The term space is used to denote the entire Universe, i.e., the Earth and if§ atmosphere, the Moon, the Sun and the rest of the Solar System with its other planets and their satellites, and all the stars and galaxies spread over the infinite skies. Outer space refers to the entire space except the Earth and its atmosphere. The outer space begins where the Earth’s atmosphere ends, and it extends in all directions from above the atmosphere of the Earth.
Since the Universe is infinite and so is the outer space, our conventional units of measurement do not suit to measure the astronomical distances, such as distances between the Earth and the Sun, the Moon or other planets or distances between other heavenly bodies. Therefore, different units of measurement, such as the light year and the astronomical unit, have been evolved to measure distances in the space.
Light travels in vacuum at a speed of 186,250 miles (299,792.5 km) per second. The distance, thus, traveled by light in one year is called the light year which equals 5880 billion miles (9460 billion km). An astronomical unit is equal to the mean distance between the Sun and the Earth, which is 92,857,000 miles or 149,597,900 km. In other words, one light year has about 60,000 astronomical units. The astronomical unit is now used to measure the distances in the Solar System.