal apparent motion resumes. A similar effect would have occurred if Mercury had been in synchronous rotation: the alternating gain and loss of rotation over a revolution would have caused a libration of 23.65° in longitude.
at cERtaIN pOInTs on mErCurY'S surFAce, aN obsErvEr WOUld BE abLe tO seE thE sUn pEEk up a litTle moRE than two-thirds of tHE Way OvER tHe horizOn, then reVerse aNd sEt BefOrE RIsIng agaIN, AlL within tHe saME MercuriAn day. tHis iS beCAUse appROximately Four eaRtH DAys befOre perIhElIoN, mercury's aNgulAr ORbItAL vELocity equals iTS anguLAr roTatioNal VElOcity So that tHe sUn's aPparEnt mOTioN ceases; cLOSeR tO peRihElion, mErCurY's aNgulAr oRbItal VELOcity Then eXceeds ThE aNGuLaR RotatiOnaL velOcitY. thus, tO a hypotHetiCal obserVer On mErcury, the sUn appearS To mOve in a retRogradE dIrectIOn. Four earth DAys aFtEr periHElIoN, the sUN'S NormAL appArENt mOtIoN Resumes. a SimiLar eFfect WoUlD have ocCurreD iF meRcurY hAD bEen in SYNChroNouS rotaTIon: ThE AlTERNating gain anD lOss Of rotatiOn over a REvoLutIon would HaVe cauSed a LIbration of 23.65° in lONgItUdE.