The Solar System has a second plane where objects orbit the Sun

Artist's impression of the ecliptic plane (yellow), and the recently-discovered "empty" ecliptic (blue) in our solar system. (Credit: NAOJ)

Almost all the objects orbiting the sun live in a particular plane, called the ecliptic plane. But a recent analysis of long-period comets reveals a second home, a so-called “empty ecliptic”. And it may be populated with comets dragged there by none other than the gravity of the Milky Way galaxy.

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Zodiacal light can be seen in the sky before sunrise or after sunset. Credit: Yuri Beletsky/ESO Paranal

Imagine you could see the position of the Sun, in the sky, relative to the stars (and galaxies, and quasars, and …). If you could, and if you plotted that position throughout the year you’d get a line; that line is called the ecliptic.

And why is it called the ecliptic? Because when the new or full Moon is very close to this, there will be an eclipse (of the Sun, and Moon, respectively).

The Earth goes round the Sun, in an orbit. That orbit defines a plane, which is an infinite two-dimensional sheet; the plane of the ecliptic.

The other planets in the solar system orbit the Sun in planes too, but those planes are slightly tilted with respect to the plane of the ecliptic … so transits of Venus (across the Sun) are quite rare (most times Venus passes either above or below the Sun, when it’s between Earth and the Sun). Mutual transits and occultations of planets are even rarer.

If you’re in a location relatively free of light pollution, on a clear, moonless night you may see zodiacal light. If you trace a line through the middle of it, you’re tracing the ecliptic (zodiacal light is due to reflection of sunlight off dust; dust in the solar system is concentrated in a plane close to the ecliptic plane).

Today astronomers use equatorial coordinates to give positions on the sky, right ascension (RA) and declination (Dec); these are like projections of longitude and latitude out into space (or onto the celestial sphere). However, in Europe ecliptic coordinates were used (up to the 17th century anyway). Here’s a curious fact: historically, Chinese astronomers used equatorial coordinates!

Universe Today stories: Plane of the Ecliptic, Vernal Equinox – Busting the Myth of Balancing Eggs, and Find the Zodiacal Light.

More: Astronomy Cast on Orbit of the Planets, and a Glow After Sunset.

Plane of the Ecliptic

Solar eclipse. Credit: NASA


Plane of the ecliptic, also known as the ecliptic plane, is a phrase you will often hear in astronomy. A basic definition is that the plane of the ecliptic is the plane of the Earth’s orbit, but that does not mean much to most people. Space is a three-dimensional vacuum, which you can think of as a kind of pool with the planets suspended in it. The Earth orbits the Sun on a particular angle and its orbit is elliptical in shape. The orbit is often shown as an ellipse made of dotted lines with the Sun at its center. If you made this ellipse a solid surface and extended it infinitively, then you would have the plane of the ecliptic. Actually our entire Solar System can be thought of as flat because all of the planets’ orbits are near or on this plane.

The ecliptic plane is used as the main reference when describing the position of other celestial objects in our Solar System. The angle between the plane of the ecliptic and the plane of an orbit is called the inclination. Until it was stripped of its status as a planet, Pluto was the planet with the most extreme inclination – 17°. Mercury is the only other planet with a significant inclination of 7°. There is also a 7° inclination between the plane of the Sun’s equator and the ecliptic plane. There are other celestial bodies that have a much greater inclination than any of the planets, such as Eris with a 44° inclination or Pallas with a 34° inclination.

The ecliptic plane got its name from the fact that a solar eclipse can only happen when the Moon crosses this plane to block out the Sun. Our Moon crosses the ecliptic about twice a month. A solar eclipse occurs when a new Moon crosses the ecliptic, and a lunar eclipse occurs when a full Moon crosses it.

Seasons on Earth are caused by our planet’s axial tilt of 23.5°, which causes variations in the amount of sunlight different parts of the Earth receive. This goes for all the other planets too. For example, Uranus rotates on its side with an axial tilt of 97.8°, which results in extreme variations in its seasons. The eclipse is also home to the constellations of the zodiac. There are twelve constellations in the zodiac, which are important symbols in astrology and can also be found in the Chinese calendar.  Here’s a list of all the zodiac symbols.

Universe Today has a number of articles including Virgo one of the zodiac signs and axial tilt.

You should also check out these articles on the ecliptic plane and ecliptic facts for more information.

Do not forget to tune into Astronomy Cast’s episode about the planet’s orbits.

NASA: The Path of the Sun, the Ecliptic