SafekipediaAstronomical unit
Adapted from Wikipedia · Adventurer experience
The astronomical unit (symbol: au or AU) is a unit of length. It equals exactly 149,597,870,700 meters. It helps scientists measure distances in space, especially in our Solar System.
Historically, the astronomical unit was thought to be the average distance between the Earth and the Sun. This was found by taking the average of the farthest point (Earth's aphelion) and the closest point (Earth's perihelion) in Earth's orbit around the Sun.
In 2012, the astronomical unit was officially changed to be exactly this number of meters. This made it easier for scientists to use. One astronomical unit is roughly equal to 499 light-seconds. This means it takes light about 499 seconds to travel this distance. The astronomical unit is also important for defining another space measurement called the parsec.
History of symbol usage
Different symbols have been used to stand for the astronomical unit. In 1976, the International Astronomical Union used the symbol A. By 2012, they said to use "au". This is the symbol most used today in science books. Other ideas like "ua" were suggested before, but "au" is now the most common.
Development of unit definition
The astronomical unit (AU) is a special distance used by astronomers to measure how far away things are in space. It was first thought of as the average distance between the Earth and the Sun.
Later, scientists found better ways to measure this distance very accurately.
In 2012, scientists decided to fix the astronomical unit to a specific number: exactly 149,597,870,700 meters. This makes it easier for everyone to use the same measurement, no matter where they are or when they measure it. This change happened because scientists wanted a simple, exact number to use when they study stars and planets.
| 1 astronomical unit | = 149597870700 metres (by definition) |
| = 149,597,870.700 kilometres (exactly) | |
| ≈ 92,955,807.2730 miles | |
| ≈ 499.004783836 light-seconds | |
| ≈ 1.58125074098×10−5 light-years | |
| ≈ 4.84813681113×10−6 parsecs |
Usage and significance
The astronomical unit is a special distance that scientists use to measure how far things are in space, especially inside our Solar System. Since 2012, this distance is exactly defined as 149,597,870,700 meters.
We use the astronomical unit to measure distances inside our Solar System, like how far planets are from the Sun or the size of disks around young stars. For bigger distances, like between stars, we use other units such as the parsec or light-year. The astronomical unit helps scientists avoid mistakes when they do calculations on computers.
Main article: Proper length
Main articles: Stellar system, Other distances in astronomy
History
Many early attempts to measure the distance between the Earth and the Sun were not very accurate. One of the first known attempts was made around 280 BC by Aristarchus. He used the angle between the Earth, Moon, and Sun to estimate this distance. His estimate was not very close to the true distance.
Later astronomers, including Ptolemy and others, also tried to estimate this distance using different methods. It wasn't until the 17th century that better tools, like telescopes, allowed for more accurate measurements. Methods such as observing the transit of Venus across the Sun helped scientists make better estimates. Over time, these measurements became more precise, leading to the modern definition of the astronomical unit.
Main article: Aristarchus
| Distance to the Sun estimated by | Estimate | In au | Percentage error | |
|---|---|---|---|---|
| Solar parallax | Earth radii | |||
| Aristarchus (3rd century BCE) (in On Sizes) | 13′ 24″–7′ 12″ | 256.5–477.8 | 0.011–0.020 | −98.9% to −98% |
| Archimedes (3rd century BCE) (in The Sand Reckoner) | 21″ | 10000 | 0.426 | −57.4% |
| Hipparchus (2nd century BCE) | 7′ | 490 | 0.021 | −97.9% |
| Posidonius (1st century BCE) (quoted by coeval Cleomedes) | 21″ | 10000 | 0.426 | −57.4% |
| Ptolemy (2nd century) | 2′ 50″ | 1210 | 0.052 | −94.8% |
| Godefroy Wendelin (1635) | 15″ | 14000 | 0.597 | −40.3% |
| Jeremiah Horrocks (1639) | 15″ | 14000 | 0.597 | −40.3% |
| Christiaan Huygens (1659) | 8.2″ | 25086 | 1.068 | +6.8% |
| Cassini & Richer (1672) | 9.5″ | 21700 | 0.925 | −7.5% |
| Flamsteed (1672) | 9.5″ | 21700 | 0.925 | −7.5% |
| Jérôme Lalande (1771) | 8.6″ | 24000 | 1.023 | +2.3% |
| Simon Newcomb (1895) | 8.80″ | 23440 | 0.9994 | −0.06% |
| Arthur Hinks (1909) | 8.807″ | 23420 | 0.9985 | −0.15% |
| H. Spencer Jones (1941) | 8.790″ | 23466 | 1.0005 | +0.05% |
| Modern astronomy | 8.794143″ | 23455 | 1.0000 | |
| Date | Method | A/Gm | Uncertainty |
|---|---|---|---|
| 1895 | aberration | 149.25 | 0.12 |
| 1941 | parallax | 149.674 | 0.016 |
| 1964 | radar | 149.5981 | 0.001 |
| 1976 | telemetry | 149.597870 | 0.000001 |
| 2009 | telemetry | 149.597870700 | 0.000000003 |
Developments
The astronomical unit connects to other important space numbers and ideas. As the Sun gives off energy, it loses mass. This means the planets move a little farther away from the Sun over time. Because of this, some people have suggested we might need a different way to measure these distances.
Scientists use the speed of light and other fixed numbers to figure out distances in space. This is becoming more common. However, there is some debate among scientists about exactly how much the astronomical unit is changing over time. Since 2010, the astronomical unit is no longer measured using the positions of the planets.
Examples
The table below shows different distances measured in astronomical units. It includes examples that are usually not expressed this way because they are either very small or very large. Distances can change over time. The examples are listed from the closest to the farthest away.
| Object or length | Length or distance in au | Range | Comment and reference point | |
|---|---|---|---|---|
| Light-second | 0.002 | — | Distance light travels in one second | |
| Lunar distance | 0.0026 | — | Average distance from Earth (which the Apollo missions took about 3 days to travel) | |
| Solar radius | 0.005 | — | Radius of the Sun (695500 km, 432450 mi, a hundred times the radius of Earth or ten times the average radius of Jupiter) | |
| Light-minute | 0.12 | — | Distance light travels in one minute | |
| Mercury | 0.39 | — | Average distance from the Sun | |
| Venus | 0.72 | — | Average distance from the Sun | |
| Earth | 1.00 | — | Average distance of Earth's orbit from the Sun (sunlight travels for 8 minutes and 19 seconds before reaching Earth) | |
| Mars | 1.52 | — | Average distance from the Sun | |
| Jupiter | 5.2 | — | Average distance from the Sun | |
| Light-hour | 7.2 | — | Distance light travels in one hour | |
| Saturn | 9.5 | — | Average distance from the Sun | |
| Uranus | 19.2 | — | Average distance from the Sun | |
| Kuiper belt | 30 | — | Inner edge begins at approximately 30 au | |
| Neptune | 30.1 | — | Average distance from the Sun | |
| Eris | 67.8 | — | Average distance from the Sun | |
| Voyager 2 | 141 | — | Distance from the Sun in September 2025 | |
| Voyager 1 | 168 | — | Distance from the Sun in September 2025 | |
| Light-day | 173 | — | Distance light travels in one day | |
| Sedna (aphelion) | 937 | — | Distance to the farthest point of the orbit from the Sun | |
| Light-year | 63241 | — | Distance light travels in one Julian year (365.25 days) | |
| Oort cloud | 75000 | ± 25000 | Distance of the outer limit of Oort cloud from the Sun (estimated, corresponds to 1.2 light-years) | |
| Parsec | 206265 | — | One parsec. The parsec is defined in terms of the astronomical unit, is used to measure distances beyond the scope of the Solar System and is about 3.26 light-years: 1 pc = 1 au/tan(1″) | |
| Proxima Centauri | 268000 | ± 126 | Distance to the nearest star to the Solar System | |
| Galactic Centre of the Milky Way | 1700000000 | — | Distance from the Sun to the centre of the Milky Way | |
| Note: Figures in this table are generally rounded estimates, often rough estimates, and may considerably differ from other sources. Table also includes other units of length for comparison. | ||||
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