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Of course, I know what the ecliptic is and what equator is. The problem is I still can not understand why we need two systems for measuring the same thing - are each of them useful only in specific occasions? For example, which is preferred in marine navigation? I am interested in this practical side of the question

Can you give an example which positions a star, using both coordinate systems?

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    $\begingroup$ I'm voting to close this question as off-topic because it appears to be a homework question $\endgroup$
    – adrianmcmenamin
    Commented Jun 13, 2017 at 9:38
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    $\begingroup$ No, it is not a homework. I am just having difficulties with understanding the principle. Funny guess by the way : )) $\endgroup$
    – Kiril Mladenov
    Commented Jun 13, 2017 at 11:03
  • $\begingroup$ Try googling and using wikipedia. If you still have specific questions, ask them. I don't think this is necessarily a homework question, but do try doing some research on your own first. $\endgroup$
    – user21
    Commented Jun 13, 2017 at 14:13
  • $\begingroup$ I did. Of course, I know what ecliptic is and what is equator. The problem is I still can not understand why we need two systems for measuring the same thing - are each of them useful only in specific occasions? For example,which is preferred in marine navigation? I am interested in this practical side of the question as I must admit (it should be obvious so far) that I do not have a degree in astronomy and in physics $\endgroup$
    – Kiril Mladenov
    Commented Jun 13, 2017 at 15:21
  • $\begingroup$ The answer is: certain things are easier to express/calculate in one coordinate system than the other. Anything involving the solar system and the motion of bodies in it, for example, is usually easier to express in ecliptic. Equatorial coordinates make stellar navigation easier, not to mention star tracking during prolonged exposures. $\endgroup$
    – Sean Lake
    Commented Jun 13, 2017 at 15:35

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To locate an object you need a system of coordinates.

You need to define a plane (to define the latitude) and a direction (from which to measure longitude). On Earth, the obvious plane is the Equator. There is no obvious direction from which to measure longitude (which in the past led to different definition of longitude in France and Britain)

There is no single, natural plane in the sky. Several are possible: You can project the equator into the sky. You can use the plane of the Earth's orbit around the sun. You could use the plane of the galactic disc.

Using the Equator is simple, but rather Earth-centric. The Ecliptic system is more natural for solar system objects. The galactic system nice for deep space objects. The lack of a single natural plane in the sky means that several different coordinates systems can co-exist.

As an example Rigel, at RA 5h14'32'', Dec −08° 12′ 05''

Has a Lat-Long of:

  • -31.11° 76.09° Ecliptic
  • -4.40° 219.66° Galactic
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  • $\begingroup$ Thank you! Could you recommend me a paper about the different definitions in the past which you mentioned? I am interested exactly in these developments of concepts: Where to measure something from? Is there another way to do it? etc. I am a historian and work with Akkadian texts. There (maybe you already know) there is a concept of Paths: Paths of Ea, Anu and Enlil (i.e. that we might call declination) and Path of the Moon (i.e. longitude) $\endgroup$
    – Kiril Mladenov
    Commented Jun 16, 2017 at 8:12

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