The Zodiac contains many of the most famous constellations in the night sky. Orion is a superstar of the night sky and he straddles the celestial equator, the imaginary line in space directly over the Earth’s equator. So why is he not part of the zodiac?
The short answer is that Orion is not near enough the ecliptic line, he is below it. But let’s unpack that.
The ecliptic line marks the passage of the sun against the stars, it is the blue line in the image above. The ecliptic also marks the band of sky that the moon and planets all follow. The ecliptic line was hugely significant to our ancestors, partly because, before Copernicus, they viewed celestial bodies as travelling on a journey around our planet. This is why the ecliptic marks the band of the night sky that contains the zodiac constellations.
The ecliptic is not the same as the celestial equator. The earth rotates on an axis which is inclined 23.5 degrees to the plane that it orbits the sun. In June, the north pole points towards the sun and in December the south pole points towards the sun. This means that when we look out at the stars near the ecliptic, we are looking well south of the equator in June and north of it in December.
Gemini and Taurus, two famous zodiac constellations, sit in this ecliptic band (see image above), north of the equator. In June, we see Scorpius straddling the ecliptic, another zodiac constellation, well south of of the celestial equator.
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