Have you ever noticed a yellow dotted line stretching across the sky in stargazing apps like Star Walk 2 or Sky Tonight? That’s the ecliptic — probably the most significant celestial reference line. In this article, we’ll explore what the ecliptic is, why it’s so important, and how it relates to the zodiac constellations, planetary alignments, eclipses, and other beautiful celestial events.
What is the ecliptic?
The ecliptic is an astronomical term that reflects the Earth’s movement around the Sun and is observed as an imaginary circle on the celestial sphere and as a plane. Let’s figure out what this term means.
The Earth’s orbital plane
As we know from school, the Earth revolves around the Sun, and it takes a year to complete one rotation. The path our planet follows around the Sun through the year is the Earth’s orbit. The plane containing the Earth’s orbit is called the Earth’s orbital plane.
The Earth’s orbital plane intersects the celestial sphere — an imaginary sphere centered on the Earth, representing the appearance of the sky from our planet. The celestial sphere is like a globe with stars, planets, and other celestial objects drawn on it. And the ecliptic is the circle at which the Earth’s orbital plane crosses the celestial sphere. In its turn, the plane containing the ecliptic is called the plane of the ecliptic — it coincides with the Earth’s orbital plane.
The apparent path of the Sun on the celestial sphere
As the Earth orbits the Sun, our star moves along the ecliptic on the celestial sphere. The Sun slowly changes its position in the sky throughout the year, shifting a little less than 1° per day. It appears higher in summer and lower in winter, returning to its original position every year. So, the ecliptic also represents the apparent path of the Sun on the celestial sphere throughout the year.
What objects can you find near the ecliptic?
Most of the Solar System bodies lie close to the ecliptic plane. This means that they orbit the Sun in roughly the same plane as the Earth.
Our Solar System may be generally perceived as a flat disk, and the reason for this is the way it was formed. Scientists believe that billions of years ago, the dust and gas surrounding our Sun were pulled into a disk by gravity; all of today’s planets and smaller objects formed out of this disk and remained more or less in the same plane.
Solar System planets
The Solar System planets lie very close to the ecliptic plane, but they are not crossed by it. The orbits of the planets tilt from the ecliptic plane by a few degrees at most. Mercury has the largest tilt (7°), and the other planets range from 0.8° to 3.2°.
Since the planets travel near the path of the ecliptic, they often meet each other in the sky, creating amazing celestial events. When two Solar System planets pass within a few degrees from each other in the sky, it’s called a close approach. When at least three planets appear close together in a small sky sector, it’s called a planetary alignment.
Do you want to explore the Solar System planets even more closely? Take our quiz and try to guess the planets by their features!
The Moon
The orbit of the Earth’s Moon tilts 5.1° from the ecliptic plane. The Moon’s orbit crosses the ecliptic plane at two points: ascending node (after which the Moon rises slightly above the ecliptic) and descending node (after which the Moon sinks below the ecliptic). If the Full Moon coincides with the moment when the Moon crosses the ecliptic, we experience a lunar eclipse; if it occurs during the New Moon, we experience a solar eclipse. The Moon crosses the ecliptic about twice a month, but not always during the New Moon or Full Moon phases, so we don’t experience eclipses every time.
The Moon and planets also meet in the sky from time to time. When our natural satellite and a planet pass within a few degrees from each other in the sky, it’s called a close approach of the Moon and a planet. Sometimes the Moon even hides a planet, which is called the lunar occultation — such events are rare and can only be seen from small parts of the world.
Small Solar System bodies — asteroids, dwarf planets, and more
Most small Solar System bodies are also located near the ecliptic — such as asteroids and dwarf planets of the main asteroid belt and the Kuiper belt. However, dwarf planets are usually inclined at larger angles from the plane of the ecliptic. For example, Pluto’s orbit is tilted 17.2° from the Earth’s orbital plane.
Short-period comets (the ones that take less than 200 years to orbit the Sun) orbit close to the plane of the ecliptic, too. But not all the comets behave this way: long-period comets (those with orbital periods longer than 200 years) are randomly oriented and do not necessarily follow the ecliptic plane.
By the way, do you know the difference between an asteroid and a comet? Test your knowledge of small Solar System bodies with our quiz!
Ecliptic constellations — the zodiac and the “forgotten” constellation
Apart from the Solar system objects, you can observe stars near the ecliptic. The ecliptic intersects 13 constellations on the celestial sphere, which are the ecliptic constellations. Twelve of them are famous for their association with astrology and are called zodiac constellations. They are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces. There is a 13th “forgotten” constellation called Ophiuchus which is not part of the zodiac but is also the ecliptic constellation.
If you want to learn more about the zodiac constellations and how they relate to the zodiac signs — read our dedicated article.
Why is the ecliptic important?
You can use the ecliptic as a guideline to find the planets, zodiac constellations, and small Solar System objects in the sky above you with stargazing apps like Star Walk 2 and Sky Tonight. In these apps, the ecliptic is pictured as a yellow dotted line stretching across the sky. The most interesting targets to spot near the ecliptic are Solar System planets. To easily find them, follow these simple instructions.
Find planets with the ecliptic in Sky Tonight
The Sky Tonight app will help you find planets in a few steps:
- Step 1: Launch the app and tap the blue compass button — the app will use your device’s location to adjust the image to the real sky above you.
- Step 2: Move your device until you find the yellow dotted line on the screen — this is the ecliptic.
- Step 3: Move your device along this line, and you’ll see several large dots on the sides of the ecliptic — these are the planets.
- Step 4: If you center your device on a planet or tap on it, you will see its name. You can observe the planets in the real sky in the directions that the app shows you.
Find planets with the ecliptic in Star Walk 2
It’s as easy to locate planets with the Star Walk 2 app. To do this, launch the app and tap the compass icon in the top right corner of the screen — the app will automatically adjust the image to match the actual sky above you. Then, follow steps 2–4 in the previous guide and enjoy observing the planets.
Celestial equator and ecliptic
The Earth’s celestial equator is the circle at which the Earth’s geographical equatorial plane crosses the celestial sphere. And it’s important to note that the celestial equator doesn’t coincide with the ecliptic.
Our planet doesn’t stand upright in its orbit: the Earth’s rotational axis is inclined at 23.5 degrees from the Earth’s orbital plane. Since the Earth’s equatorial plane is perpendicular to the rotational axis, it is inclined from the ecliptic plane at the same angle. As a result, the Earth’s celestial equator is inclined at 23.5 degrees from the ecliptic on the celestial sphere. And this causes a few more celestial events.
Solstices & Equinoxes
The celestial equator and the ecliptic intersect on the celestial sphere at two points — these points are known as equinoxes. They mark the transition from astronomical summer to fall (autumn equinox) and from winter to spring (spring equinox).
The two points on the ecliptic where it is at its farthest from the celestial equator are called solstices. They mark the transition from astronomical fall to winter (winter solstice) and from spring to summer (summer solstice). A hemisphere that is tilted toward the Sun at one of these points experiences the summer solstice, while the other hemisphere, tilted away from the Sun, experiences the winter solstice.
Axial precession and precession of the equinoxes
The Earth not only is tilted in relation to its orbit, but it also wobbles as it spins. Imagine a spinning top: when it moves slowly, you can see how its top traces a little circle in the air while moving. Similarly, the Earth traces a circle in space with its axis, completing one circle in about 25,800 years. This wobble of the Earth’s axis is known as the axial precession.
Due to the axial precession, the intersection points of the celestial equator and the ecliptic (i.e. equinoxes) shift slightly over time — this effect is called the precession of the equinoxes. As a result, the position of the Sun relative to the stars at these points gradually changes.
For example, about 2,000 years ago, at the point of the March equinox, the Sun was located in the zodiac constellation Aries. That’s why this point was also called the First Point of Aries. However, nowadays, the intersection points of the ecliptic and the celestial equator have shifted, and the Sun “visits” Pisces at the March equinox.
The axial precession also means that the position of the north celestial pole is not fixed. Currently, the star Alpha Ursae Minoris is closest to the north celestial pole and serves as our North Star. However, in about 13,000 years, the star Vega will be the next brightest North Star, as the Earth’s axis will point in its direction.
Do you know where to find Polaris and Vega in the sky? Test your knowledge with our quiz about the brightest stars and their locations!
Ecliptic coordinates
The ecliptic is the basis for one of the oldest coordinate systems in astronomy. It was documented as early as the 2nd century A.D. in the Almagest by Claudius Ptolemy, and it’s still commonly used today. The ecliptic coordinate system describes the positions and movements of the objects within our Solar System in relation to the ecliptic. Since most Solar System objects travel close to the ecliptic plane, this coordinate system is very convenient for astronomical observations. Moreover, the plane of the ecliptic does not depend on the Earth’s axial precession, unlike the celestial equator. It is fixed in space, which makes it a reliable reference for astronomers.
F.A.Q.
What does the ecliptic represent?
The ecliptic represents the circle at which Earth’s orbital plane crosses the celestial sphere. From our perspective, it’s the apparent path the Sun takes across the sky throughout the year.
Why does the Sun appear to move across the sky?
The Sun appears to move across the sky because the Earth revolves on its axis and revolves around the Sun. The Earth’s 24-hour rotation causes the Sun to rise and set, while its 365-day orbit causes the Sun’s position to shift throughout the year.
Are all the planets on the same plane?
All the Solar System planets orbit the Sun close to the same plane, called the ecliptic plane. Although only the Earth moves on the ecliptic plane, the orbits of other planets are tilted from it by just a few degrees. This means that the planets are always located in a narrow band of the sky and can easily form planetary alignments.
What is the difference between the ecliptic and orbital planes?
The term “orbital plane” can be referred to the orbit of any celestial object around its star, while the ecliptic plane is the plane of the Earth’s orbit around the Sun.
What is the difference between the celestial equator and the ecliptic?
The ecliptic is the circle at which the Earth’s orbital plane crosses the celestial sphere. The celestial equator is the circle at which the Earth’s terrestrial (geographic) equatorial plane crosses the celestial sphere. So, these are just two separate astronomical terms.
When is the Sun on the celestial equator?
The Sun crosses the celestial equator two times a year — during the vernal and fall equinoxes.
Where does the name ecliptic come from?
The term “ecliptic” is derived from the Greek word “ekleipsis,” meaning “fail to appear.” This name was given because ancient astronomers observed that the Sun “disappeared” from the sky at the points where its path intersected with the Moon’s orbit. Therefore, the Sun’s path in the sky was named “the ecliptic” after eclipses.
Bottom line
The ecliptic is the circle at which the Earth’s orbital plane crosses the celestial sphere. It is represented as the path of the Sun across the sky throughout the year. And it’s an essential reference line in astronomy. You can use it to locate planets, the Moon, zodiac constellations, and small Solar System objects. It’s like a sky highway from which you begin your journey into the world of astronomy. Locate the ecliptic with the stargazing apps Sky Tonight or Star Walk 2 and keep an eye on it to ensure that you don’t miss out on any celestial marvels!
Text Credit: Vito Technology, Inc.
