QLR

How did we learn how large the universe is? We could see the Sun, the Moon and the stars long before we knew how far away they were. Looking at the sky did not tell us whether a faint object was small and nearby or enormous and far away. Before we could measure the heavens, we needed a scale here on Earth. In the third century BCE, Eratosthenes estimated Earth’s circumference by comparing the Sun’s elevation at two locations. A difference in shadows, combined with the distance between the locations, allowed him to measure the planet without travelling around it. Ancient astronomers also used geometry to estimate celestial distances. Aristarchus investigated the relative sizes and distances of the Sun and Moon. Hipparchus later used eclipse observations to constrain the Moon’s distance. Their measurements were imperfect, but objects beyond Earth were becoming things that could be measured. In 1672, observations of Mars by Giovanni Domenico Cassini, Jean Richer and their colleagues helped establish the scale of the Solar System. From widely separated locations on Earth, Mars appeared in slightly different positions against the background stars. That small shift allowed them to calculate its distance. Combined with the relative sizes of planetary orbits, it gave an Earth–Sun distance of about 140 million kilometres, close to today’s value of 149.6 million kilometres. But the stars were much farther away. In 1838, Friedrich Bessel published a parallax measurement placing the star 61 Cygni roughly 10 light-years from Earth. Instead of observing from two locations on Earth, he used observations made from different positions along Earth’s orbit. Even that enormous baseline produced only a tiny apparent shift. Then Henrietta Swan Leavitt found a way to reach farther. In work published in 1908 and 1912, she identified a relationship between the pulsation periods and brightnesses of Cepheid variable stars. Once calibrated, this relationship allowed astronomers to estimate a star’s true luminosity, compare it with its apparent brightness and calculate its distance. In 1918, Harlow Shapley used globular clusters to investigate the Milky Way’s extent. His size estimate was too large, but his work showed that the Sun lay far from the Galaxy’s centre. A few years later, Edwin Hubble identified Cepheids in Andromeda. Their distance placed Andromeda beyond the Milky Way. The faint patch people had observed for centuries was a separate galaxy. By 1925, the evidence was being formally presented. The Milky Way was no longer the limit of the known stellar universe. As astronomers measured more galaxies, they began mapping how galaxies themselves were arranged. In 1989, Margaret Geller and John Huchra reported the Great Wall, a sheet of galaxies extending hundreds of millions of light-years. The Sloan Great Wall, announced in 2003, was reported to stretch approximately 1.37 billion light-years. These structures are parts of a cosmic network of galaxies, clusters, filaments and vast comparatively empty regions. Their boundaries depend partly on how we identify and connect their components. Calling one the “largest object in the universe” is therefore not as straightforward as comparing the sizes of planets. Today, observations combined with models of cosmic expansion put the observable universe at roughly 93 billion light-years across. That is the region from which signals have had time to reach us. It is not a measurement of the entire universe, whose total size remains unknown. We began by comparing shadows on Earth. We now measure the arrangement of galaxies billions of light-years away. And we still do not know how large the whole universe is.

??

Scammers you won a price

If I’m writing the names of my true followers Can I add you? A. YES B. NO

"Can't we all just get together and work towards an exciting future?" - Elon Musk