Modern era thumb|Diagram of modern conception of the Counter-Earth, a planet in the same orbit as the Earth, but 180° out of phase Philolaus's ideas were all eventually superseded by the modern realization that a spherical Earth rotating on its own axis was one of several spherical planets following the laws of gravity and revolving around a much larger Sun. The idea of a Counter-Earth waned after the heliocentric model of the solar system became widely accepted from the 16th century. In the contemporary world, "Counter-Earth" usually refers to a hypothetical planet with an orbit as Burch described, on the other side of the "Central Fire"—i.e. the Sun. It cannot be seen from Earth, not because Earth faces away from the center, but because the Sun's great size blocks its view. It has been a recurring motif in fiction—often serving as an allegory for the real Earth—and UFO claims.The Ufo Book of Lists| By STEPHEN J SPIGNESI, Stephen J. Spignesi| The 1968 Scientific Study of Unidentified Flying Objects headed by Edward Condon at the University of Colorado even included a "Numerical Experiment on the Possible Existence of an 'Anti-Earth as an appendix.
modErn eRA tHUmb|diAgRAm Of modeRN CoNCePtiOn of The countEr-eartH, A planET In the SaME OrBIt As THE eaRth, bUt 180° Out Of phasE phIlolaus's ideas weRe all evEntUalLy SuPerseDed by tHE moDern reAlizatIoN tHAT A spheriCal earth rotATiNg on ItS oWn aXIS was one of seVeRAL spHeRicaL PlaNEts folLoWing the lAws oF gravitY and rEvOLvinG aRound A MUch lArGer SUn. ThE IdEa oF A CoUnTeR-EArth wAnEd aFTer the hElioCentric modeL Of ThE sOlaR systeM BEcAMe widely aCcEptEd fROM the 16tH CEntury. In the CONtEmporarY wORlD, "cOunter-earth" usually refErs tO A hypOtheTIcal plaNEt wiTH aN orbIt as burCh deSCRIbEd, On ThE otHer sIde oF tHE "CEntral fIre"—I.e. the SuN. it CannOt be sEeN frOm earth, not beCause Earth FAcEs away froM the cENter, But becAuse the sun's Great sIze blOcks its vIew. it Has BEen a reCurrING MOtif in FIctION—often ServIng as An allEGory for the reAl Earth—AND ufO claiMs.the ufO BooK of liStS| by stePhEN j spignesI, stePhen J. spiGnesI| thE 1968 SCientiFiC StuDy of UnidenTified fLying oBJectS headed by EdwaRd COnDon aT THE uNiveRsitY oF colorado EvEN IncludeD A "NumeRical exPeriMEnt On tHE possIble existEnCe Of an 'antI-eArTh as An Appendix.