In astronomy, light units are a convenient way to measure distances. light travels at a speed of approximately 3 ã 10 8 m/s which means that one light second is approximately 3 ã 10 8 m. below we list seven distances in light-units and nine physical sizes. match each distance to one size. light nanosecond light millenium light second light century light hour light minute light year across a medium-sized country across a child's limb between the earth and the moon between planets in the outer solar system across the known universe between planets in the inner solar system reach of human-made radio signals out into the universe across a small galaxy between stars in the center of our galaxy