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The annual parallax of a heavenly body is the angle subtended at that body by the radius of the Earth’s orbit.

With their annual parallax, however, the case was very different.

Half of this major axis, or, what comes to the same thing, the angle which the radius of the earth's orbit subtends as seen from the star, is called the star's "annual parallax."

The annual parallax of a star is the angle under which one would see the radius, or half-diameter, of the terrestrial orbit from it.

Here we have only to do with near stars for which the annual parallax is well determined.