![]() ![]() The Sun and Earth are 149 million kilometres apart, and we give this distance a name: an astronomical unit (AU).įor an even twistier unit of distance, we use the parsec (insert Han Solo Kessel run joke here). Astronomical units and parsecsĪstronomers also use comparisons to measure how far apart things are. For example, the Andromeda galaxy is approximately three trillion times the mass of the Sun (or 3×10 41 – that’s a 3 followed by 41 zeros – Astros). On the other hand, Astro’s mass is how much stuff he’s made of – and it’s the same no matter which planet he’s on.Īstronomers use Earth and the Sun as handy units to measure mass. This would be easy to do on Earth, even easier somewhere with lower gravity like the Moon, and much harder somewhere with higher gravity like Jupiter. We can think of weight in terms of how hard it is to lift an 18kg Astro off the ground. On Earth the weight of an object, like Astro, depends on the mass of Astro and the gravitational force pulling him down to the ground. Mass is a measure of how much stuff something is made of. If we want to measure how heavy an asteroid is, we could do it with camels – but in space we’re more interested in mass than in weight. So if we want to talk about how big Betelgeuse is, it’s much more convenient to use the radius of the Sun as our unit, instead of the radius of Earth (or to describe it as 632 billion Astros). Astro the whippet contemplating the wonders of the Universe (probably). Laura DriessenĪt an even larger scale, consider the star Betelguese: its radius is 83,000 Earths, or 764 times the radius of the Sun. That number of Astros is a bit ridiculous, which is why we adjust our unit choice to one that makes more sense. Jupiter’s radius is 11.2 Earths, or 85 million Astros. Earth’s radius is about 638 million cm, or 7.5 million Astros. The basic problem is that lots of things in space are way too big for our familiar units. Stefan Kraft / Wikimedia, CC BY-SA Why do astronomers use such strange units? A platypus that is approximately 1/18th of the size of Asteroid 2023 FH7. But what about a pulsar-wind nebula with a brightness of a few milliCrab? That’s where things get weird. The idea of a planet that’s 85% the mass of Earth seems straightforward. These outlandish comparisons are the invention of Jerusalem Post journalist Aaron Reich (who bills himself as “creator of the giraffe metric”), but real astronomers sometimes measure celestial objects with units that are just as strange. Larger Unitsįor even greater astronomical distances, multiples of the parsec are used, including the kiloparsec (kpc), the megaparsec (Mpc), and the gigaparsec (Gpc), which are, , and parsecs respectively.You may have heard about an asteroid set to fly near Earth that is the size of 18 platypus, or maybe the one that’s the size of 33 armadillos, or even one the size of 22 tuna fish. In practical terms, if we observe a nearby star from two points in Earth’s orbit six months apart, and that star appears to move by one arcsecond compared to more distant stars, then that star is said to be one parsec away. The term is a portmanteau of “parallax” and “arcsecond”, reflecting its definition. ![]() The term parsec comes from the method of parallax measurements used to determine the distances of nearby stars. These relationships can be represented by the equations: In terms of light-years, a commonly used unit in astronomy, one parsec is approximately equal to 3.26 light-years. Conversion to Other UnitsĪ parsec is approximately equal to 3.09 trillion kilometers, or 1.92 trillion miles. An astronomical unit is the average distance between the Earth and the Sun, approximately 149.6 million kilometers. ![]() The parsec (pc) is defined as the distance at which one astronomical unit (AU) subtends an angle of one arcsecond. ![]()
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