Talk:Earth-centered inertial

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Comment[edit]

An ECI is not a true inertial reference frame because the reference frame accelerates with respect to the stars (and/or Sun) because the reference frame moves around the Sun in an almost circular motion. It does not rotate(the orientation of the reference frame is constant), but does accelerate. Am I correct? Someone reverted my edits because he/she said it is a true inertial reference frame. I think this is very important to explain in a simple and correct way.

The Earth does accelerate but only under the action of gravity. This makes it an inertial frame, in general relativity at least. The article does not make that clear. 2.102.212.204 (talk) 18:51, 10 September 2021 (UTC)[reply]

Vernal Equinox[edit]

The statement, "The intersection of the ecliptic and Earth's equatorial plane can be used as a principal direction for ECI frames and is called the vernal equinox", lacks precision. We must talk in terms of the intersection of the ecliptic with the celestial equator since the ecliptic intersects the earth's equatorial plane at all positions of the earth. We must speak of where the earth is in its orbit. And we must distinguish the two equinoxes. The following statements will improve precision.

An equinox occurs when the earth is at a position in its orbit such that a vector from the earth toward the sun points to where the ecliptic intersects the celestial equator. The equinox which occurs near the first day of spring is called the vernal equinox. RHB100 (talk) 00:44, 4 February 2015 (UTC)[reply]