Ecliptic: Definition and the Sun's Orbit

The ecliptic

Definition of the ecliptic

The ecliptic is a projection into the sky of the apparent orbit of the Sun around the Earth. In physical terms, it corresponds to the plane of Earth's orbit around the Sun.

Inclination and zodiac constellations

In other words, the ecliptic is a great circle inclined relative to the celestial equator by about 23.5° (the angle of the obliquity of Earth's axis). It is the path that the Sun appears to follow through the zodiac constellations over the course of the year.

Position of the planets along the ecliptic in the sky of the northern hemisphere

Motion of the planets along the ecliptic

The planets of the Solar System (Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune) move within a narrow band around the ecliptic called the plane of the ecliptic. Due to the slight inclination of planetary orbits relative to this plane, the planets are always close to the ecliptic, as in the image.

Influence of latitude and seasons

The ecliptic forms an angle of 23.5 degrees with the celestial equator (projection of Earth's equator). The proportion of the ecliptic visible above the horizon depends on the observer's latitude and the season.

The apparent motion of the ecliptic at the equator

At the equator (0° latitude), how can one visualize the apparent motion of the ecliptic over the course of the day?

• Sunrise: at sunrise, the ecliptic is almost parallel to the local horizon. The angle of inclination relative to the horizon is therefore close to 0 degrees.
• Morning: as the Sun rises in the sky, the ecliptic seems to straighten up, becoming increasingly inclined relative to the local horizon.
• Solar noon: at noon, when the Sun is highest in the sky, the ecliptic reaches its maximum angle of inclination relative to the local horizon, approximately 66.5 degrees (90° - 23.5°).
• Afternoon: after noon, the ecliptic begins to tilt back toward the local horizon, gradually returning to a more horizontal position.
• Sunset: at sunset, the ecliptic is again almost parallel to the local horizon, with an angle of inclination close to 0 degrees.
• Early night: at night, the apparent motion of the ecliptic follows a pattern similar to that observed during the day, but with the stars and planets visible instead of the Sun. As the night progresses, the ecliptic begins to tilt relative to the local horizon, becoming increasingly vertical.
• Midnight: at midnight, the ecliptic reaches its maximum angle of inclination relative to the local horizon, approximately 66.5 degrees (90° - 23.5°). This is the point where the ecliptic is most vertical relative to the horizon.
• Dawn: just before sunrise, the ecliptic is again almost parallel to the local horizon, with an angle of inclination close to 0 degrees.

The apparent motion of the ecliptic at 45° latitude

At a latitude of 45°, how can one visualize the apparent motion of the ecliptic over the course of the day?

• Sunrise: at sunrise, the angle of inclination of the ecliptic relative to the local horizon is about 23.5 degrees.
• Morning: as the Sun rises in the sky, the ecliptic seems to straighten up, becoming increasingly inclined relative to the local horizon. The angle of inclination progressively increases.
• Solar noon: at noon, when the Sun is highest in the sky, the ecliptic reaches its maximum angle of inclination relative to the local horizon, approximately 68.5 degrees (90° - 45° + 23.5°).
• Afternoon: after noon, the ecliptic begins to tilt back toward the local horizon, gradually returning to a more horizontal position.
• Sunset: at sunset, the ecliptic is again inclined relative to the local horizon, with an angle of inclination of about 23.5 degrees.
• Night: at night, the apparent motion of the ecliptic follows a pattern similar to that observed during the day, but with the stars and planets visible instead of the Sun. As the night progresses, the ecliptic begins to tilt relative to the local horizon, becoming increasingly vertical.

The apparent motion of the ecliptic at the North Pole

At a latitude of 90° (the North Pole), the apparent motion of the ecliptic over the course of the day is very different from that observed at the equator or at intermediate latitudes.

• Sunrise: at the North Pole, the concept of "sunrise" is different from that observed at lower latitudes. Due to the tilt of Earth's axis, the Sun can remain above or below the horizon for extended periods, depending on the season. During periods of polar day (midnight sun), the Sun remains above the horizon for 24 hours. During periods of polar night, the Sun remains below the horizon for 24 hours.
• Morning: during periods of polar day, the Sun describes a circular trajectory in the sky, always remaining above the horizon. The ecliptic, which is the plane of Earth's orbit around the Sun, appears as a horizontal line that circles the horizon. The angle of inclination of the ecliptic relative to the local horizon is constant and equal to 23.5 degrees, but it is always parallel to the horizon.
• Solar noon: at noon, when the Sun is highest in the sky, the ecliptic remains parallel to the local horizon. The angle of inclination of the ecliptic relative to the local horizon is still 23.5 degrees. The Sun reaches its highest point in the sky, but it remains relatively low relative to the horizon due to the tilt of Earth's axis.
• Afternoon: during periods of polar day, the Sun continues its circular trajectory in the sky, always remaining above the horizon. The ecliptic remains parallel to the local horizon, with a constant angle of inclination of 23.5 degrees.
• Sunset: at the North Pole, the concept of "sunset" is also different. During periods of polar day, the Sun does not set but continues its circular trajectory in the sky. During periods of polar night, the Sun remains below the horizon for 24 hours.
• Early night: during periods of polar night, the Sun remains below the horizon for 24 hours. The ecliptic, which is always parallel to the local horizon, is not visible because it is below the horizon.
• Midnight: at midnight, during periods of polar day, the Sun is at the lowest point of its circular trajectory in the sky, but it still remains above the horizon. The ecliptic remains parallel to the local horizon, with a constant angle of inclination of 23.5 degrees.

The circle of the ecliptic and the horizon

The ecliptic is indeed a complete circle on the celestial sphere, but this circle is not always divided equally between the part visible above the horizon and the part below. The proportion of the ecliptic visible above the horizon depends on the latitude and the season.

Observer's latitude

At the equator: the ecliptic is almost parallel to the horizon during the equinoxes. In this case, half of the ecliptic can be visible above the horizon and the other half below. However, this can change slightly over the course of the year due to the 23.5° inclination of the ecliptic relative to the celestial equator.

At 45° latitude: the ecliptic is more inclined relative to the horizon. A larger portion of the ecliptic can be visible above the horizon during part of the year, but there is always a portion below, depending on the season.

At the poles (90° latitude): the ecliptic follows a circle parallel to the horizon during the months of polar day, which means that the entire ecliptic is visible above the horizon for a 24-hour period, without setting.

The season and the position of the Sun

• At the equinoxes (March 21 and September 21), the ecliptic is almost at 0° inclination relative to the horizon at the equator, so the Sun's trajectory crosses the horizon and day and night are equal.
• At the summer solstice (around June 21), the ecliptic is higher in the sky in the northern hemisphere, which makes the upper part of the ecliptic more visible and the Sun travels higher in the sky.
• At the winter solstice (around December 21), the ecliptic is lower in the sky of the northern hemisphere, and the lower part of the ecliptic is more visible, with shorter days.

Observing the ecliptic in the northern hemisphere

Points of intersection with the celestial equator

The ecliptic crosses the celestial equator at two points called the equinoxes (vernal and autumnal points). These points define where it cuts the celestial equator. The inclination of the ecliptic reaches its maxima near the solstices, thus marking extreme angles relative to the horizon.

Observation methods

• With the naked eye: in the evening, look for an alignment of planets or the Moon; they trace the ecliptic.
• With tools: there are many sky observation applications that can help astronomy enthusiasts explore the night sky, identify constellations, planets, stars and other celestial objects (Stellarium, SkySafari, Star Walk 2, Night Sky, Google Sky Map, Celestron SkyPortal, etc.).

In summary, in the northern hemisphere, the ecliptic appears as an inclined band in the sky, clearly marked by the alignment of the planets and the zodiac constellations.