The astronomical unit is the mean distance from the Earth to the Sun. The mean is the average. It is about 92,955,807.3 miles. The International Astronomical Union recommends the abbreviation au. It is used most commonly for distances between planets.
Venus and Mars are closer to the Sun than the Earth, so their distance is less than one astronomical unit. Venus is 0.723 au from the Sun and Mercury is 0.387 au from the Sun. Earth is 1.00 au from the Sun by definition. The other planets are farther from the Sun, so their astronomical units are greater than one. Mars the red planet is the fourth farthest from the Sun at 1.524 au from the Sun. Jupiter the largest and fifth planet from the Sun is 5.2 au From the Sun. Saturn, the sixth planet from the Sun and the ringed planet is 9.54 au from the Sun. Uranus, the seventh planet from the Sun and one of the gas giants is 19.22 au from the Sun. Neptune, the eighth planet from the Sun and another of the gas giants is 30.06 au from the Sun. Pluto, the ninth and farthest known planet from the Sun is 39.48 au from the Sun.
One astronomical unit is called the astronomical constant. Light travels one astronomical unit in approximately eight minutes and nineteen seconds because it takes approximately eight minutes for light to go from the Earth to the Sun. The speed of light is approximately 186,282 miles per second or 671,000,000 miles per hour. In the metric system, it is 300,000 kilometers per second and 1.08 billion kilometers per hour. A light year is about 6 trillion miles (6,000,000,000,000 miles or 6×10^12 miles).
Aristarchus of Samos thought the distance from the Earth to the Sun to be about twenty times the distance from the Earth to the Moon, but it is about 390. At the end of the 17th century, the au was estimated to be 87 million miles (about 5 million miles off). A better approximation for the au was made at the end of the 18th century. At the start of the 20th century, an even better estimate of the au was made. And the 21st century has made even more improvements to the au using sophisticated equipment.
Some distances using the speed of light as the base using the astronomical unit are one light-second equals approximately 0.002 au, one light-minute equals approximately 0.120 au, one light-hour equals approximately 7.214 au, one light-day equals approximately 173.263 au, one light-week (approximately 1,250,000,000 miles or 1.25×10^9 miles) equals approximately 212.84 au, one light-month (approximately 500 billion miles or 500,000,000,000 miles or 5×10^9 miles) equals 5197.9 au, one light-year (approximately 6 trillion miles or 6,000,000,000,000 miles or 6×10^12 miles) equals approximately 63,241 au, and one parsec equals approximately 206,265 au.