Unit about Astronomy (in English

4. 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).

6. Selected moons (satellites).

7. The Moon

12. 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.

13. 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.

15. Lunar libration

16. GALILEO GALILEI (1564-1642) “ Galileo, perhaps more than any other single person, was responsible for the birth of modern science." 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.

18. 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.

21. 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.

24. Natural satellites of Mars: FOBOS Y DEIMOS Mars has two moons, Deimos and Fobos, which are small and irregularly shaped.

25. Elliptical orbits of the planets

26. KEPLER’S FIRST LAW 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.

27. 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).

28. Kepler’s third law Third 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).

30. 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.

35. TYPES OF ECLIPSES

40. Satellites of Uranus: UMBRIEL and ARIEL

41. SATELLITES OF URANUS: MIRANDA Y TITANIA

43. SATELLITES OF NEPTUNE: NEREID and TRITON.

44. Satellites of Neptune: THALASSA and DESPINA.

45. CONSTELLATIONS 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.

46. 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.

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

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

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

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

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

52. Constellations and brightest stars.