Astronomical unit

The astronomical unit is a length of exactly 149,597,870,700 metres, used chiefly to express distances within the Solar System.

Earth–Sun distance: The distance between Earth and the Sun, which varies over the course of Earth's orbit. The unit is close to the mean Earth–Sun distance, though it is defined exactly rather than by that changing distance.
Solar parallax: The apparent shift in the Sun's position caused by observing it from different locations on Earth. Early estimates of the unit relied on measuring this angle to infer the Earth–Sun distance.
SI base units: The seven fundamental units on which the International System of Units is based. The astronomical unit is expressed exactly in the SI base unit of length, the metre.
Lunar distance: The average distance between the centres of Earth and the Moon, about 384,400 kilometres. It measures a much smaller scale than the astronomical unit.
Solar System: The Sun and the gravitationally bound objects that orbit it. The astronomical unit is used chiefly to measure distances across this system.
Transit of Venus: An event in which Venus passes across the Sun's disk as seen from Earth. Observations of these rare events were used to estimate the scale of the Solar System.
Metre: The SI base unit of length, defined through the fixed speed of light in vacuum. The astronomical unit's exact value is stated as a number of metres.
Kilometre: A unit of length equal to 1,000 metres. Kilometres suit distances near Earth, while astronomical units keep planetary distances more compact.
Planetary orbit: The path of a planet around a star, shaped primarily by gravity. Orbital sizes are commonly reported in astronomical units.
Cassini's measurement of the solar parallax: A seventeenth-century estimate of the Sun's parallax based on observations of Mars from distant locations. It helped establish an early estimate of the Earth–Sun distance underlying the unit.

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