View Full Version : Astronomical Measurments

2003-Dec-19, 02:14 AM
Astronomical measuring units. There is light years, parsecs, miles, kilometres, c, astronomical units, kg, tonnes, solar masses, newtons. They're all different, they all have a purpose but some are a bit confusing. Solar masses, seems simple, but the mass of the sun is changing all the time, it ejects mass, it absorbs mass. Yet there are measurments which are static, eg. kilograms.


From Pluto is a planet or satellite? (http://www.universetoday.com/forum/index.php?showtopic=1319&st=30&#entry9272)

We have only to look at the number of different measures we have for celestial distances e.g. km, miles, AU, parsec (pc), etc. Different astronomer has his or her own preference.

Those all have different purposes.

I think deciding on what is a planet should be decided when we have more data on planets belonging to other stars. Posted by matthew.

Matthew, it is true that distance measures do by and large have different purposes. BUT it is NOT difficult to find within a common frame of reference e.g. the confines of our known planetary system (i.e. distance out to Pluto), different astronomers, books, journals (e.g. AJ, ApJ, AAS, Icarus, MNRAS) adopt different measures to some common object. Another area where astronomers cannot agree on is how and what to expressed Stellar Mass in. i.e. the 2 most commonly expression are in terms of kg or in terms of solar masses. Some astronomers I know prefer to keep things simple by expressing a star's mass in terms of solar masses (i.e. x Msun) and generally do NOT have much good things to say about those who prefer kg (somehow you detect some animosity between the 2 groups i.e. the astrophysicists and the other group i.e. the other astronomers (e.g. atmospheric modellers, infrared astronomers)). Posted by rajasun.

Do we need to decide what measuring units aern't really necessary? Like 1 solar mass= mass of the sun. Yet the mass of the sun is always changing, so how can that give us an accurate measurment?

2003-Dec-19, 02:56 AM
Matthew, let me share with you this little incident involving two Profs. one with a background in physics and another an observational infrared astronomer involved with SIRTF and WISE. Both have VERY different whims and fancies with regards to the use of units in formulaes and I can understand why...you mix up the systems and the derived answers will be WRONG! e.g. try the radial velocity calculations ;)

I NOT gonna state their names BUT one simply DETEST the other and vice versa because of their individual whims and fancies. It is just like the controversy over the objects found in Orion. Some would like to call them "Free Floating Planets" BUT to some like Dr. J. D. Kirkpatrick will say NO WAY! These objects Dr. J. D. Kirkpatrick says are sub-Brown Dwarfs NOT planets! Calling objects like the S Ori objects planets is NOTHING more than a scheming even devious way of grabbing attention i.e. for funding.

Hope this helps!