A light-day is the distance light travels through empty space in one day: about 25.9 billion kilometers, or about 173 times the distance from Earth to the Sun. Like the better-known light-year, it is a unit of length, not of time. Distances given in light-time also say how long light, or a radio signal, takes to cross them.

Definition

A light-day is built from two exact values. Light in a vacuum travels at exactly 299,792,458 meters per second, and the meter itself is defined from this speed: it is the distance light travels in 1/299,792,458 of a second. The International Astronomical Union (IAU), which sets astronomy's units, takes the day to be exactly 86,400 seconds.

Multiplying the two gives exactly 25,902,068,371.2 kilometers. That is about 173 astronomical units (au), where 1 au is exactly 149,597,870,700 meters, close to Earth's average distance from the Sun. The IAU defines the light-year as the distance light travels in a Julian year of 365.25 days, so a light-year is exactly 365.25 light-days.

Size comparisons

Light crosses the average distance from Earth to the Moon in about 1.3 seconds, so a light-day is about 67,000 times that distance.
A light-day is about 173 times the distance from Earth to the Sun, which sunlight crosses in about 8 minutes and 19 seconds.
It is about 5.8 times the distance from the Sun to Neptune, the outermost planet, which orbits about 30 au out.
At the farthest point of its orbit, Comet Hale-Bopp will be about two light-days from the Sun.
A car driving nonstop at 100 kilometers per hour (62 miles per hour) would need about 30,000 years to cover one light-day.

Voyager 1 at one light-day

NASA expects its Voyager 1 space probe, launched in 1977, to be one light-day from Earth on November 18, 2026. It will be the first object made by people to get that far. The trip will have taken 49 years, two months and 11 days.

At that distance, a radio command from Earth takes a full day to reach the spacecraft, and the reply takes another day to come back. Its twin, Voyager 2, took a different route to visit Uranus and Neptune, and is expected to reach one light-day from Earth in November 2035.

Light as a measuring stick

Nothing travels faster than light, which is why light-time works as a scale for the huge distances of space. The same holds for radio waves, which travel at the same speed. A radio signal between Earth and Mars takes between 3 and 22 minutes, depending on where the two planets are in their orbits.

Astronomers use a whole family of these units. The Moon is about 1.3 light-seconds away, and the Sun about 8.3 light-minutes. Proxima Centauri, the nearest star beyond the Sun, is about 4.24 light-years away, or about 1,550 light-days.

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