What is an ephemeris?
An ephemeris (plural: ephemerides) is a list of where objects in the sky are at regular times. The word comes from a Greek word meaning "daily", because the earliest ones gave positions day by day. A traditional printed ephemeris is a book of tables: each row is a date, and each column gives the position of the Sun, the Moon or a planet.
Astronomers, navigators and space missions all rely on ephemerides. So do astrologers. Before anyone can interpret a birth chart, someone has to work out where everything actually was.
A long history
People have been predicting the positions of the planets for thousands of years.
- Babylonian astronomers kept detailed records and used arithmetic rules to predict planetary positions, centuries before the common era.
- Ptolemy, working in Alexandria in the 2nd century CE, produced tables and a geometric model of the heavens that were used for over a thousand years. He also wrote one of the most influential books on astrology.
- The Alfonsine Tables, made in Castile in the 13th century, became the standard in medieval Europe.
- Printed ephemerides arrived in the 15th century. The astronomer Regiomontanus published a famous one in 1474, and it was used by sailors as well as astrologers.
- Johannes Kepler published the Rudolphine Tables in 1627, based on his discovery that planets move in ellipses. They were far more accurate than anything before.
For much of this history, astronomy and astrology were done by the same people. Over the 1600s and 1700s the two separated. Astronomy became a science of measurement and physics, while astrology remained a tradition of interpretation.
How positions are calculated today
Modern ephemerides come from precise mathematical models of the solar system, checked against observations. They take into account the gravity of the planets pulling on each other and many small effects. Simplified versions of these models are published as formulas that a computer can work through in a fraction of a second.
That is what our chart tools do. When you enter your birth details, the tool runs those formulas in your browser to work out the position of each body, seen from Earth, at the exact moment of your birth.
The result is usually given as a longitude along the ecliptic. The ecliptic is the path the Sun appears to trace across the sky over a year. It is measured in degrees from 0 to 360, and astrology divides it into twelve signs of 30 degrees each. So a position of 134 degrees becomes 14 degrees of Leo.
Why time and place matter so much
Time
The Moon moves about 13 degrees a day, or roughly half a degree an hour. The planets move more slowly. But the rising sign (the sign coming up over the eastern horizon) and the houses change much faster, because the Earth spins. The rising degree moves about one degree every four minutes on average. An error of half an hour in a birth time can shift the rising sign or move planets into different houses.
Time zones
Calculations are done in Universal Time (the time at the Greenwich meridian). Your local birth time has to be converted, including any daylight saving time in force on that date in that place. Historical time zone rules can be surprisingly complicated, which is one of the most common sources of chart errors.
Place
Your latitude and longitude decide what the horizon looked like where you were born. That is what sets the rising sign, the midheaven (the highest point of the ecliptic) and the house cusps.
If you do not know your birth time, a good tool should say so plainly. Planet signs will usually still be right, but the rising sign, houses and sometimes the Moon's sign may not be.
Tropical and sidereal zodiacs
There are two main ways to line up the twelve signs with the sky:
- The tropical zodiac, used by most Western astrologers, starts Aries at the spring equinox point (where the Sun is around 20 March). It is tied to the seasons.
- The sidereal zodiac, used in Vedic astrology, lines the signs up with the background stars.
Because the Earth slowly wobbles on its axis (an effect called precession), the equinox point drifts against the stars by about one degree every 72 years. Over many centuries the two zodiacs have drifted apart. Today the difference, called the ayanamsa, is about 24 degrees. That is why many people have a different Sun sign in a Vedic chart. Several ayanamsa values exist. Our Vedic tools use the Lahiri value, which is the most widely used in India.
Other choices a chart makes
- House system. There are many ways to divide the sky into twelve houses, such as Placidus, Whole Sign and Equal. They can give different results.
- Nodes. The lunar nodes can be calculated as "mean" (averaged) or "true" (wobbling) positions. They differ by up to a degree or two.
- Which bodies to include. Traditional astrology uses the seven visible bodies. Modern astrology adds Uranus, Neptune, Pluto and sometimes Chiron and asteroids.
A transparent tool tells you which choices it made. Ours list them in the method section of every report.
Calculation is not interpretation
It helps to keep two layers apart:
- The astronomical layer answers: where was Mars at 3:15 pm on this date, seen from this city? This is measurable and checkable.
- The astrological layer answers: what does Mars in Leo in the tenth house mean? This is tradition, built up over centuries of practice.
A precise position does not make the meaning more true. Scientific studies that have tested astrological claims, such as links between signs and personality, have generally not found the effects astrology predicts. Many people still find astrology meaningful as a language for reflection, and that is how we present it.
Try it
Our birth chart calculator shows every planet position in degrees and minutes, along with the settings it used. The kundli generator shows the same sky in the sidereal zodiac, so you can compare the two.