# Why don't astronomers use everyday units to measure distances (what is an AU or a pc)?

I am an amateur astronomer here in Georgia, and have been for several years now. I know that when we talk of distances in space we don't have to get much over the distances to our sun and several of the nearest stars to start getting into an area where normal distance measuring terms mean very little.

I know that an AU is one Astronomical Unit, and that is the distance from Earth to the Sun or about 93 million miles. I hope I am correct on that.

I have a piece of software that I sometimes use on my laptop to help me know just what I can see at that time period using my 8" Meade LX-200, and it has, in it's selection of possible units of measurement, KM for kilometer, AU for astronomical unit, and the last one that I don't understand, a PC unit of measurement.

Could you tell me what the PC unit of measurement is, and how and when it is used. Also I would like to know if in some cases one unit of measurement is used more than others and if one is actually more accurate and preferred by professional astronomers.

Thank you for this site where this type of information can be retrieved from such a good source, and also for your time.

It doesn't really matter which unit you use to measure distance - it's all a matter of convenience. It's the same as why you wouldn't measure your weight in ounces, but neither would you weigh something very light in pounds - you want a convenient number which is easy to interpret.

PC is short for parsec and it's a unit of measurement equal to 3,000,000,000,000,000,000 cm! (or 3E18 if you understand scientific notation - there are 18 zeros after the 3). It was introduced when people started measuring the distance to nearby stars using the parallax method, which involves measuring how much the star moves relative to background objects as the Earth orbits the Sun. A parsec is the distance at which a star has a parallax of 1 arcsec. I would say that it is actually the most commonly used distance measurement in Astronomy, just because it is a convenient size to measure such large distances in.

Light years are also sometimes used. 1pc is about 3 light years.

In extragalactic Astronomy people often use kpc (kilo parsecs, or 1000 parsecs) or Mpc (mega parsecs, or 1,000,000 parsecs). For example the Sun is about 8 kpc from the centre of the galaxy and Virgo (the nearest large cluster of galaxies to us) is about 15-20 Mpc away.

Good question - units in Astronomy are mostly used for historical reasons (in my experience) and can often seem very obscure when you first come across them.

Thank you so much for taking time, from what I'm sure must be a busy schedule at Cornell, to answer my question so well and so quickly. I, for some reason, didn't expect and answer as I thought if I took the time to read all the previous questions and answers I would probably find it within. And to have it answered by a professional and so quickly was such a pleasant surprise.

Your explanation was super, and I now want to do more reading on the subject so will be looking over my library for more information. I built a 4.5 inch reflector some 35 years ago, and used it much. In fact I still have it around here somewhere. I have, just in the past couple of years, rekindled that want to know and see some of what's out there, so purchased the computer controlled LX-200 and have seen more in the past two years that I ever did with the old scope. And being a professional photographer, I have put some of that knowledge to use in starting to do Astrophotography.

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