As­tro­physi­cist Dr Gemma Lavender from All About Space mag­a­zine ex­plains the sci­ence be­hind Galileo’s dis­cov­er­ies


What was geo­cen­trism?

This was a be­lief, as early as the 4th cen­tury, that the Earth was at the cen­tre of the So­lar Sys­tem, while the Moon, Sun and planets or­bited around our planet. The idea also ex­tended fur­ther out­side our so­lar neigh­bour­hood, with an­cient civil­i­sa­tions also think­ing that the stars or­bited the Earth on cir­cu­lar (not el­lip­ti­cal!) paths.

In the geo­cen­tric model, the Earth was pic­tured as a sphere at the cen­tre of the cos­mos that doesn’t move – and doesn’t even ro­tate on its axis. The sys­tem was de­vised by Greco-ro­man as­tronomer and math­e­ma­ti­cian Claudius Ptolemy who, along­side Greek philoso­pher Aris­to­tle, was seen as an author­ity on mat­ters that were en­com­passed by as­tron­omy and the nat­u­ral sci­ences.

Where did this the­ory orig­i­nate from?

The idea came about from ob­ser­va­tions of the mo­tions of the Sun dur­ing the day and the stars and planets at night. If you watch these ce­les­tial bodies mak­ing their way across Earth’s sky, it ap­pears as if they’re or­bit­ing us. We also know our planet to be spin­ning on its axis and mak­ing its way around the Sun on its or­bit. Stand­ing on its sur­face, though – and from our Earth-bound per­spec­tive – it feels like it’s sta­tion­ary. That’s how An­cient Greek, Ro­man and Me­dieval philoso­phers, led by Aris­to­tle and Plato, saw our place in the uni­verse. At the time of their stud­ies of the geo­cen­tric model, Aris­to­tle and Plato had also worked out that the Earth is a sphere.

How does he­lio­cen­trism work?

He­lio­cen­trism is the model that we use to­day and ex­plains how Earth and other planets in our So­lar Sys­tem or­bit the Sun. The the­ory was ini­tially pro­posed by Pol­ish as­tronomer Ni­co­laus Coper­ni­cus, who also be­lieved that the uni­verse was in or­bit around our near­est star. He had been made aware of the ris­ing prob­lems brought about by the geo­cen­tric model; it was un­able to make ac­cu­rate pre­dic­tions dur­ing stud­ies of the cos­mos. Coper­ni­cus pro­posed, just like the geo­cen­tric model, that or­bit­ing bodies moved around the Sun on cir­cu­lar or­bits. Of course, to­day we know these or­bits to be el­lip­ti­cal.

Why was this idea so con­tro­ver­sial at the time?

By mov­ing the Sun to the cen­tre of the model, dis­plac­ing the Earth, we – God’s creations – were no longer at the cen­tre of the uni­verse. Coper­ni­cus knew that this would greatly up­set the Church and so he didn’t pub­lish his the­ory un­til near the end of his life.

Later, in 1610, Ital­ian as­tronomer Galileo Galilei ob­served the phases of Venus and the moons of Jupiter us­ing a te­le­scope. He used his find­ings to pro­mote Coper­ni­cus’ he­lio­cen­tric model. Galileo’s dis­cov­er­ies were met with op­po­si­tion from the Catholic Church and as a re­sult, many ed­u­cated peo­ple still be­lieved that the Earth was at the cen­tre of the uni­verse. Books, teach­ings and the de­fend­ing of the he­lio­cen­tric the­ory were banned.

The new is­sue of All About Space is out now, priced £4.99. For more in­for­ma­tion, visit my­favouritemagazines.co.uk.

An early illustration of Coper­ni­can the­ory shows the Sun at its heart

Galileo en­joys a night of stargaz­ing from his pi­o­neer­ing te­le­scope, circa 1620

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