Light Travel Time Calculator

This is the simplest relation in astronomy, time equals distance divided by c, but every constant in it is now defined rather than measured. Since the 17th General Conference on Weights and Measures in 1983, the speed of light in vacuum is fixed at exactly 299,792,458 metres per second and the metre is derived from it. The International Astronomical Union fixed the astronomical unit at exactly 149,597,870,700 metres in 2012, which makes the light-time for unit distance 499.005 seconds, so sunlight reaches Earth in 8 minutes 19 seconds on average. A light-year uses the Julian year of 365.25 days and equals 9,460,730,472,580,800 metres, and the parsec was redefined in 2015 as exactly 648,000 divided by pi astronomical units, about 3.0857 x 10^16 metres. Real mission numbers show why the quantity matters operationally. Light reaches the Moon in 1.28 seconds, so lunar teleoperation has a 2.6 second round trip. Mars ranges from roughly 0.5 to 2.6 AU from Earth, giving one-way delays of about 4 to 22 minutes and making live rover driving impossible; entry, descent and landing must run autonomously. Voyager 1, beyond 165 AU, is more than 23 hours away one way. Three caveats apply. The calculation assumes a straight-line geometric distance through vacuum, whereas light slows in media by the refractive index - about 1.0003 in air, 1.33 in water and 2.42 in diamond. It ignores the small Shapiro delay near massive bodies. And at cosmological distances, light-travel time is not the same as comoving distance, because space expands while the light is in transit.

Light was widely assumed to travel instantaneously until Ole Romer, working at the Paris Observatory, noticed in 1676 that the eclipses of Jupiter's moon Io ran late when Earth was far from Jupiter and early when it was near. He interpreted the pattern as a finite crossing time for the width of Earth's orbit; Christiaan Huygens converted the result in 1678 into a speed of roughly 212,000 kilometres per second, low but the right order of magnitude. James Bradley's 1728 discovery of stellar aberration, the slight tilt of starlight caused by Earth's orbital motion, gave an independent value near 301,000 kilometres per second. Terrestrial measurement began with Hippolyte Fizeau's toothed-wheel experiment in 1849, followed by Leon Foucault's rotating-mirror apparatus, which by 1862 gave about 298,000 kilometres per second. Albert Michelson refined the rotating-mirror method for four decades, culminating in measurements over a surveyed baseline between Mount Wilson and Mount San Antonio in the 1920s. His 1887 experiment with Edward Morley, finding no ether drift, helped clear the ground for special relativity in 1905. Laser frequency and wavelength measurements at the National Bureau of Standards in the early 1970s cut the uncertainty to about one metre per second, at which point further refinement was pointless and the constant was fixed by definition. The habit of quoting stellar distances as light-travel times dates from Bessel's 1838 parallax for 61 Cygni, which he described as roughly ten years of light travel.