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Why 24 hours for right ascension?

  1. Jan 18, 2008 #1
    I'm reading Nick Strobel's astronomy notes. I should mention that it's an excellent website for an amateur like me. I can't get one thing right though. He mentions about fixed coordinates of stars measured in RA (right-ascension) and declination. He says that "Earth's rotation is broken up into 24 hours, so one hour of RA = 15 degrees of rotation" but does not mention if RA is divided into 24 hours. The same source says that stars take 23 hours and 56 minutes to make full rotation around an observer (or more exactly the earth revolves around its axis in 23 hours and 56 minutes). Doesn't this mean any star with 23 hour and 58 minutes right ascension has actually RA of 2 minutes (23 hour and 58 minutes minus 23 hours and 56 minutes=2 minutes)? Should we not divide RA into 23 hours and 56 minutes instead of 24 hours?
  2. jcsd
  3. Jan 18, 2008 #2


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    No right ascension is a part of a circle.
    You could also quote it in degrees or radians, putting into 24hours just makes dealing with time easier.
  4. Jan 18, 2008 #3


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    No. RA assumes that the stars are fixed and that the Earth rotates in 24 hours (our definition) so that we should map them in terms of a theoretical fixed Earth. The Earth's orbital movement around the Sun allows for parallax measurements to be made for the closer stars (they appear to shift in the sky [proper motion] with respect to more distant stars), though the more distant stars, galaxies, quasars, etc appear to be quite fixed on the sky.

    As the Earth orbits the Sun, day-by-day we get a slightly different viewpoint on the night sky. Today, this seems pretty archaic, but a long time ago, some telescopes were mounted on mounts that only allowed the telescope to move in declination, and they were accompanied by VERY accurate clocks. Such observatories were invaluable in mapping the sky, and providing accurate charting for navigators, etc. The problem for navigators at the time is that although these charts allowed them to gauge their latitudes quite accurately, their time-pieces did not have accuracy sufficient for them to gauge their longitudinal positions. This was HUGE problem for shipping when the British Empire was vying against the French and the Spanish trying to establish dominance of the seas in commerce and military affairs.
    Last edited: Jan 19, 2008
  5. Jan 18, 2008 #4
    Thank you for response but I can't understand still one thing. Here are two quotes from that website:
    Quote #1:
    This to me means that any given star for a given location will rise 23 hours and 56 minutes apart for each consecutive rise. This also means that for each consecutive positions of the star when it crosses meridian there's 23 hours and 56 minutes difference.

    Now look at Quote #2:
    If we extend this line of reasoning then for 2 stars 24 hour of RA apart one will see one star crossing meridian 24 hour of time before the other. Quote #1 says that first star that crosses meridian will appear on the meridian again in 23 hours and 56 minutes. So a star which is 24 RA apart from this first star will appear on the meridian 4 minutes after the first star appears second time on the meridian. So stars of 0 RA and 24 RA do not rise at the same time and do not cross meridian at the same time. Consequently this 2 RAs (0 and 24) are not the same while they should be.
  6. Jan 18, 2008 #5


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    Please realize that astronomers(Yes, even WAY back) realized that they could make measurements that were repeatable. This was the beginning of natural science/natural philosophical sciences that underlie our current beliefs and that are being extrapolated to the Standard Model of cosmology.
  7. Jan 19, 2008 #6
    What I'm about to write has mostly been said above, but it may help clarify the concepts.

    The key here is to realize that the Sun, like all stars, has an RA position. But since the Earth is orbiting the Sun, it's RA position changes throughout the year and takes one-year to return to any position (the Sun is at the same RA every January 1). Our solar clock (diurnal time) must therefore precess through the year with respect to the stellar clock (sidereal time). 24 hours divided by 365 is about 4 minutes.
  8. Jan 19, 2008 #7


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    By definition, stars cannot be 24 hours of RA apart from one another. RA is divided into 24 hours so those two stars would lie on the same line of RA and would transit your meridian at the same time.
  9. Jan 21, 2008 #8
    I think you are getting RA and time mixed up.

    RA looks like time, but it is really a coordinate system fixed on the stars.

    As the Earth rotates, it is also revolving around the sun. This causes the constellations to rise about 4 minutes earlier each night.

    Suppose a star has RA = 6hr 45min, DEC = -16 degrees, and it crosses the meridian at 7.00pm tonight. Tomorrow night, that same star will cross the meridian at 6.56pm, but its coordinates are still RA = 6hr 45min, DEC = -16 degrees.

    You can imagine RA as identical to DEC, but instead expressing 360degrees in 24 hours.
  10. Jan 24, 2008 #9
    Thanks everybody. I think I understand it now.
  11. Jan 24, 2008 #10


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    Actually, most everything written here is at least misleading...

    RA has nothing to do (at all) with rotation. Even if the Earth rotated faster or slower, RA could still be measured the same way. RA is nothing more than the concept of longitude applied to the celestial sphere. A star's RA is nothing more than a measure of its longitude, its angular distance from some chosen reference point. As has been said, you could just as easily measure this quantity in degrees or radians or any other angular unit. Historically, astronomers chose to use "hours," each hour corresponding to fifteen degrees.

    It's a useful approximation over the course of a single night, but it drifts four minutes every day.

    - Warren
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