The ecliptic
The pattern of the north-south drift described in the previous chapter is regular only if we understand it as motion along a tilted circle: a great circle set at an angle to the equator. This tilted circle is called the ecliptic.
The ecliptic is the path the Sun traces through the stars over the course of a year. It is the same circle traced by the other solar system planets, though each planet moves along it at its own pace, and the Moon follows it closely too; all these bodies stay within a narrow band centred on the ecliptic. Because the Sun moves a certain amount north of the equator and an equal amount south, the ecliptic is a great circle crossing the equator at an angle, not a small circle sitting entirely above or below the equator.
Watch the diagram above for a while and you will see the Sun slowly drifting eastward along the ecliptic, passing through one zodiac constellation after another. Notice how it also drifts north and south of the blue equator as it goes: highest when near Cancer, lowest when near Capricorn. This north-south drift is what produces the seasons.
Equinoxes and solstices
The ecliptic crosses the equator at two points and reaches its maximum distance from the equator at two others; these four points are the equinoxes and solstices. You can see them in the diagram: the two crossing points, where the gold ecliptic meets the blue equator, are the equinoxes; the point where the ecliptic is furthest above the equator (near Cancer) is a solstice, and the point where it is furthest below (near Capricorn) is the other.
These are fixed geometrical points on the celestial sphere, defined purely by the relationship between the two circles. When the Sun, in its yearly journey along the ecliptic, arrives at one of these points, we call that day an equinox or a solstice.
- On an equinox day (20 March and 22 September), the Sun sits exactly on the equator, and day and night are roughly equal in length everywhere on Earth.
- On a solstice day (21 June and 21 December), the Sun is as far from the equator as it ever gets, making it the longest or shortest day of the year, depending where you are.
The diagram above shows the sky as seen by an observer in the northern hemisphere; for this person, the Sun is highest in the sky when it is near Cancer, and that June day (when the Sun is at its northernmost point) is the longest day of the year.
In everyday language, we tend to name these events by season: the "summer solstice", the "spring equinox", and so on. But this convention is hemisphere-dependent: the summer solstice in the northern hemisphere is the winter solstice in the southern hemisphere, and vice versa. Therefore, we can also call the solstice near Cancer the "June solstice" or "solstice at Cancer", etc.
These days have been important for astronomers mostly because of how geometrically useful those four points on the celestial sphere are for measurements and calculations.
The speeds in the diagram are heavily exaggerated. Orion as well as every other constellation and star completes one full daily rotation every 13 seconds, representing about 24 hours in reality. The Sun, meanwhile, takes roughly 2¾ minutes to drift through all twelve zodiac signs. In reality, the Sun takes a full year (about 365 days) to complete this journey, so its ecliptic motion is exaggerated here by about 29 times.
The zodiac constellations
The constellations that lie along the ecliptic band are called the zodiac constellations. They are important not because they are intrinsically special (they are patterns of stars like any others) but because they happen to occupy the band of sky through which the Sun, Moon, and planets pass. They serve as a backdrop against which the motions of these wandering bodies can be tracked and described.
Orion (shown in blue) is not a zodiac constellation. It sits near the celestial equator, not on the ecliptic. It is included in the diagram as an example of how ordinary constellations behave: they participate in the daily rotation, rising in the east and setting in the west, but they do not lie on the Sun's path. The ecliptic does not have its own independent motion; it is carried along by the daily rotation just like everything else. What makes the ecliptic special is that the Sun, Moon, and planets slowly creep along it in their own time, against the background of the fixed stars.
The Sun spends roughly one month in each zodiac sign. This is the origin of astrological "star signs": to say someone is "an Aries" means they were born when the Sun was in the Aries section of the ecliptic. "Aries season" is simply the month when the Sun passes through that constellation. Whatever one thinks of astrology as a practice, the underlying astronomy (the Sun's position along the ecliptic at any given time of year) is real and observable.
How to track the Sun against the stars
You may wonder how the Sun's motion against the stars could ever be observed, given that when the Sun is visible it is daytime and the stars are washed out. This is exactly the difficulty. You cannot directly watch the Sun moving past the stars, because by definition the Sun's presence means the stars around it are invisible. Even during twilight, when some stars near the Sun become briefly visible, the Sun's daily drift along the ecliptic is far too small to notice; it moves less than a degree per day, and twilight is too short to track such a tiny shift.
Instead, the Sun's yearly journey is detected indirectly, by comparing the night sky on different nights weeks or months apart. If you note which constellations are visible at the same hour on different dates, you find that they gradually shift westward, because the Sun has moved eastward into the part of the sky that was previously dark, pushing the boundary of visibility along with it.
A Copernican update
Ptolemy understood the ecliptic as the path the Sun traces around the Earth. He was wrong about that, but not about the geometry. What Copernicus showed, and what we now know, is that the ecliptic is actually the plane of the Earth's orbit around the Sun. The Sun does not move along the ecliptic; the Earth does. The Sun only appears to drift through the zodiac because we are orbiting it: as the Earth moves, the Sun's position against the background stars shifts.
The diagram below shows the same sky from this Copernican perspective. The Sun sits at the centre. The Earth orbits it in the ecliptic plane (shown as a translucent disc cutting through the celestial sphere). The zodiac constellations line up along the ecliptic because they mark the background behind the Sun as seen from Earth at different points in its orbit.
The Copernican view. The Sun is at the centre; the Earth orbits it in the ecliptic plane. The celestial equator (blue) is tilted because the Earth's axis is tilted. Orion sits on the equator, not the ecliptic.
What we see in the night sky as the celestial equator is simply the geographical equator (the real line on maps at 0° latitude) projected outward onto the celestial sphere. The celestial equator is tilted with respect to the ecliptic because the Earth's axis of rotation is tilted from the perpendicular to its orbital plane. Determining the exact angle of this tilt is the subject of the next chapter.
Why doesn't the Earth's equator point at Orion?
If you look at the Copernican diagram, you may notice that the Earth's equator (the red ring) does not seem to "point at" Orion on the far sphere. This is because of the point-like nature of the Earth discussed in Chapter 3: the Earth is so small compared with the celestial sphere that its equator and the celestial equator are effectively the same circle. Even the Earth's entire orbit is negligibly small compared with the distance to the stars.