Knowledge of
Earth's location in the universe has been shaped by 400 years of telescopic observations, and has expanded radically in the last century. Initially, Earth was believed to be the
center of the universe, which consisted only of those planets visible with the
naked eye and an outlying sphere of
fixed stars. After the acceptance of the
heliocentric model in the 17th century, observations by
William Herschel and others showed that Earth's Sun lay within a vast, disc-shaped
galaxy of stars, later revealed to be suns like our own. By the 20th century, observations of
spiral nebulae revealed that our galaxy was only one of billions in an
expanding universe, grouped into clusters and
superclusters. By the 21st century, the
overall structure of the visible universe was becoming clearer, with superclusters forming into a vast web of
filaments and
voids. Superclusters, filaments and voids are likely the largest coherent structures that exist in the Universe. At still larger scales (over 1000 megaparsecs)<sup class="reference" id="ref_Ee">
[e]</sup> the Universe becomes homogeneous meaning that all its parts have on average the same density, composition and structure.<sup id="cite_ref-Kirshner_0-0" class="reference">
[1
</sup>Since there is believed to be no "center" or "edge" of the universe, there is no particular reference point with which to plot the overall location of the Earth in the universe.<sup id="cite_ref-Klaus2002_1-0" class="reference">
[2]</sup> The Earth is at the center of the
observable universe because
its observability is determined by its distance from Earth. Reference can be made to the Earth's position with respect to specific structures, which exist at various scales. It is still undetermined whether the universe is
infinite, and there is speculation that our universe might only be one of countless trillions within a larger
multiverse, itself contained within the
omniverse
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