The Ecliptic or the Apparent Orbit of the Sun

Image description: The planets of the solar system with their orbital inclinations or the angles in degrees at which a planet's orbit around the Sun is inclined relative to the plane of the ecliptic. Vito Technology, Inc.

The Ecliptic

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

In other words, the ecliptic is a great circle inclined relative to the celestial equator by about 23.5° (the angle of the Earth's axial tilt). It is the path that the Sun seems to follow through the Zodiac Constellations during the year.

Position of the Planets Along the Ecliptic in the Northern Hemisphere Sky

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

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

At the equator (0° latitude), how can we visualize the apparent movement of the ecliptic during 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 more and more inclined relative to the local horizon.
• Solar Noon: At noon, when the Sun is at its highest in the sky, the ecliptic reaches its maximum angle of inclination relative to the local horizon, about 66.5 degrees (90° - 23.5°).
• Afternoon: After noon, the ecliptic begins to tilt back towards 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 movement of the ecliptic follows a similar pattern 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 more and more vertical.
• Midnight: At midnight, the ecliptic reaches its maximum angle of inclination relative to the local horizon, about 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.

At a latitude of 45°, how can we visualize the apparent movement of the ecliptic during 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 more and more inclined relative to the local horizon. The angle of inclination increases progressively.
• Solar Noon: At noon, when the Sun is at its highest in the sky, the ecliptic reaches its maximum angle of inclination relative to the local horizon, about 68.5 degrees (90° - 45° + 23.5°).
• Afternoon: After noon, the ecliptic begins to tilt back towards 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 movement of the ecliptic follows a similar pattern 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 more and more vertical.

At a latitude of 90° (the North Pole), the apparent movement of the ecliptic during 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 Earth's axial tilt, the Sun can remain above or below the horizon for extended periods, depending on the season. During polar day (midnight sun) periods, the Sun remains above the horizon for 24 hours. During polar night periods, the Sun remains below the horizon for 24 hours.
• Morning: During polar day periods, the Sun describes a circular path in the sky, always remaining above the horizon. The ecliptic, which is the plane of the 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 at its 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 always 23.5 degrees. The Sun reaches its highest point in the sky, but it remains relatively low relative to the horizon due to the Earth's axial tilt.
• Afternoon: During polar day periods, the Sun continues its circular path 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 polar day periods, the Sun does not set but continues its circular path in the sky. During polar night periods, the Sun remains below the horizon for 24 hours.
• Early Night: During polar night periods, 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 polar day periods, the Sun is at the lowest point of its circular path in the sky, but it 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 does not always divide 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.

Latitude of the Observer

At the equator: The ecliptic is almost parallel to the horizon during the equinoxes. In this case, half of the ecliptic may be visible above the horizon and the other half below. However, this can change slightly over 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 polar day months, meaning that the entire ecliptic is visible above the horizon for a 24-hour period, without setting.

• At the equator: During the equinoxes, the ecliptic is almost parallel to the horizon, but due to the 23.5° inclination of the ecliptic relative to the celestial equator, half of the ecliptic may be visible above the horizon and the other half below. During the solstices, the inclination of the ecliptic means that more or less of the ecliptic is visible above the horizon.
• At 45° latitude: The ecliptic is more inclined relative to the horizon, meaning that a larger portion of the ecliptic can be visible above the horizon during part of the year. The proportion of the ecliptic visible above the horizon varies depending on the season.
• At the poles: During polar day months, the ecliptic is parallel to the horizon and the entire ecliptic is visible above the horizon for 24 hours. During polar night months, the ecliptic is below the horizon and not visible.

The Season and the Position of the Sun

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

Position of the Planets Along the Ecliptic in the Northern Hemisphere Sky

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

• 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 amateur astronomers 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 zodiacal constellations.