Unveiling the Secrets of the Ecliptic: Exploring the Path of the Sun and its Celestial Significance
Ecliptic is a term commonly used in astronomy to describe the apparent path of the Sun in the sky as seen from Earth. It is an imaginary circle formed by the intersection of the Earth's orbital plane around the Sun and the celestial sphere. The ecliptic is of great significance as it defines the plane in which the Earth, Moon, and most other planets orbit around the Sun. It serves as a reference for measuring celestial positions and is an important component in understanding the seasons, zodiac signs, and the motion of celestial objects in the night sky.
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. Let's figure out what this term(ecliptic) 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.
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.
Halley's Comet is one of the famous short-period comets. It’s located about 5° from the ecliptic. The image based on the Sky tonight data shows Halley's Comet at its next perihelion on July 29, 2061, in the Northern Hemisphere.
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.
Celestial equator and ecliptic
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.
Ecliptic coordinates
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.
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