What does Light Travel Time (LTT) measure?

Light Travel Time (LTT) specifically focuses on the concept of how long it takes for light to travel from one point to another, particularly from distant celestial objects to the Earth. When we look at stars or galaxies, we are not seeing them as they are at this very moment, but rather as they were when their light began its journey towards us, which can take years or even millions of years depending on the distance of the object. For example, if a star is 5 light-years away, the light we see from it today actually left the star 5 years ago. This concept underlines the finite speed of light and helps astronomers gauge distances in space by understanding the time it takes for light to travel that distance.

The other options do not accurately capture this concept; for instance, the physical distance between stars is measured in light-years or parsecs, not in terms of the time it takes light to travel. Likewise, the time required for the Earth to orbit the Sun or the duration of a solar day refer to time periods associated with Earth’s movements rather than any aspect of light travel. Thus, LTT is fundamentally about the travel time of light from celestial objects to Earth, which is why this choice is the correct one.