Last month The Backyard Astronomer explored The Wanderers: our Sun, our Moon, and the five naked-eye planets, Mercury, Venus, Mars, Jupiter, and Saturn.
While all the stars in the sky remained fixed in relation to one another, the Wanderers roamed the skies against the background stars. But they didn’t go just anywhere in the sky. The Wanderers all followed the same path against the background of stars. This path is called the ecliptic, which captured the imagination of the ancients and they imbued it, and the constellations along this path, with great mystical significance. These are the constellations of the zodiac.
Of course, the Wanderers are the visible members of our solar system and the reason that they don’t just go anywhere in the sky is because all the planets orbit the Sun on roughly the same plane, not willy-nilly like we envision the electrons orbiting the nucleus of an atom, for instance.
Perhaps most perplexing to our ancestors was that the ecliptic didn’t just run from the east to the west, but meanders above and below the celestial equator, i.e., Earth’s equator projected up into the sky. You may remember from science class that because the Earth’s rotational axis is tilted 23.5 degrees from the plane of the solar system, we have seasons. If the Earth’s axis weren’t tilted to the plane of the solar system, there would be no seasons to speak of. All days would receive 12 hours of sun all year everywhere on Earth and the ecliptic and the celestial equator would be the same.
As it is, we live on this goofy tilted orb that gives us seasons, midnight sun, and Daylight Savings Time. The tilted axis also gives us two solstices and two equinoxes every year. The solstices are the high and low point of the Sun’s path along the ecliptic; the equinoxes are when the sun crosses the celestial equator in spring and fall.
Soon the sun will reach his lowest point in its yearly journey here in the northern hemisphere in the constellation of Sagittarius. Sagittarius is a summer constellation because the sun is then at the opposite end of the sky at its high point in Gemini at the summer solstice.
You might remember from The Double Planet installment of “The Backyard Astronomer” that the full moon always rises at sunset because it must be opposite the sun to have 100 percent illumination. This also means that when the sun is lowest in the sky on the winter solstice, a full moon must be at its highest in the sky for full illumination. Sunlight might be in very short supply on the winter solstice, but we get a lot of extra moonlight.
The winter solstice has always been a numinous time for people of the northern hemisphere. Figuring out when the shortest day of the year was the source of great power for our ancestors and their spiritual leaders. From Stonehenge to the Tongva petroglyphs in the Simi Hills, the ancients carefully observed the skies and noted the positions of the Wanderers so they could celebrate the renewal of the winter solstice and the lengthening days.