Units in astronomy

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    As always, I am behind with my reading of journals.

    There is a very interesting article by Keith Atkin in the April 2018 edition of A&G on units in astronomy. He talks about the mixture of imperial and metric units used, and how this has come about. He also makes a good case for simplifying things.

    One, tongue in cheek I’m sure, suggestion is to get rid of arcminutes and arcseconds, and just have degrees and decimals of. 45°59’30” would become 45.9917°.

    The question is, would you miss arcminutes and arcseconds?

    Andy Wilson

    Interesting question.

    I agree, a lot of units in astronomy are a bit crazy, but I do find arcseconds are a very useful size. They are close in size to typical seeing conditions, and a small number can be used to describe the size of planets and many other celestial objects.

    Arcseconds are also quite handy when dealing with distances measured by parallax, like by the Gaia mission, as they are used to define the distance in parsecs. The distance in parsecs is defined as one divided by the parallax in arcseconds.



    Callum Potter

    I can never remember if arc-minutes and arc-seconds are ‘ and ” or vice-versa!

    Does make hunting for some objects rather problematic!

    Decimals are used on iTelescope plans, so conversion is usually needed from catalogue and internet sources…




    I had trouble with ” and ‘ but then I try and remember that seconds, are second (2nd) and therefore the magic number is 2 so they are “


    Ian Kahler

    I have  always used  minutes  and  seconds…. when it is  decimal equiv., I get frustrated.. cant tell what the  value  is  so I convert to ‘ and “.

    Dr Paul Leyland

    Andy, arcsecs are indeed a useful size for some objects but so are degrees.  For some objects, the milliarcsec and microarcsec are the appropriate measures, especially in astrometry and VLBI radio.

    This leads me to the proposal that degrees, millidegrees, microdegrees, nanodegrees, … would be the appropriate way of proceeding.  Alternatively, and this I find rather more attractive myself, radians and their SI sub-divisions must be the natural units.  You may claim that a radian is a rather large angle but the size of the farad has never seriously worried the electronics engineers who are quite happy working with pF capacitors.

    Bill Barton

    My confusion has been over the use of minutes and seconds as divisions of both degrees (of declination or right ascension) and hours (of right ascension). A complete circle is either 360 degrees or 24 hours so these minutes and seconds are actually different sizes.

    David Dunn

    Perhaps using SI units would be appropriate? Radians, Milli-radians etc. We might also ge rid of Angstroms!

    Regards David

    Alex Pratt

    To add to the fun I once saw a ‘correction’ made by an editor (non-BAA) who changed all references to Angstrom to Armstrong.   🙂

    Clear skies,


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