Chapter 16

1. Chapter 16 Earth in Space

2. The Shape of the Earth • The Earth is not a perfectly round sphere as believed by the ancient Greeks. • It is flattened at the poles and has an equatorial bulge, as do many other planets. • The distance around is larger around the equator than around the poles. This is called an oblate shape. • It is not a perfectly symmetrical oblate shape. The North Pole is slightly higher and the South Pole is slightly lower than the average surface. • It is not perfectly circular around the equator, there is a lump in the Pacific Ocean and a depression in the Indian Ocean.

5. Earth’s motions • Yearly revolution around Sun • Daily rotation on axis • Slow clockwise wobble on axis-Precession Earth follows the path of a gigantic helix., moving at a fantastic speed as it follows the Sun and the galaxy through space.

6. Fig. 16.11 A spinning top wobbles as it spins, and the axis of the top traces out a small circle. The Wobbling of the axis is called precession. http://www.geocities.com/astrologyages/precession.htm

7. Fig. 16.12 North Pole North Pole The slow, continuous precession of the Earth’s axis results in the North Pole pointing around a small circle over a period of about 26,000 years.

8. Revolution around the Sun • The average distance between the Earth and the Sun is about 93 million miles. • The Earth’s movement around the Sun is called a revolution. • It takes an average of one year for one complete circuit. • Earth’s orbit around the Sun is slightly elliptical, and it moves with a speed that varies. It moves fastest when it is closer to the Sun in January and moves slowest when it is furthest from the Sun in early July.

9. Plane of the Ecliptic • The plane (2-dimensional shape) of the Earth’s orbit around the Sun is called the Plane of the Ecliptic.

10. Even though the Earth is about 3 million miles closer to the Sun in January than in July, and the January Sun is apparently 3% larger than the July Sun, this effect is not as important as some of the other relationships that we will examine, so winter occurs in the Northern Hemisphere when the Earth is closest to the Sun.

11. Pink-Direction of motion of Earth’s axis

12. Rotation or Spinning • The Earth rotates or spins around an imaginary line called an axis which goes through the Earth’s poles. • This axis is not at a 90o angle (perpendicular) to the plane of the ecliptic. It is at a 66.5o angle. • The axis points towards the Sun for 6 months out of the year and away from the Sun for the other 6 months.

13. Effect of the Earth’s Motions • The inclination and orientation of the axis, together with the Earth’s rotation and revolution combine to produce the following effects: 1. days and nights that vary in length. 2. changing seasons 3. climates that vary with latitude (north/south)

14. Effect of the Earth’s Motions • When the North Pole is pointed towards the Sun it receives sunlight for a full 24 hours, and the South Pole is in Earth’s shadow for a full 24 hours. • This is summer in the Northern hemisphere, on June 21 or 22, with the longest daylight periods and the Sun at its maximum noon height in the sky.

15. Effect of the Earth’s Motions • Six months later, on December 22 or 23, the orientation is reversed with winter in the Northern Hemisphere, the shortest daylight periods, and the Sun at its lowest noon height in the sky.

16. Solstices • The summer solstice occurs on or about June 22 in the Northern Hemisphere. • The beginning of the summer season. • The sun has its highest altitude in the sky. (Largest distance from the Sun) Summer Solstice

17. Solstices • The winter solstice occurs on or about December 22 in the Northern Hemisphere. • The beginning of the winter season. • The sun has its lowest altitude in the sky. (Shortest distance from the Sun) Winter Solstice

18. Equinoxes • The spring or vernal equinox occurs on or about March 21. • It identifies the beginning of the spring season. Spring Equinox

19. Equinoxes • The autumnal equinox occurs on or about September 23. • It identifies the beginning of the fall season. Autumnal Equinox

20. Earth’s Revolution Spring or Vernal Equinox, all points on Earth have 12 hours of daylight.

21. Earth’s Revolution Autumnal equinox, all points on Earth have 12 hours of daylight.

22. The length of daylight during each season is determined by the Earth’s axis and the Line between the Earth and the Sun. At the equinoxes, the axis is perpendicular to the line between the Earth and the Sun. Day and night are of equal length everywhere. At the summer solstice, the North Pole points towards the Sun and is completely out of the shadow for a 24 hour day. The situation is reversed for the South Pole. At the winter solstice the North Pole is in the shadow for a 24 hour night

23. http://daphne.palomar.edu/jthorngren/tutorial.htm http://www.astro.uiuc.edu/~kaler/celsph.html

24. Identifying Place • A system of 2 straight lines can be used to identify a point, or position, on a flat, 2 dimensional surface:

25. Identifying Place • A system of curved lines can also be used to identify place on the surface of the earth. • The earth’s axis provides the north-south reference point. • The equator is a big circle around the earth that is exactly halfway between the two ends, or poles, of the rotational axis.

26. A circle that is parallel to the equator is used to specify a position north or south of the equator. A few possible ones are shown here. These lines are called parallels. The same degree numbers are repeated going south from the equator.

27. Identifying Place • Each parallel is the same distance between the equator and one of the poles all the way around the earth. • The distance from the equator to a point on a parallel is called the latitude of that point. • Latitude tells us how far north or south a point is from the equator by telling you the parallel the point is located on.

28. If you could see to the earth’ center, you would see that latitudes run from 0o at the equator north to 90 o at the North Pole (or to 90o at the South Pole).

29. Identifying Place • To identify a location you need another line, this time one that runs pole to pole and perpendicular to the parallels. • These north-south running arcs that intersect at both poles are called meridians. • There is not a naturally occurring, identifiable meridian that can be used as a point of reference such as the equator serves for parallels, so one is identified as the referent by international agreement. • The referent meridian is the one that passes through the Greenwich Observatory near London, England. This meridian is called the prime meridian. • The distance from the prime meridian east or west is called the longitude.

30. The degrees of longitude of a point on a parallel are measured to the east or to The west from the prime meridian up to 180o.

31. If you could see inside the earth you would see 360o around the equator and 180o of longitude east and west of the prime meridian.

32. Identifying Place • New Orleans has a latitude of about 30oN of the equator and a longitude of about 90oW of the prime meridian. The location of New Orleans is therefore described as 30oN, 90oW. • Locations are more precisely identified by dividing each degree of latitude into subdivisions of 60 minutes (60’) per degree and each minute into 60 seconds (60’’). • Latitudes near the equator are referred to as low latitudes and near the poles as high latitudes.

33. Identifying Place • In addition to the equator (0o) and the poles (90o), the latitudes of 23.5oN and 23.5oS from the equator are important references for climatic considerations. • 23.5oN is called the tropic of Cancer and 23.5oS is called the tropic of Capricorn. These two parallels identify the limits towards the poles within which the Sun appears directly overhead during the course of a year. • The parallel of 66.5oN is called the Arctic circle, and 66.5oS is called the Antarctic circle. These two parallels identify the limits toward the equator within which the Sun appears above the horizon all day during the summer. This starts with six months of daylight every day at the pole, then decreases as you get fewer days of full light until reaching the limit of one day of 24-hour daylight at the 66.5o limit.

34. At the summer solstice, the noon Sun appears directly overhead at the tropic of Cancer (23.5oN) and 24 hrs. of daylight occurs north of the Arctic circle (66.5oN). At the winter solstice, the noon Sun appears overhead at the tropic of Capricorn (23.5oS) and 24 hrs of daylight occurs south of the Antarctic circle (66.5oS). 23.5oN is called the tropic of Cancer and 23.5oS is called the tropic of Capricorn. The parallel of 66.5oN is called the Arctic circle, and 66.5oS is called the Antarctic circle.

35. Measuring Time • The Babylonians originated the technique of using astronomical motions for keeping time about four thousand years ago. • They marked the yearly journey of the Sun against the background of the stars, and it was divided into 12 periods, or months, after the signs of the zodiac. • It was divided also into 360 days and they invented the week and divided the day into hours, minutes, and seconds. • The week was identified as a group of 7 days, each based on one of the 7 heavenly bodies that were known at the time. • The hours, minutes, and seconds of a day were determined from the movement of the shadow around a straight vertical rod.

36. Measuring Time • As seen above the North Pole, the Earth rotates counterclockwise, turning toward the east. • On the earth, this motion causes the Sun to appear to rise in the east, travel across the sky, and set in the west. • A day is defined as the length of time required for the earth to rotate once on its axis.

37. Measuring Time • Imagine the Earth as the ancient Greek did, fixed in space as the Sun circled around it. • Then you can picture the Sun as going east to west around the Earth.

38. The standard time zones. Hawaii and most of Alaska are two hours Earlier than Pacific Standard Time.

40. Going Westward, you gain a day Going Eastward, you lose a day The international date line follows the 180o meridian in a way that land areas and island chains have the same date.

41. Length of a Year • A year is generally defined as the interval of time required for Earth to make one complete revolution in its orbit. • There are different definitions of what is meant by a year. The most common definition of a year is the interval between two consecutive spring equinoxes. This is known as a tropical year and it is 365.24220 mean solar days. • The sidereal year is defined as the interval of time required for Earth to move around its orbit so the Sun id again in the same position relative to the stars. It is slightly longer than the tropical year, by 20 minutes.

42. Length of a Year • It is the tropical year that is used as a standard time interval to determine the calendar year. In this time period Earth does not complete an exact number of turns on its axis while completing one trip around the Sun so it is necessary of periodically adjust the calendar so it stays in step with the seasons. • The calendar system initially designed to stay in step with the seasons was devised by the ancient Romans. • Julius Caesar reformed the calendar to have 365 day years with a 366 year (leap year) every fourth year. This is called the Julian calendar and it takes into account that a tropical year, the name given to the calendar year, actually has 365.24220 days, very close to 365 ¼ days.

43. Length of a Year • The Julian calendar was very close to the one used now, except the year began in March, the month of the spring equinox. • The month of July was named in honor of Julius Caesar and the following month was later named after Augustus Caesar. • The Julian calendar year was longer than the tropical year by .0078 days per year (11minutes, 14 seconds), so in 1582 Pope Gregory XIII revised the calendar and it was corrected for 10 days even though the error was on 14 days. This resulted in the Gregorian calendar, the system used today. • The accumulated error is almost .75 days per century, so every four centuries there will be an error of 3 days. As a result the leap year is dropped every three out of four century years. 2000 was a leap year but 2100, 2200 and 2300 will not be leap years.

44. The sun Illuminates the not visible side of the Moon. The Phases of the Moon The sun Illuminates the visible side of the Moon. Half of the Moon is always lighted by the Sun and half is always in the shadow. The Moon phases result from the view of the lighted and dark parts as the Moon revolves around the Earth. The period of time between two consecutive phases, such as new moon to new moon, is the synodic month, or about 29.5 days.

45. Eclipses • The event of Earth’s or Moon’s shadow falling on the other body is called and eclipse. • Most of the time eclipses don’t occur because the plane of the Moon’s orbit is inclined to the Earth’s orbit (about 5o). The shadow from the Moon or the shadow from Earth usually falls above or below the other body, too high or too low to produce an eclipse. • An eclipse occurs only when the Sun, Moon, and Earth are in a line with each other.

46. The plane of the Moon’s orbit is inclined to the plane of the Earth’s orbit by about 5o. An eclipse occurs only when the two planes meet intersect and the Earth, Moon, and Sun are in a line.

47. Solar Eclipse People in a location where the tip of the umbra falls on the surface of the Earth see a total eclipse. People in locations where the penumbra falls on the Earth’s surface see a partial solar eclipse.

48. Solar Eclipse-Occurs once every several hundred years And then lasting for less than 7 minutes.

49. Lunar Eclipse occurs when there is a Full Moon. A lunar eclipse is visible to everyone on the dark side of the Earth. These eclipses last for hours.

50. Review Exercises • Applying the Concepts p. 420-421: # 1, 3, 4, 5, 6, 7, 8, 11, 12, 17, 18, 20, 21 New Book: p. 460-463 # 1, 2, 3, 4, 5, 6, 9, 10, 18, 21, 23, 24, 25, 26, 27, 28, 31, 32, 37, 38, 41, 42, 46, 47.