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Basics of Using a Telescope - Locating Stars (RA)

  1. Nov 21, 2015 #1
    Hello pf,

    I have been trying to grasp the concept of RA. I have read through a few books and I know that it is measured eastwards from the point on the celestial sphere where the Sun crosses the celestial equator at the vernal equinox. I just want to check I understand this correctly. Does it mean when the Sun cross the celestial equator, whatever star happens to be behind the Sun at that point would serve as a RA of zero?

    My other query is whenever you look up right ascensions on the internet or a phone app they are always constantly changing (or at least for planets)? Again I tried to research this with little luck but is it because nearby objects e.g. planets appear to not stay at the same point in the sky day to day because the Earth moves around the Sun (which is why a solar day is 4 minutes longer). So a planet would change its right ascension by 4 minutes each day? But a star is so much further away it will be fixed?

    Any help is much appreciated.
  2. jcsd
  3. Nov 21, 2015 #2


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    Yes, but also because the planets themselves are moving around the sun.

    No, because the planet itself is also moving.

    Yes, approximately. But the RA of stars also change slowly for two reasons. First, the precession of the Earth's axis causes the coordinate system to rotate slowly, causing RA's to change on the order of 1 degree per century. Second, nearby stars do move slowly because they are not infinitely far away. This is called proper motion, and is typically on the order of seconds of arc per year.
  4. Nov 21, 2015 #3

    Thank you for your help it is much appreciated. When RA is quoted it is given in units of time because 360 degrees is treated as 24 hours. Are these units of time the same as arc units (e.g. is one hour of RA one arc hour) or are these units for RA different to units of arc?
  5. Nov 21, 2015 #4


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    It can be very confusing. One hour of RA is 15 degrees. This is not too bad. But one minute of RA is different from one minute of arc, and one second of RA is different from one second of arc. Attached is a brief description that I wrote for a class I taught a couple of years ago. Table 1 gives the correspondence between units of RA and angular measure.

    Attached Files:

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