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2. Earth and its Moon 2 © Zanichelli editore 2015

3. Physical data and the shape of Earth 3 © Zanichelli editore 2015

4. The measures of Earth 4 Earth is a celestial body with a relatively small mass and volume, but high density. The average surface temperature of 15 °C keeps most water in the hydrosphere in a liquid state. Earth Mass5.974 * 10 24 kg Volume 1.083 * 10 21 m 3 Average density5515 km/m 3 Average radius6371 km Equatorial radius6378 km Polar radius6357 km Average surface temperatue 15 °C © Zanichelli editore 2015

5. The shape of Earth 5 Earth rotates and has an almost spherical shape flattened at its poles, called geoid. Geographical elements of reference are the rotational axis, the N and S poles and the equator. Equator North pole South pole © Zanichelli editore 2015

6. Geographical coordinates 6 The position of a point P on Earth’s surface is determined by latitude and longitude, called geographical coordinates. © Zanichelli editore 2015

7. Cardinal points and the horizontal plane 7 The four cardinal points of the compass (North, South, East, West) can be identified on the horizontal plane, bounded by the apparent horizon. © Zanichelli editore 2015

8. Earth’s movements 8 © Zanichelli editore 2015

9. The rotation motion 9 Earth rotates from West to East in a counter-clockwise direction. The rotation period is called a sidereal day, lasting 23h 56’ 4’’. The linear speed of rotation is greatest at the equator and zero at the poles; centrifugal force depends on this value. © Zanichelli editore 2015

10. Day and night 10 Day and night alternate at each point on Earth’s surface. Only at the equator do they have the same duration throughout the year. The light zone is separated from the dark zone by the terminator, where, due to the atmosphere, the gradual passage from light to dark (and vice versa) occurs. © Zanichelli editore 2015

11. The revolution motion 11 Earth completes an elliptical orbit around the Sun in a sidereal year, which corresponds to 365d 6h 9’ 9.5’’. Earth’s axis is inclined at 66° 33’ with respect to the plane of the ecliptic and it is always stable as Earth moves. © Zanichelli editore 2015

12. Spring and autumn equinoxes 12 In spring and autumn equinoxes, day and night have the same duration everywhere and the Sun is at the zenith of the equator. © Zanichelli editore 2015

13. Winter solstice 13 In winter solstice: Sun at the zenith at the Tropic of Capricorn. © Zanichelli editore 2015

14. Summer solstice 14 In summer solstice: Sun at the zenith at the Tropic of Cancer. © Zanichelli editore 2015

15. The astronomical seasons 15 The astronomical seasons are the amounts of time that pass between a solstice and an equinox, or vice versa. They are characterized by variations in the height of the Sun above the horizon and by the day’s duration. They have different effects on the polar icecaps, in the temperate zones and in the inter-tropical zones. © Zanichelli editore 2015

16. Astronomy and the measurement of time 16 © Zanichelli editore 2015

17. The solar day 17 The solar day lasts 4 minutes longer than the sidereal day: 24h 00’ 4’’. For this reason, the Sun has an apparent movement through the celestial sphere which is slower than the stars; in the course of a year it passes through the constellations of the zodiac. © Zanichelli editore 2015

18. Local time and the time zones /1 18 Earth’s surface is divided into 24 time zones. Each time zone consists of 15 meridians wherein local time is determined by the astronomical time of the middle of the meridian. © Zanichelli editore 2015

19. Local time and the time zones /2 19 Going East from the Greenwich meridian, we add the same number of hours as the number of the time zone. Going West from Greenwich, we take away the same number. © Zanichelli editore 2015

20. The solar year, calendar year and sidereal year 20 The solar year is the period between two equinoxes or solstices of the same season; it lasts 365 d 5h 48’ 46’’, which is rounded down to 365 days in the calendar year. In order to keep them synchronized, in the western calendar one day is added every 4 years. The sidereal year is the time Earth actually takes to complete one revolution around the Sun (365d 6h 9’ 9.5’’). © Zanichelli editore 2015

21. The Moon and its movements 21 © Zanichelli editore 2015

22. Close but different: Earth and Moon 22 The Moon is the only terrestrial satellite. It differs from Earth: it lacks atmosphere, liquid water and vapor; it does not show geological activities or modeling of the surface. There is no life on the Moon. Moon Mass7.3 * 10 22 kg Radius1738 km Density3.34 g/cm 3 Rotation period27 days Revolution27 days Translation period365 days © Zanichelli editore 2015

23. The Moon’s movements 23 The Moon has three main movements: rotating on its axis; revolving around Earth; orbiting the Sun together with Earth. Rotation and revolution occur in the same direction and have the same duration; a sidereal month lasts 27 Earth days 7h 43’ (so does the lunar day). © Zanichelli editore 2015

24. The Moon’s phases /1 24 In its revolution around Earth, the Moon displays four phases of illumination: new Moon; first quarter; full Moon; last quarter. The lunar month is the period of time required for the Moon to complete its phases and lasts 29 d 12h 44’ 3’’. © Zanichelli editore 2015

25. The Moon’s phases /2 25 © Zanichelli editore 2015

26. Eclipse of the Moon 26 Eclipses of the Moon take place when the Moon is full and is located on or near an orbital node, and Earth casts a shadow over it. © Zanichelli editore 2015

27. Eclipse of the Sun 27 Eclipses of the Sun occur when the Moon is full and on or near one of the orbital nodes, blacking out the solar surface. © Zanichelli editore 2015