He wasn’t seriously positing the idea of lightyears as a unit. In his findings he mentioned that light takes 10.3 years to travel from 61 Cygni to Earth. The germ of the concept originated with Friedrich Bessel, who in 1838 made the first successful measurement of the distance to a star outside our Solar System, 61 Cygni. If you want to be precise, the IAU regards a year as 365.25 days, making a lightyear 9,460,730,472,580,800m. Credit: NASA and ESAĪ common method for measuring distance in space is to measure how far light travels in one year: known as a lightyear, which is around 9.5 trillion km. When we observe it, we are effectively looking back in time.
Lenticular galaxy NGC 4993 is about 130 million lightyears from Earth, meaning its light takes 130 million years to reach us. It wasn’t until 1695 that Christiaan Huygens made the first close guess of 24,000 Earth radii (152,904,000km) though some science historians dismiss his calculations as more luck than judgement, preferring to cite Jean Richer and Giovanni Domenico Cassini’s rigorously calculated 22,000 Earth radii (140,162,000km) as the first scientifically plausible estimate (despite the fact they were further from the mark than Huygens).
Not bad for someone who lived 2,000 years before the telescope was invented. One Astronomical Unit (AU) is equal to the radius of Earth’s orbit around the Sun: or, to be more precise, the average radius, since Earth’s orbit is elliptical.Īn AU is defined as 149,597,870,700m or 93 million miles, a value officially set by the International Astronomical Union (IAU) in 2012.Īstronomers have been trying to calculate the distance from Earth to the Sun ever since, in the 3rd century BC, Archimedes estimated it to be around 10,000 times Earth’s radius, or 63,710,000km – so he was nearly halfway there. The average distance from Earth to the Sun is 1 astronomical unit.
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