Exploring the Ecliptic Plane's Tilt

[JeffOCA]

Hi,

1/ The ecliptic plane is tilted at a constant angle (23,45°) as compared to the celestial equator. So, why the angle between ecliptic and equator is +23,45° at spring equinox and -23,45° at autumnal equinox ?

2/ On this document, ε = 23°27'. Why don't we have ψ = ε = 23°27' ? The equator circle and the little circle (which passes through the Sun S and is parallel to equator) cross the ecliptic at a constant angle 23°27' ?

Thanks for helping

 

[lippyka]

.Answer: 1/ The angle between the ecliptic and the celestial equator is +23,45° at spring equinox because the Earth's axis of rotation is tilted 23.45 degrees from the plane of its orbit around the Sun (the ecliptic plane). This tilt causes the amount of daylight to be different at different times of the year, and the Sun is directly overhead at the equator twice per year, at the spring and autumn equinoxes. During the spring equinox, the Sun is directly overhead at the equator and is also on the ecliptic plane, so the angle between the ecliptic and the equator is +23.45°. During the autumnal equinox, the Sun is still directly overhead at the equator but is on the opposite side of the ecliptic plane, so the angle between the ecliptic and the equator is -23.45°. 2/ ε is the angle between the ecliptic and the equator, which is constant at 23°27'. ψ is the angle between the equator and the little circle that passes through the Sun S and is parallel to the equator. This angle varies depending on the position of the Sun in its orbit around the Earth and is not necessarily equal to ε.