Units of Distance used in Astronomy
Interstellar distances being so huge, it makes little sense in astronomy to use kilometres as a distance measure (the actual international system unit of distance is the metre). Instead, astronomers use two other units, namely the astronomical unit and the light-year.
The astronomical unit (au) is the approximate mean distance of the Earth from the Sun, about 149.6 million kilometres. The actual modern definition of the au is rather complex, so we will not delve into this. The au is typically only used for distances of objects in our solar system. Thus, Mars lies at a distance of c. 1.5 au from the Sun, whilst Saturn lies at a distance of c. 9.5 au. The outer limits of the solar system* lie at a distance of about 75,000 to 100,000 au (which, as you can verify in a moment, equates to about 1.2 to 1.6 light-years).
The light-year (ly or lyr) is the distance that light (travelling at a speed of roughly 300,000 km per second) covers in one year. We can easily work out what 1 lyr equates to in kilometres:
In 1 second light covers a distance of c. 300,000 km
Hence, in 1 minute light covers a distance of about 300000 x 60 = 18,000,000 km
In 1 hour, light covers a distance of about 18000000 x 60 = 1,080,000,000 km
In 1 day, light covers a distance of about 1080000000 x 24 = 25,920,000,000 km
In 1 year, light covers a distance of about 25920000000 x 365.25 = 9,467,280,000,000 km
More accurately, the speed of light is 299792.458 km per second, and if we use this more accurate value in the same manner as above, we get a value for the light-year of 9,460,730,472,581 km.
Hence a light-year is roughly equivalent to 9.46 million million km.
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* Two different definitions for the boundary of the solar system have been put forward:
a) the distance at which the Sun’s gravitational pull ends and other stars’ gravity takes over and where the Oort Cloud is located.
b) the distance at which the boundary of the heliopause (the outward-flowing solar wind) is stopped by the interstellar medium.
Scale of the Cosmos
The Moon, our nearest celestial neighbour, is about 384,400 km away, so that light takes approximately 1.28 seconds to travel from the Moon to us. We can say that the Moon is 1.28 light-seconds away. Similarly the Sun may be said to be about 8 light-minutes away (since its distance from Earth is c. 150,000,000 km and 150000000/300000 gives 500 seconds or 8.33 minutes).
Now 1 au = c. 150,000,000 km and we said that the far reaches of the solar system extend to perhaps about 100,000 au. Hence, in terms of light-years, this would be equivalent to
150000000 x 100000 / (9.46 x 10^12) = 15 x 10^12 / (9.46 x 10^12) = 1.6 light-years. This is probably a maximum value.
The nearest star (to us), called Proxima Centauri (visible from the southern hemisphere) is c. 4.25 light-years away.
The brightest star, Sirius (in the constellation Canis Major and visible from both hemispheres), is c. 8.6 light-years distant.
Our Galaxy (which we call the Milky Way) is about 100,000 light-years in diameter, so that light takes one hundred thousand years to travel from one end of our galaxy to the other. We (the sun and the planets) are located about 28,000 light-years from the Galactic Centre.
Finally, some remote galaxies are situated at distances of billions of light-years!
Notes:
1. Because of the finite speed of light, we see the stars (and other celestial objects) now as they were x years ago, where x is the object’s distance in light-years. So, if an event happens on the nearest star to us (which is just over 4 light-years away), we will see the event 4 years later. Example: An event happens on Proxima Centauri in the year 2025 (when this website was launched). In this case, we won’t know that the event has happened until after 4 years because that’s how long light needs to travel to arrive from there to here.
2. Professional astronomers often also use another unit for distance called the parsec (pc). The parsec is defined as the distance at which 1 au subtends an angle of one arcsecond. It is equivalent to approximately 3.26 light-years.
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