The Vernal Point is a reference point in the Equatorial Coordinate System, from which R.A. is measured. My book says that the zero point of Right Ascension, i.e. the Vernal Point keeps moving in the sky. How can this be? If it moves then how can it be used as a reference point? Also why doesn't the dates of Vernal Equinox keep changing over the years?


2 Answers 2


It's best to think of the vernal point as the intersection of the Earth's equator with the Earth's plane of orbit. Since there are two possible choices, arbitrarily the point where the Sun crosses the equator moving North was chosen as the 0 point.

Since the Earth's poles move, obviously the vernal point will be different depending on where the Earth's pole happens to be pointing.

Now, why would we choose a coordinate system that constantly changes? It's because it makes celestial navigation easier. The vernal point doesn't move that fast, so charts are valid for long periods of time. The "geographic position" of a star can be determined rather simply. The lattitude is the declination. And the longitude is the right ascention minus Greenwhich sidereal time.

Since celestial navigation isn't used much anymore, the IAU has abandoned the "equinox method" and created the International Celestial Reference Frame. It is based on radio sources so distant, they will show no apparent movement at human timescales.



A nice explanation is given by Bob King in Sky&Telescope in the article "Right Ascension & Declination: Celestial Coordinates for Beginners". You can find it at: https://skyandtelescope.org/astronomy-resources/right-ascension-declination-celestial-coordinates/ Kind Regards Klaus

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