What is the ecliptic?
The ecliptic is the path the Sun appears to trace against the stars over a year, which is really Earth's own orbit seen from the inside. The zodiac is a band along it, measured in 360 degrees from the spring equinox. Every sign, aspect and planetary position is a point on this line.
What the word means
Stand on Earth for a year and watch where the Sun sits against the background stars (you can't see them by day, but astronomers can work it out). It creeps east about a degree a day and comes back to the same spot after a year. That track is the ecliptic. It is our own orbit, seen from the inside: the Sun only looks as if it moves because we do. 1,2
The name comes from eclipses. An eclipse can only happen when the Moon crosses this line at new or full moon, so the line of the Sun became the line of eclipses. 1
The Moon and the planets never stray far from the ecliptic, because the solar system is fairly flat. That is why the old astronomers drew a band a few degrees either side of it and watched everything happen there. The band is the zodiac. 3
Why astrologers care
Every position in a horoscope is an ecliptic longitude: how many degrees along the line, counted from where it crosses the celestial equator in spring. Zero to 30 degrees is Aries, 30 to 60 is Taurus, and so on round to 360. When a chart says the Moon is at 14° Virgo, it means 164 degrees along the ecliptic. 1
Even the rising sign is an ecliptic point: the degree of this line crossing the eastern horizon at the moment of birth. 4
How the machine uses it
Calculated facts only, worked out the same way the machine does it.
- Every planet's place on the card's chart is an ecliptic longitude worked out by an open-source astronomy library from the date, the time and the place of birth.
- Longitude 0° is the March equinox point (the tropical zodiac); each sign is the next 30°.
- The Sun moves about 1° of the ecliptic a day, the Moon about 13°.
Where the idea comes from
Era by era, with sources. More in the Reading Room.
- Ancientc. 1000–450 BCEBabylonian astronomers build the zodiac from reference stars along the path of the Moon and planets; by about 450 BCE it has become twelve equal zones of 30 units each. 5,3
- Classical2nd century CEPtolemy counts the zodiac from the spring equinox, tying the signs to the seasons rather than to particular stars. 6,7
- Today2016NASA explains that the Sun's yearly path crosses thirteen constellations, while the astrological zodiac keeps twelve equal slices. 2
Common misconceptions
“The ecliptic is the Sun going round the Earth.” It is the reverse. The Sun's yearly drift against the stars is a reflection of Earth travelling round the Sun. Astrology keeps the Earth-centred view because a horoscope describes the sky as seen from the place you were born. 1
“The ecliptic and the celestial equator are the same circle.” They are tilted about 23½° to each other because Earth's axis is tilted. They cross at two points, the equinoxes, and the spring crossing is where the zodiac starts. 1,7
Questions people ask
Why is it called the ecliptic?
Because eclipses happen on it. The Moon has to be crossing this line at a new or full moon for the Sun, Moon and Earth to line up. 1
Is the zodiac the same as the ecliptic?
The ecliptic is a line; the zodiac is the band either side of it, divided into twelve signs of 30° each. 3
Do the planets sit exactly on the ecliptic?
Close to it, never exactly. Their orbits are tilted by a few degrees, so astrologers record only how far along the line they are.
Sources
- Ecliptic, Skyscript, Glossary of Astrological Terms (Deborah Houlding). Read on .
- Constellations and the Calendar, NASA (from NASA Space Place), 20 September 2016. Read on .
- Alexander Jones, The Horoscope Casters (Archaeology Odyssey 3.6, 2000), New York University Faculty Digital Archive. Read on .
- Ascendant, Skyscript, Glossary of Astrological Terms (Deborah Houlding). Read on .
- Review of Francesca Rochberg, The Heavenly Writing: Divination, Horoscopy, and Astronomy in Mesopotamian Culture (reviewed by Eleanor Robson), BMCR 2005.06.29, Bryn Mawr Classical Review. Read on .
- Ptolemy, Tetrabiblos, Book I, chapters 10–19 (aspects, houses of the planets, triplicities), LacusCurtius, University of Chicago (Loeb Classical Library translation by F. E. Robbins, 1940). Read on .
- Precession, Skyscript, Glossary of Astrological Terms (Deborah Houlding). Read on .