词条 天文单位
描述: 天文单位（AU）是一个方便的距离单位，正好等于 149,597,870.7 千米（km）。大约是地球与太阳之间的平均距离，这也是以前对天文单位的定义。AU通常用于测量太阳系和其他行星或恒星系统中的距离。例如，海王星的轨道距离太阳约 30 AU。
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词条和定义状态： 该术语的英文原始定义已通过天文研究人员和教师的审核 该术语及其定义的译文仍待审核
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其他语言版本
- 阿拉伯语: الوحدة الفلكية
- 德语: Astronomische Einheit
- 英语: Astronomical Unit
- 西班牙语: Unidad Astronómica
- 法语: Unité astronomique
- 北印度语: खगोलीय इकाई (AU)
- 意大利语: Unità astronomica
- 日语: 天文単位 (外部链接)
- 韩语: 천문단위
- 马拉地语: खगोलशास्त्रीय एकक
- 巴西葡萄牙语: Unidade astronômica
- 繁体中文: 天文單位
相关图表
Astronomical Unit
图注： An astronomical unit (AU) is a convenient unit of distance equal to exactly 149,597,870.7 kilometers (km). This is approximately the average distance between the Earth and the Sun, which was a previous definition of the AU. The AU is often used to measure distances in the Solar System and in other planetary or stellar systems.
来源： Danielle Futselaar/IAU OAE
Annual Parallax
图注： Distance determination has historically been a challenge for astronomy. One of the primary ways to measure distance is to use annual parallax. The Earth orbits around the Sun over the course of a year meaning that it moves from one side of the Sun (shown here as position A) to the other side of the Sun (position B) over the course of six months. It then moves back to its original position over the remaining six months. This movement subtly changes the perspective an observer on Earth sees the night sky from. This is similar to the change in viewing perspective you may get when viewing a scene from your left eye and then your right eye. The change of viewing perspective causes nearby objects to shift in position in your vision. The annual motion of the Earth around the Sun changes the perspective of the observer enough to shift the observed positions of celestial objects. How big this effect is depends on the distance to the celestial object. Nearby stars will have bigger shifts in observed position than more distant stars.
The positional shift is known as the trigonometric or annual parallax (which we will call α here) and is defined as the shift in position of a star compared to what an observer at the center of the Solar System (the Sun) would see. In this diagram we see the star viewed from perspectives six months apart (positions A and B). When observed from position A the star’s shift in position will be α while when observed at position B it will be –α. Thus the relative difference in the stars position between being observed at position A and position B will be 2α.
The size of the trigonometric or annual parallax in arcseconds is approximately 1 divided by the distance in parsecs. An arcsecond (often represented by a ″ symbol) is the angular diameter a one-metre-long stick would have when viewed from 206 km away. A parsec (often abbreviated to pc) is 3.26 light years or 30.86 trillion kilometres. This is 206,265 astronomical units (the typical distance between the Earth and the Sun). No other star is closer than 1 pc to the Sun so all stars in the sky have trigonometric parallaxes less than one arcsecond.
While trigonometric parallaxes have long been used to measure the distances to objects in our Solar System or nearby stars, recent advances have pushed the boundaries of these distance measures further. The Gaia satellite has pushed the boundaries of parallax measurements to over a thousand parsecs. Arrays of radio telescopes can also very accurately measure the positions of very distant objects and thus their trigonometric parallax.
Note the Earth and Sun are not to scale here and the Earth’s axial tilt is not accurately represented.
来源： Aneta Margraf/IAU OAE
