什么是 The ecliptic？

The ecliptic is the great circle of the celestial sphere along which the Sun appears to move in a year, the trace of the plane of the Earth’s orbit. It is inclined to the celestial equator by the obliquity of the ecliptic $\varepsilon$, the tilt of the Earth’s axis to its orbit. The ecliptic crosses the equator at two points; the vernal equinox point $\gamma$ is the one where the Sun crosses the equator northward, in March. The Sun’s ecliptic longitude $\lambda_\odot$ is its angle along the ecliptic from $\gamma$, counted eastward.