Time, Day, Month, and the Moon

Transcription

1 Time, Day, Month, and the Moon

2 Announcements o First Homework will start on Tue Sept 20st; due on Thu, Sept 29th. o Accessible through SPARK

3 Assigned Reading n Units 7 and 8

4 Goals for Today n To discuss time and time keeping. n To discuss the day, and month n To Introduce the Moon and the Lunar Phases

5 What is time?

6 - Time has length. Properties of Time - Time is believed to be continuous. - Time is relative (not absolute). - two people traveling at different speeds experience the flow of time differently! - Time may have a beginning. - Time has a direction (unlike the spatial dimensions).

7 Physical Time Direction The Three arrows of time 1. Thermodynamic: the direction in which disorder increases 2. Psychological: the direction in which we remember the past and not the future 3. Cosmological: the direction in which the universe expands or contracts. Stephen Hawking, A Brief History of Time

8 The Thermodynamic Argument n The 2 nd law of thermodynamics says that: degree of disorder in the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium This actually makes intuitive sense imagine a jigsaw puzzle on a vibrating table. Or an ice crystal melting. So the direction of time can be determined by the change in disorder in an isolated system that is not in equilibrium.

9 Thermodynamic time It measures `irreversible systems, i.e. systems (the Earth, a gas balloon, you) for which doing an action in one direction cost more `work than doing it in the opposite direction. A system that has performed an `irreversible action is said to have increased its disorder 1. Reversible: You drive from Amherst to Boston. 2. Irreversible: You let air out of your bike s tire.

10 Survey Question Which of the following demonstrates a direction to the flow of time? 1) the Earth rotates counter-clockwise when viewed from above the North Pole 2) the Moon orbits the Earth 3) light travels from the Sun to us in 8 minutes 4) the lava flows out of a volcano.

11 Some ways to measure the passage of time Astronomical - Appearance of seasons - Appearance of constellations - Appearance of Moon (phases) - Rising and setting of the Sun Mechanical - The motion of a pendulum - The vibration of a crystal Atomic - The decay of unstable isotopes - The emission of radiation by all elements

12 Seconds (MKS) Old Definition: The hour / minute / second system New Definition: Hour- 1/24 of a mean solar day Minute 1/60 of an hour Seconds 1/60 of a minute Seconds now defined by one form of radiation given off by the cesium atom (9,192,631,770 Hz)

13 Day Day = 1 full rotation of the Earth on its axis Local Noon sun crosses the celestial meridian Ante meridiem (A.M.) and post meridiem (P.M.) Sidereal day consecutive meridian crossings of a particular star (23:56:4) Solar day consecutive meridian crossings of the Sun (noon to noon) Four minutes longer than a sidereal day (Solar day = 86,400 s)

14 Solar 86,400s Sidereal 86,164s Why are they different?

15 Solar and Sidereal Days Need to rotate this much more for 1 solar day 360º rotation = 1 sidereal day Direction to Noon

16 Survey Question A sidereal day is 4 minutes shorter than a solar day. If the Earth s spin were in the opposite direction then a sidereal day would 1) not change 2) change, but still be shorter than a solar day 3) be longer than a solar day 4) be the same as a solar day

17 Backwards Solar and Sidereal Days Need to rotate this much less for 1 solar day 360º rotation = 1 sidereal day Direction to Noon Rotating backwards!

18 Survey Question If you put a sundial in your back yard, you want to point the 0 point in which direction? 1) North 2) South 3) East 4) West

19 Time Zones in the U.S. Local Noon changes approximately one minute for every 12 1/2 miles E-W Solar local noon in Northampton occurs about 45 seconds after it happens in Amherst. In general, time zones were created to synchronize our schedules with the daylight and to make time uniform over a large region (for the railroads). In Michigan, there are days in the summer when it is still light out at 10:00pm!

20 Universal Time

21 Daylight Saving Time n Just when things were getting easy n Idea is to shift clock during the year to minimize the use of artificial lights. n Most of the world observes DST, with laws and regulations dating back to the early 1900 s. n But non-uniformities can make this quite complicated (Move to Arizona if you don t like setting your watch!)

22 Year Tropical Year interval between spring equinoxes 365 days, 5 hours, 48 minutes, 46 seconds, or solar days Sidereal Year interval for the sun to return to same apparent position relative to stars m.s.d. (about 20 minutes longer than tropical year) precession of the Earth s rotation axis (a.k.a precession of equinoxes) Calendar year = 365 days (with leap year of 366 days every 4 years except when century changes unless century is divisible by 400)

23 Survey Question The year 1900 was a leap year: 1) Yes 2) No

24 Month Linked to the phases of the moon Sidereal month days, two consecutive crossings of any star Synodic month 29.5 days, new moon to new moon Calendrical month 1/12 year, days The Egyptians are credited with the first 12 month solar year.

25 Sidereal and Synodic Month

26 Sidereal and Synodic Month Sidereal month (27.33 days) Synodic month complete cycle of moon phases (29.5 days)

27 Survey Question If the Moon s orbit (radius) were twice as big but the Moon went around at the same speed as it currently does, the difference between a sidereal month and a synodic month would be 1) smaller 2) larger 3) the same

28 Earth & Moon The Moon is ~1/4 of the Earth in diameter and ~1 percent in mass. The rotation of the Moon and its revolution around the Earth takes 27 days, 7 hours, and 43 minutes. This is the Sidereal Period of the Moon s orbit. The period of time it takes the moon to go through its cycle of phases is called its synodic period. This is 29.5 days or one month. We only see one face of the moon!

29 Lunar Phases Why can we see the Moon at all? The Moon is Illuminated by the Sun! Like planets, the moon does not emit its own light.

30 sunlight

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33 Survey Question A third quarter moon is rising. What time is it? 1) sunset 2) sunrise 3) midnight 4) noon

34 Survey Question A third quarter moon is rising. What time is it? 1) sunset 2) sunrise 3) midnight 4) noon

35 Earth Rising on the Moon Apollo 11

36 Survey Question In this picture of the Earth rising, what is the predominant direction of the sun in relation to the observer? 1) above the observer 2) below the observer 3) behind the observer 4) behind the Earth

37 Survey Question In this picture of the Earth rising, what is the predominant direction of the sun in relation to the observer? 1) above the observer 2) below the observer 3) behind the observer 4) behind the Earth

38 Solar Eclipses

39 The Sun is physically 400 times larger than the Moon. Why can it be obscured by the Moon?

40 Angle diameter (in radians) = diameter/distance Full circle = 2π (in radians)=360 degrees=360 o 1 o =60 arminutes = 60 1 = 60 arcseconds = 60

41 Angular Size Some Examples: Horizon to zenith (point overhead) 90 o Your fist at arm's length 10 o Sun or Moon seen from Earth 0.5 o = 30' Smallest detail visible to naked eye 1' = 60'' Smallest detail visible by a single telescope from Earth's surface: 1'' Angular size of Pluto: 0.15 Betelgeuse (largest star) seen from Earth 0.004''

42 Geometry of Moon and Sun Both the Moon and the Sun have angular diameter of about 0.5 degree, the exact value at a given time depending on the exact distance. Therefore, we can periodically have solar eclipses

43 Geometry of Solar Eclipses

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50 Looking Back on an Eclipsed Earth; 1999 August 11 Credit: Mir Space Station Crew

51 Total and Annular Eclipse

52 Annular Eclipse October 3, 2005

53 It takes about an hour for the moon to cover the sun. Why? The moon moves at an angular speed of 360 deg/(28x24 hr) = 0.5 deg/hr. The angular size of the moon is about 0.5 deg.

54 How come there isn't an eclipse every full moon and new moon? The main reason is that the moon s orbit is tilted from the earth s orbit. However, twice a year the nodes of the moon s orbit will align with the Earth-Sun direction, and total eclipses will be possible. A total eclipse from a given point on the surface of the Earth is not a common occurrence. For example, next solar eclipse in the USA will occur on 21 August 2017; the following one will be on 8 April 2024.

55 Survey Question The phase of the moon at a solar eclipse is? a) new b) 1 st quarter c) full d) it depends on the year

56 Survey Question The phase of the moon at a solar eclipse is? a) new b) 1 st quarter c) full d) it depends on the year

57 Lunar Eclipse (the Moon in the Earth s Shadow)

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