Science 9
Astronomy

Two areas where much of modern Science concentrates its investigations are at the opposite ends of our vision world - the unimaginably tiny revealed by the microscope and the unimaginably huge revealed by the telescope. In 1610, the Italian astronomer Galileo used a two lens telescope to examine the moons of Jupiter and so, led us into the world of the enormous. In 1674, Anton van Leeuwenhoek, using a single lens microscope, led us into the world of the minute with his diagrams of microscopic life.

Ancient Astronomy Looking up has been an important activity for thousands of years. The locations of and patterns of motion of heavenly bodies, also called celestial bodies, have helped humans in numerous ways - predict weather, predict the best time for farming activities, tell where they were on the earth, navigate across land and sea - the list is long. Ancient records and, in some cases, ruins, tell us of the construction of astronomical observatories all over the globe. Stonehenge, about 130 km west of London, England, the Pyramids of Egypt, the earth and rock mounds found in the Eastern United States, the temples built by the Maya of Mesoamerica - all are popular destinations for historians of astronomy.

As time went by, sky watchers in some civilizations became an important segment of society and their body of knowledge and skills gradually developed into what we call astrology. Although astrology has long ceased to be a part of science, it holds a key place in the history of astronomy.

The Motions of The Heavens The casual viewer will notice that heavenly bodies parade east to west across the day and night sky, sometimes in a predictable pattern, sometimes apparently not. For example, as the uncountable stars all move across the night sky in a slow majestic way, they seem to remain in the same locations, relative to each other. In contrast, the Sun, Moon and planets can be found in a slightly different location each time we look for them. Why is this? For thousands of years, it seemed obvious to most observers that the Earth was motionless and the heavenly bodies revolved around it. The Earth was thought to be at the center of a huge sphere to which all the stars, Sun, Moon and planets were attached. As the sphere revolved, it carried them along, moving them through the sky. More precisely, the heavens contained a number of transparent spheres, all sitting one inside the other, with the Earth at the center. As each sphere turned, it carried with it the heavenly body attached to it. Objects that looked close to Earth and whose positions changed noticeably from day to day were located on spheres closest to Earth. Stars, whose positions appeared not to change, were located on the outermost sphere, called the "firmament of fixed stars" or the celestial sphere. To move all the celestial objects easily seen by ancient observers, 55 spheres concentric spheres were required. This Earth-centered (geocentric) model of heavenly motions is based on the ideas of the famous Greek philosopher Aristotle. His hypotheses was widely accepted and lasted about 2 000 years.

During the Renaissance (the 14^th to the 17^th centuries in Europe) the Polish astronomer Nicholas Copernicus suggested the planets travelled across the heavens on a huge imaginary flat disc called the solar plane with the Sun at its center. This Sun-centered (heliocentric) model provided a more accurate description of how the planets moved and was a one of the key ideas in the history of astronomy. In fact, the ideas of Copernicus were so powerful they started a revolution in scientific thinking called the Copernican Revolution.

The Solar System The Sun is the heart of the Solar System. It's gravity holds the whole thing together and provides significant heat and light to the inner planets. From the outermost planets, the Sun is just a dot much like our view of distant stars from here on Earth. The Sun is huge, 110 times the size of Earth. It is an extremely hot ball of gas, mostly hydrogen and its surface, always churning, often sends up huge arcs of gas called solar prominences. The surface is mottled with patches of cooler, darker material called Sun spots. Solar flares often erupt next to these spots and spew huge amounts of high energy subatomic particles into space. These particles create the solar wind which has negative effects on power and communications when it reaches Earth.

The Inner Planets Mercury is about 0.4 Earth size. It thin crust is heavily cratered and surrounds a mostly solid, iron rich core. It has almost no atmosphere and its closeness to the Sun means the average temperature is about 350⁰. The orbital period (time to circle Sun) is only about 0.24 years.

Venus is 0.95 Earth size and has a rocky cratered crust surrounding a semi-solid iron-nickel core. The thick atmosphere made mostly of CO₂ holds in huge amounts of the Sun's heat and the average temperature is 480⁰. The orbital period is about 0.6 years.

Earth's rocky crust surrounds a two-layer core. An outer molten zone covers the solid inner one, rich in iron and nickel. The atmosphere contains about 80 % nitrogen and 20% oxygen. Although it is as thin as a layer of plastic food wrap on an apple, it is active enough to have erased most traces of craters. Oceans help moderate temperature variations and the average is 22⁰.

Mars is about 0.5 the size of Earth. It has a thin crust over rocky mantle surrounding a solid core. The crust's surface shows signs of ancient weathering. Mars' red color is due to the natural rusting of iron in the surface rocks. The thin atmosphere is about 95% CO₂. The average temperature is only - 23⁰ and the orbital period is about 1.9 years.

The Outer Planets Jupiter is a giant: 11 times the size of Earth and 2.5 times as massive as all the other planets combined! It has a rocky core surrounded by solid hydrogen, then a layer of liquid hydrogen and finally gaseous hydrogen. Its distinctive bands of clouds are produced by its rapid rotation. A distinguishing feature among the cloud bands is a huge eddy called the Great Red Spot, perhaps a storm that has lasted for thousands of years. The average temperature is - 150⁰ and the orbital period is about 12 years.

Saturn is 9.5 times the size of Earth. A key feature of this large yellowish planet is its system of rings, made of ice coated rock chunks and dust. Its structure is similar to Jupiter's: a rock and ice core covered with solid hydrogen, then liquid hydrogen and lastly gaseous hydrogen. And like Jupiter, its rapid rotation creates a banded atmosphere. Note: Jupiter and Saturn are so much gas, the term "atmosphere" has no meaning. The average temperature is - 180⁰ and the orbital period is about 29.5 years.

Uranus is 4 times the size of Earth. Its rocky core and icy mantle are covered with a mostly hydrogen atmosphere which gives the planet a blue look. The average temperature is - 214⁰ and the orbital period is 84 years.

Neptune is sometimes called the twin of Uranus because its structure and contents are so similar. One notable surface feature is the Giant Dark Spot. The average temperature is - 220⁰ and the orbital period is about 165 years.

Pluto is the most distant planet; some think it is not a planet at all, just a huge chunk of material left over from the creation of the solar System! Its average temperature is - 230⁰ and its orbital period is about 248 years.

Other Solar System Bodies Asteroids are millions of irregularly shaped chunks of rock orbiting between mars and Jupiter. The largest, Ceres, has a diameter of 1 000 km. Some think asteroids are the remains of a planet that was pulled to pieces by unbalanced gravity tugs from the neighboring planets and the Sun. Asteroids are a potential hazard if their orbit brings them close to Earth.

Comets are icy balls of rock and dust though to be travelling in huge orbits far from the Sun. Some times, an unusual combination of gravity tugs will direct them toward the Sun. As they approach, its heat vaporizes their outer layers. As matter leaves the comet's surface, it is stretched out by the solar wind into beautiful tails thousands of kilometers long. The tails always point away from the Sun. Many comets have been discovered by amateur astronomers but this is becoming less common as computer controlled telescopes automatically scan the skies looking for asteroids whose orbits may bring them too close to us. Because so much sky is scanned so often, the computer is getting credit for numerous comet discoveries.

Meteors are rocky fragments that happen to enter the Earth's atmosphere. Friction with the air heats them until they begin to burn. Meteorites are meteors large enough to survive the fiery trip down to the Earth's surface. The surfaces of the larger meteorites bear the signs of incredible heating and partial melting. Some meteorites are thought to have originated on the surface of Mars, blasted into space when a body impacted its surface. Chance and gravity then brought them to Earth.

The Formation of The Solar System Although scientists may not agree on all the details, one hypothesis is that there existed a huge cloud of dust and gas. Over time, it to begin to revolve and condense. Pockets began to grow here and there and their gravitational fields pulled in still more matter. Eventually, the cloud organized itself into a huge mass of condensed dust and gas at the center with smaller, orbiting masses of dust and gas. As the orbiting masses raced along in their huge arcs around the core, they scooped up more and more dust and gas, growing larger all the time. Eventually, due to time and gravity, the enormous central mass became the Sun and the orbiting masses formed the planets. A planet's distance from the Sun and it's awesome radiation determined if it became a small, rocky Inner Planets or an Outer Planet, one of the gas giants.

Just How Big Is Big? Infinity is a word often used in descriptions of the size of the universe. The problem is, the meaning of the term is very difficult to grasp. One simple definition for it is the "largest possible." But, this doesn't help. Think of a huge number with many zeros. Now double it, again, and so on. Where does it stop? It doesn't and that's why the idea of infinity is hard to understand.

How does all this relate to our investigation of space? Astronomers seem to be unable to find a place where the universe stops. There seems to be no end to space around, since humans are used to boundaries, this idea is unsatisfying. Each time a newer, more powerful telescope is built and pointed up, it captures "older" light than the previous "most powerful" telescope. Think: all light reaching Earth left some glowing material out there somewhere. Some of it left objects so far away, it has taken thousands and thousands of years to get here. In fact, some of the light captured by telescopes had been flying through space for millions of years. FYI: telescope images of glowing gas clouds and swirling galaxies are some of the most beautiful, inspiring pictures you will come across.

Space may be very cold and dark in many areas but it is alive with motion. Matter rushes along, sometimes straight ahead, sometimes in huge swirling eddies, everything on its way somewhere. Not only does the universe seem to be limitless already, it is getting bigger. How can something that appears to have no maximum size get "larger"?

But, why do we think the universe is expanding? Starlight tells us. White light contains a secret - all the beautiful rainbow colors are hidden in it. Passing white light through a prism allows us to see the spectrum of colors, i.e., the pattern of these hidden colors. The rainbow spectrum is: red, orange, yellow, green, blue, indigo and violet. (Think of a person named Roy G. Biv.) Light from a star will most likely pass through at least a bit of cold dust on its way to Earth and some of the color in the starlight will be blocked out by the dust. Then, here on Earth, when we look closely at the star's color spectrum, we will see some dark lines where the missing colors would have been if the dust had not trapped them. If the star is moving toward us, the dark lines are located in the blue end of the spectrum and if it is receding from us, its dark lines are located in the red end of the spectrum. The location of the dark lines in the spectra of many stars indicate they are moving away. By the way, a star's spectrum also tells us what chemicals it contains and the color of its light indicates the temperature. Red stars are about 3 000⁰ while blue stars are about 25 000⁰.