Astronomy - Brief History of Ancient Astronomy

A Brief History of Ancient Astronomy



Nebra Sky Disc 1600 BC


Astronomy is the oldest of the natural sciences, dating back to antiquity, with its origins in the religious, mythological, cosmological, calendrical, and astrological beliefs and practices of prehistory:

The origins of Western astronomy can be found in Mesopotamia, the "land between the rivers" Tigris and Euphrates, where the ancient kingdoms of Sumer, Assyria, and Babylonia were located.



A form of writing known as cuneiform emerged among the Sumerians around 3500–3000 BC. Our knowledge of Sumerian astronomy is indirect, via the earliest Babylonian star catalogues dating from about 1200 BC.



Classical sources frequently use the term Chaldeans for the astronomers of Mesopotamia, who were, in reality, priest-scribes specializing in astrology and other forms of divination.

The first evidence of recognition that astronomical phenomena are periodic and of the application of mathematics to their prediction is Babylonian. Tablets dating back to the Old Babylonian period document the application of mathematics to the variation in the length of daylight over a solar year.

The Venus tablet of Ammi-saduqa, which lists the first and last visible risings of Venus over a period of about 21 years and is the earliest evidence that the phenomena of a planet were recognized as periodic.

A significant increase in the quality and frequency of Babylonian observations appeared during the reign of Nabonassar (747–733 BC).

The systematic records of ominous phenomena in Babylonian astronomical diaries that began at this time allowed for the discovery of a repeating 18-year cycle of lunar eclipses, for example.

The last stages in the development of Babylonian astronomy took place during the time of the Seleucid Empire (323–60 BC).

In the 3rd century BC, astronomers began to use "goal-year texts" to predict the motions of the planets.

These texts compiled records of past observations to find repeating occurrences of ominous phenomena for each planet.

About the same time, or shortly afterwards, astronomers created mathematical models that allowed them to predict these phenomena directly, without consulting past records.

Babylonian astronomy was the basis for much of what was done in Greek and Hellenistic astronomy, in classical Indian astronomy, in Iran, in Byzantium, in Syria, in Islamic astronomy, in Central Asia, and in Western Europe.

Greek Astronomy

Greek astronomy is astronomy written in the Greek language in classical antiquity. Greek astronomy is understood to include the ancient Greek, Hellenistic, Greco-Roman, and Late Antiquity eras.

It is not limited geographically to Greece or to ethnic Greeks, as the Greek language had become the language of scholarship throughout the Hellenistic world following the conquests of Alexander.

The development of astronomy by the Greek and Hellenistic astronomers is considered by historians to be a major phase in the history of astronomy.

Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena. Most of the constellations of the northern hemisphere derive from Greek astronomy, as are the names of many stars, asteroids, and planets.

The Ancient Greeks developed astronomy, which they treated as a branch of mathematics, to a highly sophisticated level. The first geometrical, three-dimensional models to explain the apparent motion of the planets were developed in the 4th century BC by Eudoxus of Cnidus and Callippus of Cyzicus. Their models were based on nested homocentric spheres centered upon the Earth. Their younger contemporary Heraclides Ponticus proposed that the Earth rotates around its axis.

The Antikythera Mechanism

An analogue computer from 150–100 BC designed to calculate the positions of astronomical objects.

A different approach to celestial phenomena was taken by natural philosophers such as Plato and Aristotle.

They were less concerned with developing mathematical predictive models than with developing an explanation of the reasons for the motions of the Cosmos.

Plato described the universe as a spherical body divided into circles carrying the planets and governed according to harmonic intervals by a world soul.

Aristotle proposed that the universe was made of a complex system of concentric spheres, whose circular motions combined to carry the planets around the earth.

Egyptian Astronomy

The precise orientation of the Egyptian pyramids affords a lasting demonstration of the high degree of technical skill in watching the heavens attained in the 3rd millennium BC. It has been shown the Pyramids were aligned towards the pole star.

Astronomy played a considerable part in Egyption religious matters for fixing the dates of festivals and determining the hours of the night.

The titles of several temple books are preserved recording the movements and phases of the sun, moon and stars.

The Astrologer's instruments (Horologium and palm) are a plumb line and sighting instrument. They have been identified with two inscribed objects in the Berlin Museum; a short handle from which a plumb line was hung, and a palm branch with a sight-slit in the broader end.

Astronomical ceiling decoration in its earliest form can be traced to the Tomb of Senenmut The tomb and the ceiling decorations date back to the 18th Dynasty of ancient Egypt (ca. 1473 B.C.).

The Celestial Diagram consisted of a northern and a southern panel which depicted circumpolar constellations in the form of discs; each divided into 24 sections suggesting a 24-hour time period, lunar cycles, and sacred deities of Egypt.

Egyptian Celestial Diagram

Indian Astronomy

Historical Jantar Mantar observatory in Jaipur, India

Astronomy in the Indian subcontinent dates back to the period of Indus Valley Civilization during 3rd millennium BC, when it was used to create calendars.

The oldest Indian astronomical text is the Vedanga Jyotisha, dating from the Vedic period. Which describes rules for tracking the motions of the Sun and the Moon for the purposes of ritual.

Aryabhata (476–550), propounded a computational system based on a planetary model in which the Earth was taken to be spinning on its axis and the periods of   the planets were given with respect to the Sun.

He accurately calculated many astronomical constants, such as the periods of the planets, times of the solar and lunar eclipses, and the instantaneous motion of the Moon.

Astronomy was advanced during the Shunga Empire and many star catalogues were produced during this time. The Shunga period is known as the "Golden age of astronomy in India".

It saw the development of calculations for the motions and places of various planets, their rising and setting, conjunctions, and the calculation of eclipses.

Indian astronomers by the 6th century believed that comets were celestial bodies that re-appeared periodically.

And by the 10th-century astronomer Bhattotpala listed the names and estimated periods of certain comets, but it is unfortunately not known how these figures were calculated or how accurate they were.

Chinese Astronomy

Su Song's star maps the oldest existent ones in print

Astronomy in China has a long history. Detailed records of astronomical observations were kept from about the 6th century BC, until the introduction of Western astronomy and the telescope in the 17th century. Chinese astronomers were able to precisely predict eclipses.

Much of early Chinese astronomy was for the purpose of timekeeping.

The Chinese used a lunisolar calendar, but because the cycles of the Sun and the Moon are different, astronomers often prepared new calendars and made observations for that purpose.

Astrological divination was also an important part of astronomy. Astronomers took careful note of "guest stars" which suddenly appeared among the fixed stars.

They were the first to record a supernova, in the Astrological Annals of the Houhanshu in 185 AD. Also, the supernova that created the Crab Nebula in 1054 is an example of a "guest star" observed by Chinese astronomers, although it was not recorded by their European contemporaries.

Maya Astronomy

El Caracol observatory temple Mexico

Maya astronomical codices (book) include detailed tables for calculating phases of the Moon, the recurrence of eclipses, and the appearance and disappearance of Venus as morning and evening star.

The Maya based their calendric in the carefully calculated cycles of the Pleiades, the Sun, the Moon, Venus, Jupiter, Saturn, Mars, and also they had a precise description of the eclipses.

A number of important Maya structures are believed to have been oriented toward the extreme risings and settings of Venus.

To the ancient Maya, Venus was the patron of war and many recorded battles are believed to have been timed to the motions of this planet.

Mars is also mentioned in preserved astronomical codices and early mythology.

Although the Maya calendar was not tied to the Sun, it has been proposed that the Maya calculated the solar year to somewhat greater accuracy than the Gregorian calendar.

Both astronomy and an intricate numerological scheme for the measurement of time were vitally important components of Maya religion

Islamic Astronomy

The Arabic and the Persian world under Islam had become highly cultured, and many important works of knowledge from Greek astronomy and Indian astronomy and Persian astronomy were translated into Arabic, used and stored in libraries throughout the area.

An important contribution by Islamic astronomers was their emphasis on observational astronomy. This led to the emergence of the first astronomical observatories in the Muslim world by the early 9th century.

In the 10th century, Abd al-Rahman al-Sufi (Azophi) carried out observations on the stars and described their positions, magnitudes, brightness, and colour and drawings for each constellation in his Book of Fixed Stars.

He also gave the first descriptions and pictures of "A Little Cloud" now known as the Andromeda Galaxy.

In the late 10th century, a huge observatory was built near Tehran, Iran, by the astronomer Abu-Mahmud al-Khujandi who observed a series of meridian transits of the Sun, which allowed him to calculate the tilt of the Earth's axis relative to the Sun.

He noted that measurements by earlier (Indian, then Greek) astronomers had found higher values for this angle, possible evidence that the axial tilt is not constant but was in fact decreasing.

Other Muslim advances in astronomy included the collection and correction of previous astronomical data. 

The invention of numerous astronomical instruments, including the development of the universal latitude-independent astrolabe

Arabic astrolabe from 1208 AD

Muhammad Mūsā believed that the heavenly bodies and celestial spheres were subject to the same physical laws as Earth.

The first elaborate experiments related to astronomical phenomena, the introduction of exacting empirical observations and experimental techniques, and the introduction of empirical testing, which produced the first model of lunar motion which matched physical observations.  

Medieval Western European Astronomy

9th century diagram of the positions of the seven planets on 18 March 816

Western Europe entered the Middle Ages with great difficulties that affected the continent's intellectual production.

The advanced astronomical treatises of classical antiquity were written in Greek, and with the decline of knowledge of that language, only simplified summaries and practical texts were available for study.

In the 6th century Bishop Gregory of Tours noted that he had learned his astronomy from reading Martianus Capella, and went on to employ this rudimentary astronomy to describe a method by which monks could determine the time of prayer at night by watching the stars.

In the 7th century the English monk Bede of Jarrow published an influential text, On the Reckoning of Time.

By the 9th century rudimentary techniques for calculating the position of the planets were circulating in Western Europe; medieval scholars recognized their flaws, but texts describing these techniques continued to be copied, reflecting an interest in the motions of the planets and in their astrological significance.

Building on this astronomical background, in the 10th century European scholars began to travel to Spain and Sicily to seek out learning which they had heard existed in the Arabic-speaking world.

There they first encountered various practical astronomical techniques concerning the calendar and timekeeping, most notably those dealing with the astrolabe.

The renaissance came to astronomy with the work of Nicolaus Copernicus, who proposed a heliocentric system, in which the planets revolved around the Sun and not the Earth.

Nicolaus Copernicus

His De revolutionibus provided a full mathematical discussion of his system, using the geometrical techniques that had been traditional in astronomy since before the time of Ptolemy. His work was later defended, expanded upon and modified by Galileo Galilei and Johannes Kepler.


Astronomy - Saturn and its Moons


Diameter 74,000 miles

Distance from Sun 0.89 billion miles 9.54 AU

Its polar diameter is 90% of its equatorial diameter, this is due to its low density and fast rotation.

Saturn turns on its axis once every 10 hours and 34 minutes giving it the second-shortest day of any of the solar system’s planets.

Saturn orbits the Sun once every 29.4 Earth years.

At least 62 moons are known to orbit Saturn, of which 53 are officially named.

Saturn is the sixth planet from the Sun and the most distant that can be seen with the naked eye. the other four being Mercury, Venus, Mars and Jupiter.

Saturn is the second largest planet in the Solar System, after Jupiter.

Like Jupiter, Saturn is a gas giant and is composed of similar gasses including hydrogen, helium and methane.

Saturn Earth Size Comparison


Saturn was known to the ancients, including the Babylonians and Far Eastern observers.

It is named after the Roman god Saturnus who was the god of agriculture. And was known to the Greeks as Cronus.

Ancient Chinese and Japanese culture designated the planet Saturn as the "earth star" (土星).

Saturn ring system was first observed in 1610 by the astronomer Galileo Galilei. He thought of them as two moons on Saturn's sides.

It was not until Christiaan Huygens used greater telescopic magnification that this notion was refuted. Huygens discovered Saturn's moon Titan

Cassini later discovered four other moons: Iapetus, Rhea, Tethys and Dione. In 1675, Cassini discovered the gap between A and B ring now known as the Cassini Division.

Four spacecraft have visited Saturn.

Pioneer 11, Voyager 1 and 2, and the Cassini-Huygens mission have all studied the planet.

Pioneer 11 made the first flyby of Saturn in September 1979, when it passed within 20,000 km of the planet's cloud tops.

Images were taken of the planet and a few of its moons, although their resolution was too low to discern surface detail. It also measured the temperature of Titan.

In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, its rings and satellites. Surface features of various moons were seen for the first time.

Voyager 1 performed a close flyby of Titan, increasing knowledge of the atmosphere of the moon.

In August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings.

On 1 July 2004, the Cassini–Huygens space probe performed the SOI (Saturn Orbit Insertion) manoeuvre and entered orbit around Saturn.

The orbiter completed two Titan flybys before releasing the Huygens probe on 25 December 2004.

Huygens descended onto the surface of Titan on 14 January 2005, sending a flood of data during the atmospheric descent and after the landing.

Starting in early 2005, scientists used Cassini to track lightning on Saturn. The power of the lightning is approximately 1,000 times that of lightning on Earth.

Saturn’s upper atmosphere is divided into bands of clouds.

The top layers are mostly ammonia ice. Below them, the clouds are largely water ice.

Below that are layers of cold hydrogen and sulphur ice mixtures.

Saturn has oval-shaped storms similar to Jupiter’s.

The region around its north pole has a hexagonal-shaped pattern of clouds.

Scientists think this may be a wave pattern in the upper clouds.

The planet also has a vortex over its south pole that resembles a hurricane-like storm.

Eventually, deep inside, the hydrogen becomes metallic. At the core lies a hot interior.

Saturn has the most extensive rings in the solar system.

The rings are made mostly of chunks of ice and small amounts of dust.

The rings stretch out more than 120,700 km from the planet, but are amazingly thin: only about 20 meters thick.



Saturns Moons






Largest Moon

Diameter 3,200 miles

Distance from Saturn 759,228 miles

Orbital Period 15.5 days

Titan was discovered on March 25, 1655 by the Dutch astronomer Christiaan Huygens. Huygens was inspired by Galileo's discovery of Jupiter's four largest moons in 1610 and his improvements in telescope technology.

Titan is the largest moon of Saturn. It is the only moon known to have a dense atmosphere, and the only object in space other than Earth where clear evidence of stable bodies of surface liquid has been found.

Titan is the sixth gravitationally rounded moon from Saturn. Frequently described as a planet-like moon, Titan is 50% larger than Earth's Moon, and it is 80% more massive.

It is the second-largest moon in the Solar System, after Jupiter's moon Ganymede, and is larger than the smallest planet, Mercury.

11.4 times larger in the sky of Saturn than the Moon from Earth.

Titan Earth Moon Size Comparison

Titan is primarily composed of water ice and rocky material.

Information from the Cassini–Huygens mission in 2004, including the discovery of liquid hydrocarbon lakes in Titan's polar regions.

The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found.

Surface of Titan From Huygens Probe

Two new studies from Cornell University in  New York show that the liquid lakes and seas on Titan follow a constant elevation relative to Titan’s gravitational pull.

In other words, just as Earth’s oceans lie at an average elevation that we call sea level, so do Titan’s seas.

Its lakes and seas are filled with hydrocarbons rather than liquid water, and water ice overlain by a layer of solid organic material serves as the bedrock surrounding these lakes and seas.

The new study suggests that elevation is important because Titan’s liquid bodies appear to be connected under the surface in something akin to an aquifer system at Earth.

The atmosphere of Titan is largely nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate including wind and rain creates surface features similar to those of Earth.

Such as dunes, rivers, lakes, seas and deltas, and is dominated by seasonal weather patterns as on Earth.


Diameter 949 miles

Distance from Saturn 327,387 miles

Orbital Period 4.5 days

The second-largest moon of Saturn and the ninth-largest moon in the Solar System.

It was discovered in 1672 by Giovanni Domenico Cassini.

Rhea has a rather typical heavily cratered surface, with the exceptions of a few large fractures (wispy terrain) on the trailing hemisphere (the side facing away from the direction of motion along Rhea's orbit)


Rhea Earth Moon Size Comparison

Rhea is an icy body with a density of about 1.236 g/cm3. This low density indicates that it is made of ~25% rock (density ~3.25 g/cm3) and ~75% water ice (density ~0.93 g/cm3).

Although Rhea is the ninth-largest moon in The Solar System, it is only the tenth-most-massive moon.

Its surface can be divided into two geologically different areas based on crater density; the first area contains craters which are larger than 40 km in diameter, whereas the second area, in parts of the polar and equatorial regions, has only craters under that size.

This suggests that a major resurfacing event occurred some time during its formation.

Earlier it was assumed that Rhea had a rocky core in the center. However, measurements taken during a close flyby by the Cassini orbiter in 2005 cast this into doubt.

Now considered that Rhea has an almost homogeneous interior (with some compression of ice in the center).



Diameter 892 miles

Distance from Saturn 2,212,889 miles

Orbital Period 79.3 days

Iapetus is the third-largest natural satellite of Saturn, eleventh-largest in the Solar System.

Iapetus was discovered by Giovanni Domenico Cassini, in October 1671.

He had discovered it on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful.

Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.


Iapetus Earth Monn Size Comparison

The low density of Iapetus indicates that it is mostly composed of ice, with only a small (~20%) amount of rocky materials.

The orbit of Iapetus is somewhat unusual. Although it is Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites.

Unlike most of the large moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles; also, its unique equatorial ridge is so high that it visibly distorts Iapetus's shape even when viewed from a distance.

These features often lead it to be characterized as walnut-shaped.

Equatorial Ridge


Diameter 697 miles

Distance from Saturn 2,212,889 miles

Orbital Period 2.7 days

It was discovered by Giovanni Domenico Cassini in 1684. It is named after the Titaness Dione of Greek mythology.

About two thirds of Dione's mass is water ice, and the remaining is a dense core, probably silicate rock.

Data gathered by Cassini Orbiter indicates that Dione has an internal liquid water ocean.

Dione Earth Moon Size Comparison

Gravity and shape data points to a 99 ± 23 km thick ice shell crust on top of a 65 ± 30 km internal liquid water global ocean.

Dione's ice shell is thought to vary in thickness by less than 5%, with the thinnest areas at the poles, where tidal heating of the crust is greatest.

Dione is very similar to Rhea. They both have similar features and varied terrain, and both have dissimilar leading and trailing hemispheres.

Dione's leading hemisphere is heavily cratered and is uniformly bright. Its trailing hemisphere, however, contains an unusual and distinctive surface feature: a network of bright ice cliffs.

The Cassini probe flyby of December 13, 2004, produced close-up images. These revealed bright ice cliffs created by tectonic fractures showing that some of them are several hundred metres high. Dione has been revealed as a world riven by enormous fractures on its trailing hemisphere.

On April 7, 2010, instruments on board the Cassini probe, which flew by Dione, detected a thin layer of molecular oxygen ions  around Dione, so thin that scientists prefer to call it an exosphere rather than a tenuous atmosphere.


Diameter 650 miles

Distance from Saturn 183,100 miles

Orbital Period 1.89 days

Discovered by Cassini in 1684 observed using a large aerial telescope he set up on the grounds of the Paris Observatory. And is named after the titan Tethys of Greek mythology.

Tethys has a low density of 0.98 g/cm3, the lowest of all the major moons in the Solar System, indicating that it is made of water ice with just a small fraction of rock. This is confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material.

The surface of Tethys is very bright, being the second-brightest of the moons of Saturn after Enceladus, and neutral in colour.

Tethys is heavily cratered and cut by a number of large faults/graben. The largest impact crater,  is about 400 km in diameter, whereas the largest graben, is about 100 km wide and more than 2000 km long.

The surface of Tethys is one of the most reflective (at visual wavelengths) in the Solar System. The high albedo indicates that the surface of Tethys is composed of almost pure water ice with only a small amount of a dark material.

This very high albedo is the result of the sandblasting of particles from Saturn's E-ring, a faint ring composed of small, water-ice particles generated by Enceladus's south polar geysers.

The surface of Tethys has a number of large-scale features distinguished by their colour and sometimes brightness. The trailing hemisphere gets increasingly red and dark as the anti-apex of motion is approached.

The leading hemisphere also reddens slightly as the apex of the motion is approached, although without any noticeable darkening.

On the leading hemisphere of Tethys spacecraft observations have found a dark bluish band spanning 20° to the south and north from the equator.

The band has an elliptical shape getting narrower as it approaches the trailing hemisphere.


Diameter 310 miles

Distance from Saturn 148,000 miles

Orbital Period 1.4 days

Enceladus is the sixth-largest moon of Saturn. It is about 500 kilometres (310 mile) in diameter, about a tenth of that of Saturn's largest moon, Titan.

Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System.


Enceladus Earth Moon Size Comparison

Consequently, its surface temperature at noon only reaches −198 °C (−324 °F), far colder than a light-absorbing body would be.

Despite its small size, Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains that formed as recently as 100 million years ago.

Enceladus was discovered on August 28, 1789, by William Herschel but little was known about it until the two Voyager spacecraft, Voyager 1 and Voyager 2, passed nearby in the early 1980s.

In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail.

Cassini discovered water-rich plumes venting from the south polar region. Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space.

Over 100 geysers have been identified. Some of the water vapor falls back as "snow"; the rest escapes, and supplies most of the material making up Saturn's E ring.

In 2014, NASA reported that Cassini found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km (6 mile).

These geyser observations, along with the finding of escaping internal heat and very few (if any) impact craters in the south polar region, show that Enceladus is currently geologically active.


Diameter 246 miles

Distance from Saturn 115,289 miles

Orbital Period 0.94 days

Mimas was discovered by the astronomer William Herschel on 17 September 1789.

He recorded his discovery as follows: "The great light of my forty-foot [12 m] telescope was so useful that on the 17th of September, 1789, I remarked the seventh satellite, then situated at its greatest western elongation.“

It is named after Mimas, a son of Gaia in Greek mythology.

A number of features in Saturn's rings are related to resonances with Mimas. Mimas is responsible for clearing the material from the Cassini Division, the gap between Saturn's two widest rings, the A Ring and B Ring.

They orbit twice for each orbit of Mimas. The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap.

The surface area of Mimas is slightly less than the land area of Spain. The low density of Mimas, 1.15 g/cm3, indicates that it is composed mostly of water ice with only a small amount of rock.

Due to the tidal forces acting on it, Mimas is noticeably prolate; its longest axis is about 10% longer than the shortest.

Mimas's most distinctive feature is a giant impact crater 130 km (81 miles) across, named Herschel after the discoverer of Mimas. Herschel's diameter is almost a third of Mimas's own diameter; its walls are approximately 5 km (3 miles) high, parts of its floor measure 10 km (6 miles) deep, and its central peak rises 6 km (4 miles) above the crater floor.

If there were a crater of an equivalent scale on Earth (in relative size) it would be over 4,000 km (2,500 miles) in diameter, wider than Australia.

The impact that made this crater must have nearly shattered Mimas: fractures can be seen on the opposite side of Mimas that may have been created by shock waves from the impact travelling through Mimas's body.

The surface is saturated with smaller impact craters, but no others are anywhere near the size of Herschel.

Although Mimas is heavily cratered, the cratering is not uniform.

Most of the surface is covered with craters larger than 40 km (25 miles) in diameter, but in the south polar region, there are generally no craters larger than 20 km (12 miles) in diameter.

Astronomy - Extraterrestrial Life

Extraterrestrial Life


Definition of Life

Encyclopaedia Britannica

Life, living matter and, as such, matter that shows certain attributes that include responsiveness, growth, metabolism, energy transformation, and reproduction.


From a physics perspective, living beings are thermodynamic systems with an organized molecular structure that can reproduce itself and evolve as survival.

Extra-terrestrial life also called alien life, is life that occurs outside of Earth and that probably did not originate from Earth.

These hypothetical life forms may range from simple single cell organism to beings with civilizations far more advanced than humanity.

Since the mid-20th century, there has been an ongoing search for signs of extra-terrestrial life.

This encompasses a search for current and historic extra-terrestrial life, and a narrower search for extra-terrestrial intelligent life.

Depending on the category of search, methods range from the analysis of telescope and specimen data to radios used to detect and send communication signals.

Alien life, such as microorganisms, has been hypothesized to exist in the Solar System and throughout the universe.

This hypothesis relies on the vast size and consistent physical laws of the observable universe.

According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, as well as well-regarded thinkers such as Winston Churchill, it would be improbable for life not to exist somewhere other than Earth.

Drake equation

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.

The Fermi Paradox

The Fermi paradox or Fermi's paradox, named after physicist Enrico Fermi, is the apparent contradiction between the lack of evidence and high probability estimates for the existence of extra-terrestrial civilizations.

The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old.

Life may have emerged independently at many places throughout the universe. Alternatively, life may have formed less frequently, then spread by meteoroids, for example between habitable planets.

In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth.

According to these studies, this process may occur outside Earth on several planets and moons of the Solar System and on planets of other stars.

Since the 1950s, scientists have proposed that "habitable zones" around stars are the most likely places to find life.

Numerous discoveries in such zones since 2007 have generated numerical estimates of Earth-like planets.

On 4th November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way.

Life on Earth requires water as a solvent in which biochemical reactions take place. Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth.

More generally, life based on ammonia (rather than water) has been suggested, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane.

About 29 chemical elements play an active positive role in living organisms on Earth.

About 95% of living matter is built upon only six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur.

These six elements form the basic building blocks of virtually all life on Earth, whereas most of the remaining elements are found only in trace amounts.

Should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of extremophile microorganisms.

According to NASA's 2015 Astrobiology Strategy, "Life on other worlds is most likely to include microbes, and any complex living system elsewhere is likely to have arisen from and be founded upon microbial life.

In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggest DNA and RNA components, building blocks for life as we know it, may be formed extra terrestrially in outer space.

In October 2011, scientists reported that cosmic dust contains complex organic matter that could be created naturally, and rapidly, by stars. One of the scientists suggested that these compounds may have been related to the development of life on Earth.

In August 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system.

The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid, or RNA.

This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets.

Scientists search for biosignatures within the Solar System by studying planetary surfaces and examining meteorites. Some claim to have identified evidence that microbial life has existed on Mars.

An experiment on the two Viking Mars landers reported gas emissions from heated Martian soil samples that some scientists argue are consistent with the presence of living microorganisms. Lack of corroborating evidence from other experiments on the same samples, suggests that a non-biological reaction is a more likely.

Projects such as SETI are monitoring the galaxy for electromagnetic interstellar communications from civilizations on other worlds.

If there is an advanced extra-terrestrial civilization, there is no guarantee that it is transmitting radio communications in the direction of Earth or that this information could be interpreted as such by humans.

The length of time required for a signal to travel across the vastness of space means that any signal detected would come from the distant past.

Astronomy - Observing The Night Sky

Observing The Night Sky

The Earth rotates once a day and orbits the Sun once each year.

The first motion causes sky objects to move from east to west, and the second causes different constellations to appear in each season’s sky.

Amateur astronomers use star hopping to go from stars and constellations they know to ones they don’t know yet. First, look for noticeable patterns on the sky’s dome.

One very easy pattern to find at this time of year is the constellation Orion the Hunter.

Orion is easy to find because it contains a very noticeable pattern of three medium-bright stars in a short straight row. These stars represent Orion’s Belt.

If you can find Orion, you can use it to star hop to Sirius, the sky’s brightest star, in the constellation CanisMajor.



The celestial coordinate system, which serves modern astronomy so well, is firmly grounded in the faulty world-view of the ancients.

They believed the Earth was motionless and at the center of creation.

The sky, they thought, was exactly what it looks like: a hollow hemisphere arching over the Earth like a great dome.

The celestial dome with its starry decorations had to be a complete celestial sphere, early sky watchers realized, because we never see a bottom rim as the dome tilts and rotates around the Earth once a day.

Part of the celestial sphere is always setting behind the western horizon, while part is always rising in the east. At any time half of the celestial sphere is above the horizon, half below.

Whenever you want to specify a point on the surface of a sphere, you'll probably use spherical coordinates. In the case of Earth, these are named latitude and longitude.

Imagine the lines of latitude and longitude ballooning outward from the Earth and printing themselves on the inside of the sky sphere.

They are now called, respectively, declination and right ascension.


Directly out from the Earth's equator, 0° latitude, is the celestial equator, 0° declination.

If you stand on the Earth's equator, the celestial equator passes overhead.

Stand on the North Pole, latitude 90° N, and overhead will be the north celestial pole, declination +90°.

On Earth, 0° longitude has long been defined as a line engraved on a brass plate set in the floor under a position-measuring telescope at the Old Royal Observatory in Greenwich.

In the sky, 0h ("zero hours") right ascension is defined as where the plane of the Earth's orbit around the Sun (the ecliptic) crosses the celestial equator in Pisces the March equinox . This point is called, for historical reasons, the First Point of Aries.



Right Ascension is measured in hours (h), minutes (m) and seconds (s) and is similar to longitude on Earth. As the Earth rotates, stars appear to rise in the East and set in the West just like the Sun.

Declination is measured in degrees (°), arc-minutes (') and arc-seconds ("), and is similar to latitude on Earth.

There are 60 arc-minutes in a degree and 60 arc-seconds in an arc-minute.

Declination measures how far overhead an object will rise in the sky, and is measured as 0° at the equator, +90° at the North Pole and -90° at the South Pole.


Hand at Full Arms Length

The word "constellation" seems to come from the Late Latin term cōnstellātiō, which can be translated as "set of stars", and came into use in English during the 14th century.

A constellation is a group of stars that are considered to form imaginary outlines or meaningful patterns on the celestial sphere, typically representing animals, mythological people or gods, mythological creatures. The 88 modern constellations are defined regions of the sky together covering the entire celestial sphere.

In 1928, the International Astronomical Union (IAU) ratified and recognized 88 modern constellations, defined regions of the sky together covering the entire celestial sphere. With contiguous boundaries defined by right ascension and declination.

Therefore, any given point in a celestial coordinate system lies in one of the modern constellations.

Locating Stars


A complete twenty-four-hour time cycle is marked on the rim of the overlay. A full twelve months of calendar dates are marked on the rim of the starchart. The window is marked to show the direction of the eastern and western horizons.

The disk and overlay are adjusted so that the observer's local time of day on the overlay corresponds to that day's date on the star chart disc.

The portion of the star chart visible in the window then represents (with a distortion because it is a flat surface representing a spherical volume) the distribution of stars in the sky at that moment for the planisphere's designed location.

Users hold the planisphere above their head with the eastern and western horizons correctly aligned to match the chart to actual star positions.



Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just as you would see with the naked eye, binoculars or a telescope.


Experience Earth and our solar system, the universe and the spacecraft exploring them, with immersive apps for Mac, PC and mobile devices.



WorldWideTelescope is an open source set of applications, data and cloud services, originally created by Microsoft Research but now an open source project hosted on GitHub. The.NET

American Astronomical Society