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Welcome to our Physics lesson on Ecliptics, this is the fourth lesson of our suite of physics lessons covering the topic of Orientation in the Sky and Constellations, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
We have described the ecliptics as a big circle of celestial sphere obtained by its intersection with the orbital plan of the Earths revolution around the Sun. We assume all planets of the solar system as being on the ecliptics or very close to it, as their orbit forms a very small (negligible) angle to that of the Earth. The Moon also moves very close to ecliptics. We have explained that eclipses occur when the Moon is exactly on ecliptics (hence the name "ecliptics").
The Sun is at different points of ecliptics in various periods of year. It completes one West-to-East revolution in a year that is a whole ecliptic per year. Since ecliptics does not fit to celestial equator, everyday the Sun occupies different positions in the celestial sphere. In other words, the values of right ascension and declination of the Sun (as a common star) are never the same during a year. We have to wait until the same day of the following year to obtain the same identical values of RA and Dec. The same thing is true for planets as well. Moreover, the movement of planets is more complicated than that of the Sun (because of synodic period etc.). Last, the Moon completes one ecliptic every 29 days due to its period of revolution around the Earth.
Vernal points represent the intercept of ecliptic with the celestial sphere. There are two vernal points, which correspond to the two equinoxes (spring and autumn ones).
You have reached the end of Physics lesson 22.8.4 Ecliptics. There are 8 lessons in this physics tutorial covering Orientation in the Sky and Constellations, you can access all the lessons from this tutorial below.
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