We can measure the passage of time on earth by observing the rising and setting of the Sun and Moon or marking the passing of days, weeks, months, and years. In other words, we measure time based on the Earth’s rotation, and its orbit around the Sun.
Once you leave the Earth, this yardstick of measurement no longer exists. In space, there is no day or night. Therefore, the concept of weeks and years does not exist.
The distances between the stars are so vast that it boggles the imagination.
Our normal clocks and measuring devices are totally useless for measurements in space. So, astronomers have come up with a new unit to measure distances in space. It is called the light year.
The light year is a unit that expresses how far a ray of light travels in one year.
1 light year= 9.461 × 10¹² kilometres
Throughout the Universe, all light travels at exactly the same speed.
The main reason for using light years, however, is because the distances we deal with in space are immense. If we stick to kilometres, we quickly run into unwieldy numbers.