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Astronomical Unit
The astronomical unit (symbol: AU, a.u., or ua) is a unit of length used for expressing astronomical distances. It is defined by the International Astronomical Union (IAU) as exactly 149,597,870,700 metres. It is approximately equal to the distance from the Earth to the Sun, about 93 million miles or 150 million kilometres. The AU is used to measure distances within the Solar System and to exoplanets.
History
The concept of an astronomical unit dates back to the 17th century, when astronomers began to estimate the distance to the Sun. In 1672, Giovanni Domenico Cassini used observations of Mars during its opposition to estimate the Sun's distance at about 87 million miles. This was later refined by James Bradley in the 18th century to about 95 million miles.
In the 19th century, the development of parallax measurements allowed for more accurate determination of the Sun's distance. Friedrich Bessel calculated the Sun's distance to be 153 million kilometres (95 million miles) in 1838.
The term "astronomical unit" was first used in the early 20th century. In 1952, the International Astronomical Union (IAU) adopted the AU as a standard unit of length for astronomical distances. Its value was then set at 149,500,000 kilometres.
In 2012, the IAU redefined the AU to be exactly 149,597,870,700 metres. This definition is based on the SI system of units and is independent of any specific astronomical observation.
Definition
The astronomical unit is defined as the average distance between the Earth and the Sun. This distance varies slightly throughout the year due to Earth's elliptical orbit around the Sun. The Earth is closest to the Sun at perihelion, which occurs in early January, and farthest at aphelion, which occurs in early July.
The IAU's 2012 definition fixes the AU at precisely 149,597,870,700 metres. This value is derived from the speed of light and the definition of the second, ensuring its stability and precision.
Usage
The astronomical unit is primarily used to measure distances within the Solar System. For example, the average distance from the Sun to Jupiter is about 5.2 AU, while the distance to Neptune is about 30.1 AU.
The AU is also used to measure distances to exoplanets, which are planets orbiting stars other than our Sun. For instance, the closest known exoplanet, Proxima Centauri b, orbits its star at a distance of about 0.049 AU.
While the AU is a convenient unit for measuring distances within the Solar System, larger units like the light-year and the parsec are used for measuring interstellar and intergalactic distances. One light-year is the distance light travels in one year, approximately 9.46 trillion kilometres, or about 63,241 AU. A parsec is equivalent to about 3.26 light-years, or approximately 206,265 AU.
Significance
The astronomical unit is a fundamental unit in astronomy. It provides a common scale for understanding the vast distances involved in the Solar System and beyond. Its definition has evolved over time, reflecting advancements in measurement techniques and scientific understanding. The current precise definition ensures consistency and accuracy in astronomical calculations and research.
Category:Redirects from plurals
This category is for pages that are redirects from a plural noun to its singular form.
Purpose
The primary purpose of this category is to organize and manage redirects that arise from the use of plural forms of words. When a user searches for a term in its plural form, such redirects ensure they are directed to the correct, singular-form article. This maintains a consistent and organized structure within Wikipedia.
Usage
Redirects from plural to singular are generally created for convenience. For example, a search for "apples" might be redirected to the article on "apple." This is a common practice to improve user experience and searchability.
The standard template for tagging such redirects is {{R from plural}}. This template is applied to the redirect page itself, categorizing it appropriately.
Guidelines
- Mainspace Redirects: The
{{R from plural}}template is intended for redirects within the main article space of Wikipedia. - Other Namespaces: For redirects from plural forms found in namespaces other than the main article space (such as Wikipedia talk pages or User pages), a different template,
{{R from modification}}, should be used. This ensures proper categorization across different parts of the wiki. - Link Formatting: While these redirects are useful, it is often preferable to include the plural form directly after the singular link in the text. For instance, instead of just linking to
[[apple]], it might be written as[[apple]]s. This practice, sometimes referred to as "linking the plural," enhances readability and ensures that the plural form is explicitly mentioned. - Maintenance: The guideline WP:NOTBROKEN advises against changing existing links to avoid breaking functionality unless the page is being updated for other reasons. Therefore, these plural redirects should generally be left in place unless there's a compelling reason to modify them.
Examples
- A redirect from "planets" to "planet."
- A redirect from "stars" to "star."
- A redirect from "galaxies" to "galaxy."
These redirects ensure that users searching for these common terms are seamlessly directed to the relevant encyclopedic content. It's a small detail, but it contributes to the overall usability and comprehensiveness of the encyclopedia.