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B Is right ascension only measured once a year?

  1. Sep 8, 2018 #1
    So I'm learning some basics about astronomy...and this right ascension value is defined as being measured eastward along the celestial equator from the vernal equinox....now as I understand it, the vernal equinox only occurs once a year (the first day of spring, in the northern hemisphere), so is any object's right ascension value only measured on one day of the year, and then kept constant until the vernal equinox comes around again in a year?
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  3. Sep 8, 2018 #2


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    You gotta set the zero point somewhere in the sky and astornomers chose the vernal equinox as the zero point for the measurement known as right ascension. if you think of the sky as a gigantic clock face it all makes sense. Since there are 24 hours in a day and obviously you can only see 12 hours of the sky at any given time of day [horizon to horizon]. The sun is at the zenith, as viewed from the equator, at precisely noon on the vernal equinox, and this is labeled 0 hours right ascension, Thus the eastern horizon is at 6 hours right ascension at noon on the vernal equinox.
  4. Sep 8, 2018 #3


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    no, the right ascension (and declination) of a galaxy, star, nebula is the same (to a respectable precision) over many years.
    The only thing that affects it is "stellar drift". If the RA and DEC changed significantly every year, then all our star maps would be out of date every year.
    Star atlases (maps) get updated roughly every 50 years to take the drift into account

    Last edited: Sep 10, 2018
  5. Sep 8, 2018 #4
    It's the mean year moment that's overhead, not the Sun. The Sun will be 7+ minutes late according to the equation of time.
  6. Sep 9, 2018 #5
    In the equatorial coordinate system, the vernal equinox can be seen as a point (indeed one of two, as we have also autumnal equinox) on the celestial sphere, where ecliptic (annual apparent path of the Sun across the sky) is intersecting the celestial equator, i.e. directly on zenith as seen from locations at equator. It was chosen as the zero point with coordinates (0, 0) in this system. However due to precession of Earth's rotation axis, the right ascension and declination are very slowly changing, therefore an epoch is usually specified when a position of an object is listed, in catalogue for example. Currently used epoch is designated J2000.0, which means that coordinates listed are such as it would be measured on Jan 1st 2000, at noon GMT. You can than calculate the actual coordinates of the object based on these values and the time elapsed since the reference date/time. Note that the corrections are really small on the human timescalles, something on the order of 0.05 degrees per 5 years.
  7. Sep 11, 2018 #6


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    You know Earth's orbit around the Sun. You can calculate its current orientation relative to the vernal equinox, and that allows you to measure the right ascension of things in the sky at any point in time.
  8. Sep 12, 2018 #7


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    Declination and RA are basic and both use a reference. Polaris is always up there and it can be used as an fairly good Celestial North Pole (its Dec is 89°) That origin hardly changes at all. If we use an Equatorial Mount for a telescope, we can very often just point our mount at Polaris and that's good enough Turn on the tracking (used to be a clockwork motor) and you will keep any deep sky object in the eyepiece, once you have found it. RA has been taken care of by finding the star by other means. But RA is needed if you want to find an object that isn't in atlases or your GoTo mount's library.
    The Origin for RA (The Vernal Equinox or First Point of Aries) is which part of the celestial sphere your telescope would be pointing at if you aimed it directly upwards from Greenwich UK at Noon on the Equinox. I agree that the time based term "Equinox" is a strange way to describe a point on the celestial circle and I feel that First Point of Aries would make more sense. Wherever you happen to be in the world and at whatever time, the coordinates that you use to find an object relate to the angle between its position relative to the First point of Aries and the local time relative to GMT. For Navigation purposes, they use the Greenwich Hour Angle to specify a point in the sky but it means virtually the same thing and it still uses the same reference origin.
    It's 'Right' Ascension because stars rise on the Right when you are looking at Polaris. But the Sun rises on the left when you are looking at is (i.e. South). It's almost more confusing than electrons going the wrong way in a circuit ain't it?
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