Newton s Universal Law of Gravitation and planetary orbits. Gravity (cont.) / Night Sky / Seasons 1/23/07

Transcription

1 Newton s Universal Law of Gravitation and planetary orbits

2 Announcements The first homework due Thursday (at the start of class). The next assignment will be posted on the website on Thursday Weekly preceptor-led study group Every Monday 10:30AM-12:00Noon Mission update volunteers?

3 Tonight's Public-Lecture Series: Dr. Martin Tomasko Mission to the Surface of Saturn s Moon Titan (7:30 PM, 308 of Kuiper this room)

4 STEREO first light image of Comet McNaught

5 Today s Topics Gravity continued Newton s Universal law of Gravitation orbits Diurnal and seasonal variations The motion of stars and planets in the sky What is the reason for the seasons?

6 Newton s Law of Universal Gravitation Gravitation force between two bodies G = Gravitational Constant = 6.67x10-11 N m 2 / kg 2 m 1 and m 2 = the masses of the two objects r = the distance between the two (more precisely, the distance between the two centers)

7 Example #1 A boy with a mass of 25 kg experiences how much less of a gravitational force by the Earth than a man with a mass of 100 kg? (possibly) Useful info M Earth = 5.97x10 24 kg R Earth = 6378 km

8 Example #2 Suppose that Earth were at 10 AU. How much less of a gravitational force from the Sun would it experience? Note 1AU = 1.5x10 8 km M Sun = 2x10 30 kg

9 Example #3 How much would a man weight on Mars if he weighs 200 pounds on Earth? Useful info: The mass of Mars = 10 times less than Earth The radius of Mars = ½ times smaller than Earth

10 Orbits The law of universal gravitation accounts for planets not falling into the Sun nor the Moon crashing into the Earth Paths A, B, and C do not have enough horizontal velocity to escape Earth s surface whereas Paths D, E, and F do. Path E is where the horizontal velocity is exactly what is needed so its orbit matches the circular curve of the Earth

11 Physics of spaceflight: Weightlessness Astronauts are actually still falling they are falling with the spacecraft They sometimes train for missions in large airplanes.

12 New Subject: The Night Sky The night sky changes in a number of ways Diurnal variations Seasonal variations Pole-star wander These variations can be understood in terms of the combined effects of the spinning of the Earth on its axis, the orbit of Earth about the Sun, and the precession of Earth s spin axis.

13 Diurnal motion Diurnal means having a period of 1 day. Stars appear to rise in the east, slowly rotate about the earth and set in the west. This diurnal or daily motion of the stars is caused by the ~24-hour rotation of the Earth on its axis.

14 Diurnal Motion

15 Diurnal Motion

16 Seasonal variations in the night sky The stars also appear to slowly shift in position throughout the year This is due to the orbit of the Earth around the Sun, relative to the (essentially) fixed stars If you follow a particular star on successive evenings, you will find that it rises approximately 4 minutes earlier each night, or 2 hours earlier each month

17 Constellations Orion A Winter/Spring Constellation Sagittarius A Summer Constellation Zodiac Constellations are those that the planets and Sun move through Near the ecliptic plane! Leo, Sagittarius, Scorpio, Taurus, etc. The constellation that the Sun is in on your birth date defines which zodiac sign you are

18 What causes the change of seasons?

19 The source of energy that heats Earth s surface the Sun

20 Proximity to the Sun partly determines the temperature of a planet s surface The closer a planet is to the Sun, the warmer its surface will be The received solar energy decreases with the square of the distance to the Sun If Earth were at 2AU, it would receive 2 2 = 4 times less solar energy

21 These Solar magnetograms show that the size of the Sun as seen from Earth changes depending on where Earth in its orbit July, 1989 January, 1989 The Earth is closer to the Sun in January

22 Recall that Earth s orbits is very nearly circular The Earth s orbital eccentricity = This is too small to effect the seasons The solar energy at Earth s orbit at: Perihelion = 1420 W/m 2 Aphelion = 1330 W/m 2 This is not enough of a difference to lead to a change in seasons

23 The seasons are caused by the tilt of Earth s axis of rotation The Earth s axis of rotation is not perpendicular to the plane of the Earth s orbit It is tilted about 23½ away from the perpendicular The Earth maintains this tilt as it orbits the Sun, with the Earth s north pole pointing toward the north celestial pole

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26 In Tucson, when the Sun at its highest point in the sky on Dec 21 (the winter solstice) is at an angle of 35 degrees above the horizon Solar flux absorbed by Earth per square meter = 450 W/m 2 In Tucson, when the Sun at its highest point in the sky on June 21 (the summer solstice) is at an angle of 82 above the horizon Solar flux absorbed by Earth per square meter = 1350 W/m 2 Thus, by far, the angle of incidence is the most important factor in leading to a change in seasons on Earth

27 Reading Assignment: Please read the textbook discussions of the following topics (this is in Chapter 2) as they will not be covered in lectures Celestial Coordinates Right Asscension Declination Zenith and Meridian Solstices and Equinoxes Stonehenge, England Sun Dagger, Chaco, NM

28 Reading for next class Chapter 3 pp Also, re-read section 4-2 and box 4-2 (pages 65-69).

29 Additional slides

30 The Earth precesses as it rotates on its axis, much like a top Precession Has a period of about 19,000 years Causes the pole star to change

31 What is the North Star? Polaris -- for now, but it wont be forever! Vega will be the pole star in about 14,000 years