When the short-term periodic oscillations of this motion are averaged out, they are considered "mean" as opposed to "true" values. It is due to shorter-period fluctuations in the strength of the torque exerted on Earth's equatorial bulge by the sun, moon, and planets. Nutation is the smaller amplitude shorter-period (< 18.6 years) wobble that is superposed on the precessional motion of the Celestial pole. The gravitational attraction of the Sun and Moon on the Earth's equatorial bulge cause the rotational axis of the Earth to precess in space similar to the action of a top. Locations of objects in space can also be represented using Cartesian coordinates in an ECI frame. Right ascension and declination are spherical coordinates analogous to longitude and latitude, respectively. The location of an object in space can be defined in terms of right ascension and declination which are measured from the vernal equinox and the celestial equator. ![]() ![]() The fundamental plane for ECI frames is usually either the equatorial plane or the ecliptic. The Sun lies in the direction of the vernal equinox around 21 March. The vernal equinox can be used as a principal direction for ECI frames. The equinox which occurs near the first day of spring (with respect to the North hemisphere) is called the vernal equinox. The angle between the Earth's equatorial plane and the ecliptic, ε, is called the obliquity of the ecliptic and ε ≈ 23.4°.Īn 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 Earth's orbit plane is called the ecliptic, and it does not coincide with the Earth's equatorial plane. It is convenient to define the orientation of an ECI frame using the Earth's orbit plane and the orientation of the Earth's rotational axis in space. The closer the observed object is to the ECI-origin, the less significant the effect of the gravitational disparity is. For example, the Moon's gravitational influence on a high-Earth orbiting satellite is significantly different than its influence on Earth, so observers in an ECI frame would have to account for this acceleration difference in their laws of motion. The extent to which an ECI frame is actually inertial is limited by the non-uniformity of the surrounding gravitational field. ECEF: not inertial, accelerated, rotating w.r.t stars useful to describe motion of objects on Earth surface. ![]()
0 Comments
Leave a Reply. |