Ecliptic

The ecliptic or ecliptic plane is the path the Sun appears to follow in the sky as seen from Earth. This happens because of the way the Earth moves around the Sun. Ancient people used this path to help them understand the stars, make calendars, and even predict special events like eclipses, which only happen when the Moon crosses this path.

We can see the Sun moving against the stars, especially along a group of constellations called the Zodiac. The planets in our Solar System also seem to move along this same path because their orbits are very close to Earth's. The Moon is nearby too, but its path is tilted just a little bit, crossing the ecliptic at special points called the lunar nodes.

The ecliptic is very important for scientists. It helps us measure angles in the sky and understand how Earth's axis is tilted. By comparing the ecliptic to another plane called the equatorial plane, ancient scientists discovered that Earth's tilt is about 23.4 degrees. This knowledge helps us today in studying the sky and space.

Sun's apparent motion

The ecliptic is the apparent path of the Sun throughout the year. Because Earth takes one year to orbit the Sun, the Sun appears to move across the sky, completing a full circle in that time. Each day, the Sun moves a little less than 1° eastward against the background stars. This movement causes the time when the Sun is directly overhead to shift slightly each day.

The speed at which Earth orbits the Sun changes a bit during the year, which means the Sun's apparent speed along the ecliptic also changes. For example, the Sun spends about 185 days north of the celestial equator and about 180 days south of it each year. Additionally, the motion of Earth around the Earth–Moon center of mass causes the Sun's path to wobble slightly each month.

Inclination to the plane of the Solar System

Main article: Solar System

Most of the big objects in our Solar System go around the Sun in almost the same flat space. Scientists think this happened because these objects formed from a big cloud of dust and gas that spun around the Sun in one direction. The middle flat space of the Solar System is called the invariable plane, and it goes through the center of all the objects and is perpendicular to their spinning motion. Earth's path around the Sun, called the ecliptic, leans just a little more than 1° from this middle flat space. Other planets lean only a little bit more, so most objects in the Solar System seem very close to the ecliptic when we look up at the sky.

The invariable plane is found by adding up all the spinning motions of everything in the Solar System. Because we don't know exactly where this plane is, and because the ecliptic is clearly shown by the Sun's apparent movement, we use the ecliptic as our main flat space for measuring in the Solar System. The only problem with this is that, over very long times, the ecliptic will shift its position against faraway stars in the sky.

Inclination to the galactic plane

The ecliptic leans 60° from the flat space that our whole galaxy makes.

Top and side views of the plane of the ecliptic, showing planets Mercury, Venus, Earth, and Mars. Most of the planets orbit the Sun very nearly in the same plane in which Earth orbits, the ecliptic.

Referencing Earth's equator

Further information: Axial tilt

Because Earth's rotational axis is not straight up and down compared to its path around the Sun, Earth's equatorial plane does not line up perfectly with the ecliptic plane. Instead, it leans at an angle of about 23.4°. This angle is called the obliquity of the ecliptic. If we imagine the equator stretching out into space, it meets the ecliptic at two special points called the equinoxes. At these points, the Sun appears to cross from one side of the equator to the other. One crossing happens in March, moving from south to north, and the other in September, moving from north to south.

Celestial reference plane

The ecliptic is one of the main planes used to describe positions in the sky, with the other being the celestial equator. The points directly above and below the ecliptic are called the ecliptic poles. When we talk about where objects are in the sky using the ecliptic, we use special directions called ecliptic longitude and latitude. Longitude tells us how far east along the ecliptic an object is, starting from the March equinox. Latitude tells us how far north or south from the ecliptic the object is.

Using ecliptic directions works well for objects in our Solar System because most planets stay close to the ecliptic. Since Earth's path changes very little, the ecliptic stays almost the same compared to the stars. However, because of a slow wobble in Earth's axis, called precession, the positions we measure using the ecliptic keep changing over time. To know exactly where something is, we need to say which date's equinox we are using, called an epoch.

Change of inclination

The angle between Earth's orbit and its axis, called the obliquity, is slowly getting smaller. It changes by about half an arcsecond every hundred years because of the influence of other planets.

Scientists figure out this angle by watching how Earth and the planets move over many years. They use this information to make better predictions and calculations for things like the position of the stars. Different methods have been used over time, from older calculations to modern computer models, to get more accurate results. These models help us understand the sky very precisely, even thousands of years into the past or future.

Equinoxes and solstices

Main article: Equinox (celestial coordinates)

The times called equinoxes and solstices happen when the Sun's path in the sky reaches special points. These moments occur when the Sun's position is at 0°, 90°, 180°, and 270° along its path. Because Earth's orbit is not perfect and our calendar has some unusual patterns, the exact dates of these events can change a little each year.

Eclipses

Main article: Eclipse

Eclipses happen when the Moon crosses the path that the Sun appears to follow in the sky. This path is called the ecliptic. Even though the Moon's orbit is tilted a little, eclipses only occur when the Moon is near certain points called nodes while the Sun and Moon are lined up. This is why we don't see eclipses every month.

In the constellations

The ecliptic passes through thirteen special groups of stars called constellations. These are known as the zodiac, and they include Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus, and Aquarius.

There are twelve more constellations close to the ecliptic where the Moon and planets can sometimes appear. These include Cetus, Pegasus, Aquila, Scutum, Serpens, Hydra, Corvus, Crater, Sextans, Canis Minor, Auriga, and Orion. Venus is special because it can move through all twenty-five of these constellations.

Astrology

Main article: Astrology

The ecliptic is important in the zodiac, which is a band of sky about 20° wide. The Sun, Moon, and planets always seem to move through this band. Long ago, people divided this band into 12 parts, called signs, each covering about one month of the Sun's path. These signs were named after constellations that line up with the ecliptic. One special point, called the "First Point of Aries", was named when the Sun seemed to be in the constellation Aries during the March equinox. But because of Earth's slow wobble called precession of the equinoxes, this point has shifted into the constellation Pisces.