Nothing you can see is happening now. Light is fast but finite, so every image is a delivery from the past — a second late from the Moon, eight minutes from the Sun, twenty-six thousand years from the centre of the galaxy. Put a distance in below and the calculator will tell you how old the picture is.
Light takes
8 min 19 s
to cross 1.00 AU. Anything you see at that distance is that far into the past.
149,597,871 km ÷ 299,792.458 km/s = 499.00 s
Things this site has written about
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The Kordylewski clouds sit at the same distance, at the Earth–Moon L4 and L5 points. Read it
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Parker Solar Probe flies far inside this; its telemetry is correspondingly fresher. Read it
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BepiColombo took seven years to cover a distance light crosses in four minutes. Read it
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The reason a rover cannot be driven live: the round trip is over six minutes at best. Read it
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Every command sent to an instrument at Jupiter is half an hour old on arrival. Read it
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The nearest single star to the Sun, and the super-Earth found around it. Read it
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The matter watched falling towards the galactic centre fell before agriculture existed. Read it
The speed of light in vacuum is 299,792.458 km/s — not a measurement but a definition, since 1983 the metre has been defined from it. Distances to planets are given at closest approach and change through the year; distances to stars are the published parallax figures and carry their own uncertainty. Nothing you type here leaves your browser.
Why the speed is exact and the distance is not
Since 1983 the metre has been defined as the distance light travels in 1/299,792,458 of a second. That makes 299,792.458 km/s not a measurement with an error bar but a definition — the uncertainty moved out of the speed and into the ruler.
The distances are where the uncertainty actually lives. A planet’s distance changes continuously as both orbits move, so the figures above are closest approach and are labelled as such; at conjunction Mars is more than six times further away. Stellar distances come from parallax — the tiny apparent shift of a star as the Earth crosses its orbit — and every one carries an error bar that grows with distance. The 26,000 light years to Sagittarius A* is a round number standing in for a measurement that is still being refined.
What the delay actually costs
- Nobody drives a Mars rover. At best the round trip is a little over six minutes, and at conjunction it is more than forty. Rovers are given instructions for the day and left to execute them.
- A spacecraft at Jupiter is on its own. Half an hour each way means any command written in response to a problem arrives long after the problem has resolved itself, one way or the other.
- Deep-sky astronomy is history, not news. When a telescope watches matter fall towards the black hole at the galactic centre, it is watching something that finished before anyone farmed anything.
The units, and when to use which
- Kilometres — useful out to about the Moon, then the zeros take over.
- Astronomical units (AU) — one AU is the Earth–Sun distance, 149,597,870.7 km, also exact by definition since 2012. The natural ruler for the Solar System.
- Light years — 9.46 trillion km. Not a time despite the name; it is the distance light covers in a Julian year.
- Parsecs — 3.26 light years, and the unit astronomers actually use, because it falls straight out of the parallax measurement rather than out of a conversion.
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