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A light-year is the distance light travels in one Julian year, around 9461 billion kilometres, 5879 billion miles, or 0.3066 parsecs. In round figures, a light year is nearly 10 trillion kilometres or nearly 6 trillion miles. Proxima Centauri, the closest star to Earth after the Sun, is around 4.2 light-years away. [89]
By timing the eclipses of Jupiter's moon Io, Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth's orbit around the Sun. [1] Using modern orbits, this would imply a speed of light of 226,663 kilometres per second, [2] 24.4% lower than the true value of 299,792 km/s. [3]
1810 – François Arago observes that the speed of light of stars – measured with stellar aberration – may be independent of the relative motion of stars and the Earth; or at least, no differences are observable with a naked eye. [4] 1818 – Augustin-Jean Fresnel proposes his model of partial aether dragging to explain Arago’s finding. [5]
The Earth and Moon are very likely destroyed by falling into the Sun, just before the Sun reaches the top of its red giant phase. [115] [note 3] Before the final collision, the Moon possibly spirals below Earth's Roche limit, breaking into a ring of debris, most of which falls to the Earth's surface. [117]
This is equivalent to a linear increase of at least 10 26 times in every spatial dimension—equivalent to an object 1 nanometre (10 −9 m, about half the width of a molecule of DNA) in length, expanding to one approximately 10.6 light-years (100 trillion kilometres) long in a tiny fraction of a second.
For example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is further away. [9]
Visible range or light spans 380 to 700 nm. [24] As the name suggests, this range is visible to the naked eye. It is also the strongest output range of the Sun's total irradiance spectrum. Infrared range that spans 700 nm to 1,000,000 nm (1 mm). It comprises an important part of the electromagnetic radiation that reaches Earth.
The most distant space probe, Voyager 1, was about 18 light-hours (130 au,19.4 billion km, 12.1 billion mi) away from the Earth as of October 2014. [29] It will take about 17 500 years to reach one light-year at its current speed of about 17 km/s (38 000 mph, 61 200 km/h) relative to the Sun.