Hipparchus facts for kids

This page is about the Greek astronomer. For other uses, see Hipparchus (disambiguation).

Quick facts for kids Hipparchus

Born	c. 190 BC Nicaea, Kingdom of Bithynia (modern-day İznik, Bursa, Turkey)
Died	c. 120 BC (around age 70) Rhodes, Roman Republic (modern-day Greece)
Occupation	Astronomer Mathematician Geographer

Hipparchus (/hɪˈ[unsupported input]kəs/; Greek: Ἵππαρχος, Hipparkhos; c. 190 – c. 120 BC) was an important Greek astronomer, geographer, and mathematician. He is often called the "father of trigonometry" because he helped create this field of math. He is also famous for discovering something called the precession of the equinoxes. Hipparchus was born in Nicaea, which is now part of Turkey. He likely died on the island of Rhodes, Greece. He worked as an astronomer between 162 and 127 BC.

Many people think Hipparchus was the greatest ancient astronomer. He was the first to create accurate models for how the Sun and Moon move. He used observations and math ideas from the Babylonians and other Greek thinkers. His work helped predict solar eclipses more reliably. He also created the first known detailed star catalog in the Western world. Some even believe he invented the astrolabe, a tool used to measure star positions.

Life and Discoveries
- Babylonian Influence
Math and Tools
- Measuring the Sky
Moon and Sun's Paths
Star Catalog and Precession
Geography
Legacy
See also

Life and Discoveries

Hipparchus was born in Nicaea, a city in ancient Bithynia. We don't know the exact dates of his life. But we know he made important observations between 147 and 127 BC. Many of these were made in Rhodes, where he likely spent his later years. His birth year, around 190 BC, was figured out from clues in his writings.

In ancient times, coins were even made in his honor in Bithynia. These coins showed his name and a globe.

Not much of Hipparchus's own writing has survived. He wrote at least fourteen books, but only one is still around. This book is a commentary on a popular poem about stars by Aratus. Most of what we know about Hipparchus comes from other ancient writers. These include Strabo, Pliny the Elder, and Ptolemy.

Hipparchus's only surviving work is called "Commentary on the Phaenomena of Eudoxus and Aratus." In this book, he carefully reviewed a poem by Aratus that was based on the ideas of Eudoxus of Cnidus. Hipparchus also made a list of his main works, which included about fourteen books. His famous star catalog was later used by Ptolemy. Hipparchus is known as "the father of trigonometry" because he created the first tables for this type of math.

Babylonian Influence

Earlier Greek astronomers learned from Babylonian astronomy. Hipparchus was probably the first to really use Babylonian knowledge and methods in a big way. For example, he used the Babylonian idea of dividing a circle into 360 degrees. He also used their unit called the "cubit" for measurements.

Hipparchus likely made a list of Babylonian observations of the sky. Historians believe that Ptolemy learned about many ancient eclipse records from Hipparchus's list. This shows how important Babylonian knowledge was to Hipparchus's work.

Math and Tools

Hipparchus was the first mathematician known to have a trigonometric table. He needed this to figure out the shapes of the Moon's and Sun's paths. This table helped him measure the length of a straight line inside a circle. He calculated these lengths for different angles.

Trigonometry was a huge step forward. It allowed Greek astronomers to solve problems with triangles. This made it possible to create detailed models and predictions about the sky using geometry.

Hipparchus also used math ideas from the Chaldeans, an ancient people from Mesopotamia. He was one of the first Greek mathematicians to do this. This helped him expand the tools available for astronomers and geographers.

Some clues suggest Hipparchus knew about spherical trigonometry. This is a type of math used for shapes on a sphere, like the Earth or the sky. However, the first clear writings about it came later from another mathematician.

Measuring the Sky

Hipparchus and earlier astronomers used tools like the gnomon, astrolabe, and armillary sphere.

Hipparchus is believed to have invented or improved several tools for observing the sky. He might have made the first "astrolabion," which could have been an armillary sphere. Or it might have been an early version of the flat astrolabe. With an astrolabe, Hipparchus was the first to measure latitude and time using fixed stars. Before him, people used the shadow of a gnomon or other simple tools.

Ptolemy mentioned that he used a tool similar to Hipparchus's, called a dioptra. This tool helped measure the apparent size of the Sun and Moon. It was a long rod with a scale and a movable wedge to block out the Sun or Moon.

Hipparchus also observed the equinoxes. These are the times when day and night are equal. He might have used an equatorial ring for this. This ring casts a shadow on itself when the Sun is directly over the equator.

Moon and Sun's Paths

Hipparchus studied how the Moon moves. He confirmed accurate measurements for its motion that Babylonian astronomers already knew. For example, he knew that a synodic month (the time from one new moon to the next) is about 29.53 days. He also knew that 251 synodic months are roughly equal to 269 anomalistic months (the time it takes for the Moon to return to its closest point to Earth).

Hipparchus checked his calculations by comparing eclipses from his own time with older Babylonian records. This helped him confirm the accuracy of these ancient measurements.

Moon's Changing Speed

People had known for a long time that the Moon's speed isn't always the same. This is called its anomaly. Hipparchus used geometry to explain this. He used two main models:

The Moon moves in a circle, but the Earth is not exactly at the center.
The Moon moves on a smaller circle (an epicycle) that itself moves on a larger circle (a deferent) around the Earth.

Hipparchus showed that these two models worked mathematically. He used observations of lunar eclipses to figure out the sizes of these circles. He found some results that didn't quite match up. But his work was a big step forward in understanding the Moon's complex motion.

Sun's Apparent Motion

Hipparchus also studied the Sun's apparent motion. He found that the lengths of the seasons are not equal. For example, spring (from spring equinox to summer solstice) was longer than summer. This meant the Sun couldn't be moving around the Earth in a perfect circle at a steady speed.

Hipparchus's idea was that the Earth was not exactly at the center of the Sun's path. This model explained why the seasons had different lengths. Today, we know that planets, including Earth, move in ellipses around the Sun. But Hipparchus's model was the best explanation for his time.

He also calculated the length of the tropical year (the time it takes for the Sun to return to the same point in the seasons). He found it was about 365 days, 5 hours, and 55 minutes. This was very close to the actual value for his time.

Distances to the Moon and Sun

Hipparchus also tried to figure out the distances and sizes of the Sun and Moon. He used a tool called a diopter to measure their apparent sizes. He noticed that the Moon's size changes as it moves, but the Sun's size seemed constant. He found that the Sun and Moon look about the same size from Earth.

He also knew that the Moon has a noticeable parallax. This means it appears to shift slightly from its true position depending on where you observe it from. This shift happens because the observer is on the Earth's surface, not at its center. By measuring this parallax, he could estimate the Moon's distance in Earth radii. For the Sun, he couldn't see any parallax, so he assumed it was very far away.

Using a solar eclipse, Hipparchus estimated the Moon's closest distance to be 71 Earth radii and its farthest to be 81 Earth radii. He also used lunar eclipses to estimate the Sun's distance. His results were the best for his time. The actual average distance to the Moon is about 60.3 Earth radii, which is close to his findings.

Star Catalog and Precession

Around 135 BC, Hipparchus created his famous star catalog. For centuries, scholars have been looking for it. In 2022, parts of it were found hidden in an old manuscript.

Hipparchus also built a celestial globe that showed the constellations. He might have been inspired to create his catalog after seeing a supernova (a new, bright star). Or, as Ptolemy suggested, it might have been because he discovered something called precession.

Hipparchus wrote a commentary on a poem about stars by Aratus. This book is his only surviving work. It contains many star positions and times for when constellations rise and set. These were likely based on his own measurements. Roman sources say Hipparchus used scientific tools to measure about 850 stars for his catalog.

Star Brightness

Hipparchus is thought to have created a system for ranking how bright stars appear. He supposedly used a scale from 1 (brightest) to 6 (faintest). This idea comes from a vague statement by Pliny the Elder. We don't know for sure if Hipparchus invented this system, but it was definitely used by Ptolemy later on. This system was later made more precise and is still used today.

Star Coordinates

It's debated which coordinate system Hipparchus used for his star catalog. Ptolemy's catalog, which came from Hipparchus's, uses ecliptic coordinates. Some historians believe Hipparchus used a mix of ecliptic and equatorial coordinates. His surviving work mentions different ways to describe star positions.

Hipparchus's star catalog was later used and improved by Ptolemy. Some historians believe Ptolemy copied most of Hipparchus's catalog. However, the catalog was likely updated and added to over 265 years.

Celestial Globe

Hipparchus's celestial globe was like an ancient computer. He used it to figure out when stars would rise, set, and reach their highest point in the sky. His globe had a horizontal base and a meridian ring with a scale. It also had lines dividing the celestial equator into 24 "hour lines." This allowed him to tell the time using the stars. The globe also showed the ecliptic and the 12 zodiac "signs."

Hipparchus's work had a lasting impact. His catalog was later updated by astronomers like al-Sufi and Copernicus. It was only replaced in the late 1500s by Tycho Brahe and Wilhelm IV. They used better tools and math, even before the telescope was invented. Hipparchus is considered the greatest observer of the sky from ancient times until Brahe.

Precession of the Equinoxes

Hipparchus is famous for discovering the precession of the equinoxes around 127 BC. He wrote two books about this topic. Ptolemy tells us that Hipparchus measured the position of bright stars like Spica and Regulus. By comparing his measurements with older data, he noticed that these stars had shifted slightly.

He also compared the length of the tropical year (the time for the Sun to return to an equinox) and the sidereal year (the time for the Sun to return to a fixed star). He found a small difference. Hipparchus realized that the equinoxes were slowly moving through the zodiac. He estimated this movement to be at least 1 degree every century.

Geography

Hipparchus also wrote a book criticizing the work of the geographer Eratosthenes. This book is now lost. From Strabo, we know that Hipparchus thought maps should be based on careful astronomical measurements. He believed that latitude and longitude should be determined by observing stars and using triangulation for distances.

Hipparchus made three big improvements in geography:

He was the first to use a grid system based on degrees.
He used star observations to find latitude, not just the Sun's height.
He suggested using simultaneous observations of lunar eclipses in different places to find longitude. This method would work, but it was hard to do accurately with the tools available back then.

Hipparchus also listed the latitudes for many places. He improved Eratosthenes's measurements for places like Athens and Sicily. He used a very accurate value for the tilt of the Earth's axis, which was 23°40'. Other ancient writers used a less accurate value.

Hipparchus believed that the Atlantic Ocean, Indian Ocean, and Caspian Sea were not all connected as one big ocean. He also thought that people could live closer to the equator and the Arctic Circle than others believed. Hipparchus's ideas greatly influenced Ptolemy's later work on geography.

Legacy

Hipparchus might be shown in Raphael's famous painting The School of Athens. He is often seen as the figure holding a celestial globe, representing astronomy.

The ESA named its Hipparcos Space Astrometry Mission after him. This satellite was designed to measure the positions of stars very precisely.

The lunar crater Hipparchus and the asteroid 4000 Hipparchus are also named in his honor.

Historians like Jean Baptiste Joseph Delambre considered Hipparchus one of the greatest astronomers of all time. He is also featured on the Astronomers Monument at the Griffith Observatory in Los Angeles, California. This monument honors six of the greatest astronomers in history, and Hipparchus is the only one from ancient times.

Johannes Kepler greatly respected Tycho Brahe's work. He saw Brahe as a "new Hipparchus" who would build the foundation for modern astronomy.