![]() In other words, the Earth and the Sun mid-distance varies in a single year. As the earth orbits the sun with a varying orbital distance, we need to consider the average distance, therefore, one astronomical unit is the average distance between the Earth and the Sun. Astronomical Unit DefinitionĪccording to the prevailing astronomical convention, 1 astronomical unit is equal to 149,597,870.7 kilometres (or 92,955,807 miles). One astronomical unit is equal to 92955807 miles. The astronomical unit is quite instrumental in formulating and understanding the distance between stellar objects and is crucial in the calculations and computation involving astronomical problems. It is also an essential element in the definition of another unit of astronomical length, the parsec. The astronomical unit is applied primarily for measuring distances around other stars or within the Solar System. The definition of AU also means distances in astronomical units to pre-eminent solar system objects. An AU is nearly 93 million miles (150 million km). ![]() We can say that one astronomical unit (AU) outlines the mean distance between the Earth and our Sun for general reference. So, how far is a parsec? 1 pc = 3.261631 ly.How to define astronomical unit? Astronomers use astronomical units – or AU – to represent the distance of the solar system. An equivalent way to define the parsec is to say it is the distance at which 1 AU subtends an angle of 1 arcsecond, also shown in a diagram on the next page. Actually, the nearby star’s parallax across the entire baseline of the Earth’s orbit (2 AU) would be twice as much: 2 arcseconds at a distance of one parsec. What this means is at a distance of one parsec, a nearby star would undergo a one arcsecond (1/3600˚) trigonometric parallactic shift relative to very distant stars across a one AU baseline, as shown in the diagram on the next page. What’s a parsec, then? Parsec is an abbreviation of parallax arcsecond. An astronomical unit is very nearly equal to the average distance of the Earth from the Sun, i.e. It could disappear today (highly unlikely), and you wouldn’t see it disappear until 8 years and 7 months from now! In the solar system-which is much smaller than a light year-astronomers like to use the astronomical unit as the distance unit of choice. So, when you look at Sirius, you are seeing the star as it looked 8 years and 7 months ago. The brightest star in the night sky, Sirius, for example, is 8.6 light years away. When we look out into space, we also look back in time. A very useful property of using the light year as a measure of distance is that it also tells you how many years ago the light left the object you are observing. In a year, then, light travels 9.5 trillion kilometers, which is 5.9 trillion miles. ![]() Even so, the nearest known star beyond our solar system, Proxima Centauri, is 4.23 ± 0.01 ly away. The speed of light in terms you may be more familiar with is 186,282.4 miles per second, or a whopping 671 million miles per hour. In fact, this speed defines the exact length of a meter. The speed of light is very fast: 299,792,458 meters per second, exactly. A light year is the distance light travels in an average Gregorian year (365.2425 days) in the near-vacuum of space. ![]() But before we define the parsec, let’s look at some more familiar units of astronomical distance: the light year (ly), and the astronomical unit (AU). ![]() Download What's a Parsec? and more Astronomy Study notes in PDF only on Docsity!What’s a Parsec? A parsec (pc) is a unit of distance that is often used by astronomers. ![]()
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