Astronomy, one of the oldest sciences of humankind, always provided orientation in space and time: Cardinal directions (east, north, west, south) are defined and obtained by basic astronomical measurements.
Time and calendar issues are also definable and measurable by astronomical observations: One “year” is the period the earth needs for one full revolution around the sun (originally, before the Copernican revolution, it was seen the other way around), and one “month” is roughly the time our moon needs to orbit the earth.
The currently most widely used calendar system, the Christian calendar in use in Europe, North and South America, and many other parts of the earth, is based mainly on the motion of the earth with respect to the sun. Other cultures have developed slightly different calendars based either on the moon (e.g., the Moslem calendar) or a combination of sun and moon (e.g., the Jewish calendar).
We count seven days per week because, a long time ago, people considered “seven” objects as “planets” or “planetlike objects,” namely the real planets, which could be observed by the naked eye before the invention of the astronomical telescopes: Mercury, Venus, Mars, Jupiter, and Saturn, as well as the other two large visible bodies in the solar system, the sun and our moon, together seven objects, hence also the names of the seven days of the week:
Sunday, as the original first day of the week, is the day of the sun, the brightest object in the sky, often even worshipped as a god in several ancient cultures.
The word Monday obviously refers to the moon.
god of War
Tuesday is named after Mars, the god of War (notice in French, Italian, and Spanish, the words for Tuesday are still close to that for the Roman God Martius (for Mars), namely Mardi, Martedi, and Martes, respectively) and it originally comes from Tiwes dag or Tyr dag, from the old Teutonic word Tyr for Mars.
Wednesday is named after the Roman God Mercury (in Romanian, the day is still known as Miercuri), and the word Wednesday itself comes from Wodan dag for the Teutonic god Wodan.
In Roman times, the fifth day of the week (Thursday) was known as dies Jovis, after their god of thunder and chief of the gods, Jupiter, where Thursday itself comes from Thunor dag, the day of the Teutonic God Thor.
The Romans named another day after their goddess of beauty, Venus, and called it dies Veneris (still similar in French). When Germanic tribes invaded England more than 500 years ago, they imposed their goddess upon that day and called it Frigedaeg, now Friday.
And finally, Saturday is obviously called after Saturn.
Nowadays, both time and the unit second are defined by the speed of light. Previously, a second was defined by the atomic clocks, and also earlier as one certain small part of a day, that is, one small part of a revolution of the earth. Still, astronomical observations are important for fixing “time”: Due to tidal interaction among the sun, earth, and moon, the rotation period of the earth is very gradually slowing down.
The definition of a planet has changed over the centuries, always following new astronomical observations and new understanding. The word planet comes from the Greek word for wanderer, meaning a wandering or fastmoving starlike object (e.g., the old Arabic name for the Egyptian capital Cairo is “Al Qahira” for “the backwards wandering,” meaning Mars).
As mentioned above, a few hundred to 3,000 years ago, people could see, by the naked eye, seven objects apparently moving fast in the sky (compared to the “fixed stars”), incorrectly thought to orbit around the earth in the center, namely the sun, Mercury, Venus, the moon, Mars, Jupiter, and Saturn.
The next planet known today behind Saturn, called Uranus, is also visible to the naked eye during clear and dark nights, and may be visible when Uranus is close to the sun and the earth is roughly in between the sun and Uranus. This is called opposition: when an outer planet like Uranus is brightest as seen from the inner planet like Earth, but no such reports are known so far , possibly because Uranus moves only slowly and is quite faint.
During the Renaissance period in general and the socalled Copernican revolution in particular, it became clear, through a number of new observa tions, that the old theory placing the earth in the center of the universe was not perfect.
Those observations became possible with the invention of the astronomical telescope. In 1609, Galileo Galilei observed the phases of Venus and craters on our moon and discovered moons around Jupiter (first called “Medici planets” or “Medici stars” after his supporters of the Italian Medici family, and now known as “Galileian moons”).
All this together favored an alternative explanation, putting the sun in the center of our solar system and having the planets orbiting around the sun. At this moment, it also became clear that the earth is orbiting the sun and, hence, was now seen as a planet. Later, two more planets were discovered beyond Saturn, namely Uranus and Neptune.
While it was always possible to estimate the orbital periods of planets around the sun by their periodic appearance and disappearance in the sky, it was originally difficult to measure distances between the planets, or from Earth to either its moon or the sun. The distance between Earth and the sun is now called the astronomical unit, which is about 150 million kilometers.
The first good estimates of such distances were obtained a few centuries ago by observing eclipses of the sun by the inner planets Venus and Mercury, which happen only very rarely (usually only once or a few times per century): One has to measure exactly either the angular distance between the apparent path of the planet across the solar disk, as seen from two different locations on Earth, or the exact times of ingress and egress of the planet moving in front of the sun.
These four socalled contacts must be observed and measured from different locations on Earth with as large as possible a distance in between them, for example from South Africa and Europe. A few centuries ago, it was still difficult to coordinate such efforts and also to run precise clocks. A first observation was done in 1639, a Venus transit.
After several attempts, the first good values for the distance between Earth and the sun were obtained in 1761 and 1769—these values also giving evidence about the size of the sun. Together with the laws of gravity just determined by Isaac Newton and their application to the solar system by Johannes Kepler, these values immediately yielded all distances between each of the planets and the sun.
Toward the end of the 18th century, the socalled TitiusBode law was found and discussed: According to this law, the distance from planet to planet roughly doubles with each planet reached as one moves further away from the sun; for example, Saturn is roughly twice as distant from the sun as Jupiter, Uranus is roughly twice as distant as Saturn, and so on.
However, from Mars to Jupiter, the distance increases roughly by a factor of four, so that there would be space for one more planet. Even the famous philosopher G. W. F. Hegel wrote his dissertation about this problem at the University of Jena in Germany.
Many astronomers were already hunting for this new planet. Then, in January 1801, an object was found at the expected distance from the sun, called Ceres, and celebrated as a new planet. However, soon afterward, more similar objects were found, all at a similar distance; a few decades later, the solar system had more than 20 “planets.”
It was also found that these new objects were smaller than all other previous planets, so it was decided to call them “minor planets” (a new class of objects). Hence, objects celebrated and counted as planets were removed from the list of planets by a new definition.
Early in the 20th century, another new object was discovered and celebrated as a new, ninth planet, called Pluto, located most of the time beyond Neptune, but sometimes crossing its orbit.
The Solar System
There are now eight planets in the solar system.
Mercury, the innermost known planet, is also the smallest known planet in our solar system with a diameter of less than 5,000 kilometers. (Pluto is smaller, but it is not a planet anymore according to the new definition.)
It has a rotation period of 59 days, which is about two thirds of its orbital period around the sun (88 days); hence one “Mercuryday” is equal to two “Mercuryyears.” Mercury does not have an atmosphere that is comparable to that of Earth, and its surface is similar to that of the moon. Two thirds of its material and mass is made of iron.
According to Einstein’s general theory of relativity, the orbit of Mercury should change slowly: The location of the perihelion (the point in the planet’s orbit at which it is the smallest distance from the sun) moves by a small angle of 43 seconds or arc per century, which has been confirmed observationally.
theory of relativity
Venus needs 225 days for orbiting the sun (compared to 365 days for one Earth orbit around the sun). The rotation of Venus around its own axis is retrograde, that is, in the rotational direction opposite to the direction in which it revolves around the sun, and one such “Venusday” lasts 243 days; that is, it is longer than one “Venusyear.”
Venus has a dense atmosphere consisting mostly of carbon dioxide and nitrogen, and it has strong pressure on the surface, from where one would never be able to see the stars in the night sky through the dense clouds.
Mercury and Venus, as planets inside the earth’s orbit, orbit the sun faster than the earth does and are often close to the sun, as seen from Earth. Hence, they are observable either in the evening sky just after sunset or in the morning sky just before sunrise; that is, Venus is also called the “morning star” or the “evening star.” The Greeks called Mercury “Hermes” when it appeared as the evening star and “Apollo” when it appeared as the morning star; Venus was similarly called either “Hesperus” or “Phosphorus,” respectively.
Earth is the third planet from the sun; it needs 365 days for a complete orbit around the central star and 24 hours for one rotation. Its atmosphere consists mainly of oxygen and nitrogen. This planet is the only one known so far to harbor living beings like plants, animals, and intelligent life.
The fourth planet is called Mars. It has an orbital period of 687 days and a rotation period of 24.6 hours, so that a “Marsday” is only slightly longer than a day on Earth. Its thin atmosphere consists mostly of carbon dioxide and nitrogen, but this atmosphere is not identical to that of Earth.
There is frozen carbon dioxide and water ice at the poles, but no fluid water has yet been detected. However, some surface structures look like dry river beds and may indicate that fluid water was present some billions of years ago. It is not impossible that life has formed on Mars, too, but no clear evidence for life on Mars has been found yet.
Phobos and Deimos
Mars is orbited by two small moons, called Phobos and Deimos, with 8hour and 30hour orbital periods, respectively. Like the moon of Earth, their rotation is bound: Their orbital period equals their rotational period; they are rotating around themselves only by orbiting their planet and always show the same side to their planet.
The innermost four planets are also called terrestrial planets, as they are all made mostly of solid material like Earth (terra). Between Mars, the fourth planet, and Jupiter, the fifth planet, there is a large gap where many small bodies are orbiting the sun. These are called minor planets or sometimes planetoids, because they are physically like terrestrial planets; that is, they are rocky objects.
They are also called asteroids, because in the sky they look like the stars looked when they were discovered, namely pointlike (as opposed to the planets of our solar system, which appeared to be extended on the sky even in nakedeye observations, because of their larger size and smaller distance from observers).
The four outermost known planets (Jupiter, Saturn, Uranus, and Neptune) all are larger in size than the terrestrial planets, mostly because of their large atmospheres and only small solid or fluid cores. (In the case of Jupiter, there may not even be a core at all.) Hence, they are called the “gaseous giant planets.”
Jupiter is the largest planet in our solar system; it has a diameter of 143,000 kilometers and a mass of 318 times the mass of Earth. It needs almost 12 years for one orbit around the sun, but only 10 hours for a rotation around itself, as can be observed with even a small telescope because of the moving large red spot in its outer atmosphere.
Given its diameter, mass, and composition (mostly molecular hydrogen), it is not absolutely clear whether it has a solid or fluid core or possibly even no core, that is, no solid surface. If it has a core, the core could have a mass of a few or maybe 10 Earth masses. Due to contraction, Jupiter is still radiating more energy to outer space than it is receiving from the sun.
This giant planet also has a small ring system and a large number of moons, probably a few dozen; new small moons are still being discovered. The four largest moons were originally discovered by Galileo, when he observed Jupiter for the first time with a telescope. These four moons (Io, Europa, Ganymede, and Callisto) are called the Galileian satellites.
Saturn is twice as far from the sun as Jupiter is. Saturn is known mostly for its large ring system. It also has a large number of moons. Saturn needs 29.5 years to orbit the sun and has a rotation period of 10 hours and 40 minutes.
It has a solid core of a few Earth masses and a large atmosphere made mostly of molecular hydrogen gas. Saturn is the second largest planet (120,000 kilometers in diameter) and the second most massive (95 times the mass of Earth) in our solar system.
All planets from Mercury to Saturn (including Earth) have been known for several thousand years to most cultures on Earth, because they can be observed by the naked eye. The outermost planets, Uranus and Neptune (as well as Pluto), were discovered after the invention of the telescope. While Jupiter and Saturn are called “gas giants,” Uranus and Neptune are also gaseous planets that can be seen as “ice giants.”
Uranus was discovered (and recognized as a planet) in 1781 by William Herschel. Others had observed it before but did not recognize that it as a planet. Uranus is also a gaseous planet with a central solid core, but in total it is only 15 times as massive as Earth.
Uranus needs 84 years to circle around the sun, and one “day” on Uranus lasts around 17 hours. Uranus’s atmosphere consists of 83% hydrogen, 15% helium, and 2% methane. So far, 21 moons have been discovered (and astronomers are still counting). Uranus also has a small ring system as discovered by the Voyager satellites.
Neptune is the outermost known (and accepted) planet. It was observed by Galileo in 1612, but he did not recognize it as a planet. Because of apparent deviations in the orbit of Uranus, both John Couch Adams and Urbain Le Verrier predicted the existence of another planet theoretically and tried to forecast its rough location in the sky.
Later, in 1846, the observer Johann Gottfried Galle in Berlin, Germany, searched that area of the sky for a small moving object and discovered Neptune within a few hours. Neptune needs 165 years for one full circle around the sun.
One “day” on Neptune lasts 16 hours. Neptune has a small solid core, a large gaseous atmosphere composed mostly of molecular hydrogen, and a total mass of 17 times the mass of Earth. Neptune, like all gaseous planets in our solar system, has moons and rings.
The object Pluto was discovered in 1930 and celebrated as a new planet, but it was deleted from the list of planets in the 2006 definition of planet by the International Astronomical Union.
The new definition of planet is formulated for the solar system, but it can and should be applied analogously to other planetary systems around other stars. However, there is as yet no consensus or definition for the upper mass limit of planets. Such an upper mass limit, however, would be very important for extrasolar planets, to be able to decide whether they are planets or socalled brown dwarfs.
In history, the two definitions for a planet worked for about 200 years: The first definition worked from the Copernican revolution to the discovery of Ceres and other minor planets (which now form the asteroid belt between Mars and Jupiter); the second definition, excluding the minor planets, was in effect again for about 200 years until 2006.
Both the problem regarding Pluto and the missing upper mass limit for planets may very well lead to a new definition at one of the next meetings of the International Astronomical Society, which holds a general assembly every three years.