Basic astronomy

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ASTRONOMY The word “astronomy” comes from the Greek language “αστρονομία” which means “Stars law”, and it’s an old science that studies celestial objects as well as the formation and development of the universe.

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Unit about Astronomy (in English

Transcript of Basic astronomy

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The word “astronomy” comes from the Greek language “αστρονομία” which means “Stars

law”, and it’s an old science that studies celestial objects as well as the formation and

development of the universe.

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The Solar System• The Solar System consists of the Sun and the celestial

bodies within its gravitational influence, which includes the eight planets and their satellites, the dwarf planets (as Pluto), asteroids, comets, and interplanetary dust.

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The four smaller inner planets; Mercury, Venus, Earth and Mars, also called the “terrestrial planets”, are primarily composed of rock and metal. The four outer planets, Jupiter, Saturn, Uranus and Neptune, also called the “gas giants”, are composed largely of hydrogen and helium and are far more massive than the terrestrials.

(Pluto is no longer considered as a planet).

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Satellites A natural satellite, also known as “moon” is a small celestial body orbiting a larger one. Written with a capital “M”, the Moon refers to Earth unique natural satellite. Every planet in the Solar System, except Mercury and Venus, has at least one natural satellite in orbit about it. The discovery of planetary satellites was important because measurement of the period and dimensions of a satellite’s orbit enables to determine the mass of its planet (or strictly, the combined mass of the planet and satellite).

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Satellites of the planets.

There are 167 moons orbiting six of the eight planets:

The large gas giants have many moons, including half a dozen comparable in size to Earth's moon: - The four Galilean moons of Jupiter.- Saturn's Titan- Neptune's Triton.

Of the inner planets:- Mercury and Venus have no moons at all.- Earth has one large moon, known as the Moon - Mars has two tiny moons, Phobos and Deimos.

It has been suggested that a few moons, notably Europa, one of Jupiter's Galilean moons, may harbour life, though there is currently no direct evidence to support this claim

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Selected moons (satellites).

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The Moon

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The Moon- The Moon is the Earth’s

only natural satellite. – Because of its proximity,

it is the brightest object in the sky, apart from the Sun. It is at a mean distance of only 384,000 km.(distance goes from 356,000 km. to 406.000 km.)

– Much of the Moon’s surface is covered in a fine dust. Because there is no wind or weathering, this material remains just as it settles, and the marks of small impacts, as well as astronauts with lunar carts, remain more or less permanently.

Picture of the surface of the Moon.

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-- The length of its diameter (3476 km.) is less than a third of the Earth’s diameter.- - The Moon has about 1/14 of the Earth’s area and about 1/50 of its volume.-

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• The Moon also has two different kind of motions:

• - An orbital motion around the Earth that takes about 28 days.

• - A rotational motion around its axis that also takes around 28 days.

• - As its orbital period around is the same as its axial rotation period, an observer on the Earth always sees the same side of the Moon (“the nearside”) .

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The invisible side of the Moon

● “Everyone is a Moon, and has a dark side which he never shows to anybody.”– Mark Twain

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The Moon orbiting the Earth. Phases of the Moon.

A common misconception is that the phases of the Moon are caused by the shadow of the Earth but, actually, the phases of the Moon are the result of varying viewing angles throughout the month.

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The rotational motion of the Earth combined with the orbital motion of the Moon around the Earth implies that it takes 50 minutes more each day for the Moon to appear in the sky.The following diagram shows the Moon at sunset over its 4 weeks orbit around the Earth.

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Lunar Libration● You might expect that if the Moon always keeps the same

face pointed toward Earth, that we can only see 50% of the Moon.– However, the Moon appears to rock back and forth. This

back and forth rocking is called libration.● In its elliptical orbit around Earth, the Moon moves

faster when close to the Earth and slower when farther. Since its rotation is fairly constant, we get to see a little bit around each edge.

● In addition, the orbit of the Moon is tilted with respect to the ecliptic, so we get to see somewhat beyond the poles as the Moon moves above and below the ecliptic.

● 59% of the Moon is visible to us on Earth.

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Lunar libration

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“Galileo, perhaps more than any other single

person, was responsible for the birth of modern


Stephen Hawking.

Italian astronomer, physicist and mathematician.

He improved the telescope and made important astronomical observations with it.

Galileo discovered mountains and craters on the Moon, four satellites of Jupiter known as the “Galilean satellites”, and he also observerd the phases of Venus.

He supported the heliocentric theory which places the Sun at the centre of the universe with planets orbiting around the Sun instead of around the Earth.

This brought him into conflict with the Catholic Church (that at the time considered heliocentrism “contrary to Scripture”) and led him to trial and house arrest for the last eight years of his life.

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Venus• Venus is the second-closest

planet to the Sun.

• It orbits the Sun every 224.7 days.

• The planet is named after Venus, the Roman goddess of love and beauty.

• After the Moon, it is the brightest object in the night sky.

• Venus and Earth are similar in size, gravity and composition.

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The phases of Venus

From September 1610 Galileo Galilei observed that Venus displayed a full set of phases similar to the phases of the Moon (that’s because Venus is nearer to the Sun than the Earth as you can see in the diagram). So, Venus shows its illuminated hemisphere to the Earth when it is on the opposite side of the Sun, while you can’t see it when it is in between the Sun and the Earth. This discovery of the phases of Venus helped to affirm Copernicus’ heliocentric theory.

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JUPITER● Jupiter is the fifth planet from the

Sun and the largest planet in the Solar System.

● It is the third-brightest object in the night sky after the Moon and Venus, though Mars can be brighter than Jupiter in certain points of its orbit.

● Its orbital period is 11.86 years.● Jupiter is primarily composed by

hydrogen an helium, and may also have some rocky elements

● In march 2011 it will reach its nearest position to the Sun, which implies that in September 2010 you will be able to see it especially bright.

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● Galilean satellites (Jupiter): Io, Europa, Ganymede and Callisto

On 7 January 1610 Galileo observed with his telescope what he described at the time as "three fixed stars, totally invisible by their smallness" close to Jupiter. Observations on following nights showed that the positions of these "stars" relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars. On 10 January Galileo noted that one of them had disappeared, an observation which he attributed to be hidden behind Jupiter. Within a few days he concluded that they were orbiting Jupiter. He had discovered three of Jupiter's four largest satellites (moons): Io, Europa, and Callisto. He discovered the fourth, Ganymede, on 13 January. This was the first proof that not all motion in the Solar System is centred in the Earth. So these small heavenly bodies orbiting a planet became the first step towards the acceptance of the heliocentric theory.

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KEPLER Johannes Kepler (1571-1630) was a German astronomer and mathematician. He is best known for his three laws of planetary motions around the Sun. Observing the movement of Mars he concluded that Mars moves in an elliptical orbit. He also made important contributions to optic, improved the telescope, and helped to legitimize the discoveries of his contemporary Galileo Galilei.

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MARS• Mars is the fourth

planet from the Sun in the Solar System. It is also known as “The Red Planet” because of its reddish appearance due to iron oxide dust prevalent on its surface.

• It has a thin atmosphere. In 1971, American spacecraft Mariner 9 observed a great dust storm during months.

Mars in 2001 as seen by Hubble Space Telescope.

It is suggested than there are some sedimentary rocks on its surface that had formed when liquid water existed on the planet’s surface. It is believed that polar caps have water ice.

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Surface of Mars

Mars has a thin atmosphere which is extremely tenous. In 1971, American spacecraft Mariner 9 observed a great dust storm during months.

Below you can see a picture of sunset in Mars where there is very little refraction of light. This picture was taken by Mars Pathfinder, an American spacecraft that was launched on 1996 December 4 and arrived at Mars at 1997 July 4, using an airbag method to land at 1 km from the initial impact site. Two days later a small vehicle rolled down a ramp on to the rocky surface, took many images and carried out several chemical analyses of rocks

and soil

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Natural satellites of Mars:FOBOS Y DEIMOS

Mars has two moons, Deimos and Fobos, which are small and

irregularly shaped.

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Elliptical orbits of the planets

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First law of planetary motion(1609): The orbit of every planet is an ellipse with the Sun at a focus.

Put a pin in the position of each focus, and attach a string to them. Put a pencil point against the string and pull the string taut with the pencil, and keeping the string taut, move the pencil in a large arc. The pencil will draw the desired ellipse. Advice: draw the upper and lower halves of the ellipse separately. As you see, the sum of the distances of any point of ellipse to the foci is always the same.

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Kepler’s second law.Second Law (1609): A line joining a planet and the

Sun always sweeps out equal areas in equal intervals of time.

The two shaded sectors EF and CD have the same surface areas.Therefore, if a planet moves from E to F in the same time that it moves from C to D (equal areas covered in an equal time), then the planet must move faster when it is near the Sun (from C to D) than when it is far from the Sun (from E to F).

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Kepler’s third lawThird Law (1618): For any planet, the square of its orbital period T (the time going around the Sun until it comes back to its original position) is directly proportional to the cube of the semi-major axis of its orbit (a).

So, the nearer a planet is to the Sun (less distance to the Sun), the faster it orbits the Sun (less orbital period).

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EARTH• The Earth is the third

planet from the Sun, and the fifth planet by size. It’s the only planet known to support life in the universe.

• Its mean distance from the Sun is defined as one “astronomical unit”.

• Its orbit around the sun defines the plane of the “ecliptic”

kmUA 610·6.1491

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Elliptical movement of the Earth around the Sun.

It takes 365.26 days for the Earth to orbit the Sun, with a mean orbital speed of 29.76 km/s. In this period the Earth travels 930.000.000 km.

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ROTATION:• It takes 23 hours and 56

minutes for the Earth to complete one revolution about its rotational axis.

• Imagine the line, which join north and south poles, and about which the Earth rotates or has rotational symmetry. The angle between this spin axis and the perpendicular to its orbital plane is called the axial inclination, which in the case of the Earth is 23.67º.

• This inclination creates long periods of either darkness or light in the poles. Besides it is the cause of the seasons since the angle of solar radiation changes.

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Because the Earth’s axis is inclined to its plane of orbit, one of its hemispheres gets more light and has longer days, and so experiences summer, whereas the other hemisphere experiences winter. Six months later the situation is reversed.

In summer solar rays are almost perpendicular to the Earth’s surface; that means that Sun’s light (and heat) is located in a small terrestrial area.

Besides, in summer the daylight is longer.

• • In winter solar rays are more

inclined, so that its energy (heat) spreads over a bigger area.

We already know that the distance between the Earth and the Sun is bigger in summer than in other seasons. Then, why is it hotter in summer?

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Lunar eclipses• The Moon revolves around the

Earth once each month. Since the Moon's orbital plane is inclined by five degrees, we do not have lunar eclipses each month because the Moon passes either above or below the Earth's shadow. However, every few months the Moon passes either partially or entirely into the Earth's shadow, resulting in a partial or total lunar eclipse. This can only happen when the Earth lies between the Moon and the Sun when the Moon is in its full phase.

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Solar eclipses

Why do we not see an eclipse when other objects pass in front of the Sun?

• Like the Moon, the planets Mercury and Venus also pass between the Earth and the Sun from time to time. However, they are much further away from the Earth than the Moon. They look much smaller in the sky, and never block the light of the Sun completely.

• In fact, it is an unlikely coincidence that we see total solar eclipses at all. The Moon is about 400 times smaller than the Sun, but also happens to be about 400 times closer. Therefore, the two bodies look about the same size in the sky.

• The Moon is moving slowly away from the Earth (at a rate of a couple of centimetres a year.) In a billion years' time, the Moon will appear too small for there to be any total solar eclipses at all.

A solar eclipse occurs when the Sun, the Moon and the Earth all line up. The Moon blocks the light of the Sun, and casts a shadow on the Earth.During a total solar eclipse, the sky appears as dark as night, and you can see the Sun's glowing corona.

Why don't you see a solar eclipse every month? The Moon goes around the Earth once every four weeks. However, it revolves at an angle to Earth's path around the Sun. So the Earth, Moon and Sun do not end up in a straight line every month.

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● Mercury is the smallest planet and nearest to the Sun.

● Mercury orbits the Sun in 87969 days.

● It completes three rotations about the axis for every two orbits.

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Mercury• As Mercury and Venus are

nearer to the Sun than the Earth, sometimes you can’t see them because they display phases (as Moons phases).

• In this diagram, you can’t see Mercury when it is at position (1), and its “full” at position (3). At positions (2) and (4) you can partially see it.

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Saturn is the second largest planet of the Solar System.

It has a wonderful system of rings, first seen by Galileo in 1610, even though he thought they were moons.

Saturn is composed of hydrogen and helium.

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● Uranus is the seventh planet in the Solar System, and the first to be discovered telescopically by William Herschel in 1781.

● It has at least 21 satellites.

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Satellites of Uranus:UMBRIEL and ARIEL

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NEPTUNE● After Uranus was discovered in

1781, its orbit was calculated in order to follow the planet’s motion. Before about 1820, however, Uranus was found to get ahead of its predicted position. It was suggested that this anomaly was a result of the gravitational influence of a unknown planet orbiting farther from the Sun. From observed irregularities in the motion of Uranus, mathematicians John Couch Adams, working in England, and Urbain Le Verrier, working independently in France, both predicted the location of the unseen planet. It was Le Verrier who eventually persuaded the German astronomer Galle in Berlin to search the region of the sky in which he had calculated that the new planet would lie. On 1846 September 23, Galle located the new planet very close to the predicted position.

Neptune appears blue because methane in its atmosphere absorbs red light.

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Satellites of Neptune:THALASSA and DESPINA.

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A constellation is an arbitrary group of stars. Today astronomers reconized 88 constellations, that are based on the ancient Greek culture (in fact their names are based on Greek mythology).

A constellation has no real significance as their stars are at very different distances from us, and appear close together only because of a line-of-sight effect.

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A star describes a complete circle in the sky, once per day, due to the Earth’s rotation around its axis. However it can disappear below the observer’s horizon if it is not sufficiently close to the celestial pole. In the diagram you can see such apparent circle for “Alkaid”, the end star of constellation Ursa Major, while Arcturus (one of the brightest stars in the sky) may dissapear for some hours.

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Night sky at 0.00 , 2010-08-02 (S=South)

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Night sky at 0.00, 2010-08-02 (Águila constellation lies to the South, above Escudo)

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Night sky at 0.04, 2010-08-02(E=East)

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Night sky at 0.0, 2010-08-02(Osa Menor is close to Casiopea)

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Night sky at 0.02, 2010-08-02(N=North)

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brightest stars.