You’ll hear that word mentioned a lot when you read about astronomy and the stars, but what is the Ecliptic?

Let me demonstrate by an illustration:

What is the Ecliptic

Simply put, this is the journey the Sun seems to take across the celestial sphere over the course of one year. As we know, it’s actually the Earth moving around the Sun over the course of a year – but relative to our place on Earth observing the sky above, the Sun appears to move across the stars.

The path reaches its most northerly point at the Tropic of Cancer, and its most southern at the Tropic of Capricorn, and crosses the celestial equator at the green points above, at the vernal and autumnal equinox.

Tilt of the Earth compare to the Ecliptic

The equator as we know it, or to give it it’s proper term, the Celestial Equator, runs between the northern and southern hemisphere of the Earth. However as the axis of the Earth is tilted at an angle of 23.45° against the plane of the ecliptic, as it goes around the Sun every year, the Sun appears from our position on Earth, to move northward and southward (or wobble) against the stars we see in the background during the course of the year.

Why do we have seasons

This tilt of the Earth is the reason we have seasons. For 6 months of the year, the northern hemisphere is closer to the Sun than the southern hemisphere and is therefore warmer as a result (Spring and Summer).  For the other 6 months, it is further away – leading to Autumn and Winter. The seasons are completely reversed in each hemisphere for this reason.

If the Earth was not tilted on its axis, then the Ecliptic and Celestial equator and their respective poles, would be identical. If that were the case, then the Sun would not appear to rise higher during some months and lower in others – and the stars that it passed across (in our view) would be the same throughout the year.

I hope this has helped answer any questions you have about what the ecliptic is, and why we have seasons. Please let me know in the comments below if you’ve any questions, or better definitions of exactly what is the ecliptic.