Light needs time to cross space. At a distance of one astronomical unit, its journey takes about 8 minutes and 19 seconds. Earth's distance from the Sun changes through its orbit, so the actual travel time varies.
Calculate the travel time
Use the relation time = distance ÷ speed. One astronomical unit is 149,597,870,700 metres. Divide it by the speed of light in vacuum, 299,792,458 metres per second, and the result is about 499 seconds.
Eight minutes account for 480 of those seconds. The remaining 19 seconds complete the picture. This is a calculation at a defined distance, rather than a measurement of today's Earth–Sun separation.
What a light-year measures
The same idea works in reverse: multiply light's speed by a chosen time interval and you get a distance. A light-year describes how far light travels in a year. Looking at a distant object means receiving light that left it earlier.
Our diagram compresses the distance and exaggerates the sizes of the bodies for readability. It is a scale explanation, not a photograph or a view of their present positions.
Sources & model notes
Diagrams are explanatory illustrations. EXOVAS is independent and is not affiliated with NASA or JPL.