![]() ![]() In fact I still have it around here somewhere. I built a 4.5 inch reflector some 35 years ago, and used it much. 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. And to have it answered by a professional and so quickly was such a pleasant surprise. 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. 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. 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. 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. In extragalactic Astronomy people often use kpc (kilo parsecs, or 1000 parsecs) or Mpc (mega parsecs, or 1,000,000 parsecs). ![]() 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. A parsec is the distance at which a star has a parallax of 1 arcsec. 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. 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'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. It doesn't really matter which unit you use to measure distance - it's all a matter of convenience. Thank you for this site where this type of information can be retrieved from such a good source, and also for your time. 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. 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.Ĭould you tell me what the PC unit of measurement is, and how and when it is used. 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 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. The less an object appears to have moved, the farther it is from the Sun, and vice versa.I am an amateur astronomer here in Georgia, and have been for several years now. In this case, the object's distance in parsecs is numerically equal to the reciprocal of half the number of arcseconds by which its position appears to change. ![]() After six months, the Earth is at the opposite end of its orbit around the Sun, a distance equal to twice the base of the right triangle described above. It becomes quite convenient for astronomers to determine the object's distance by simply measuring its apparent motion in the sky over a 6-month interval. The angle can be measured by observing the object's precise location in the sky. Now, if the angle formed between the line drawn from the object to the Earth and the line drawn from the object to the Sun is exactly one arcsecond, then the object's distance from the Sun would be exactly one parsec. In other words, imagine that a straight line is drawn from the object to the Earth, a second line is drawn from the Earth to the Sun, and a third line is drawn from the object to the Sun that is perpendicular to the line drawn from the Earth to the Sun. A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond. The name parsec is "an abbreviated form of 'a distance corresponding to a parallax of one arcsecond'." It was coined in 1913 at the suggestion of British astronomer Herbert Hall Turner. The parsec is a unit of length used in astronomy, equal to about 30.9 trillion kilometres or 3.26 light-years. Freebase (0.00 / 0 votes) Rate this definition: ![]()
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