HNRS 227 Fall 2007Chapter 14

Earth in Spacepresented by Prof. Geller

25 October 2007

Key Points of Chapter 14

• Shape, Size and Motions of the Earth– Rotation and Revolution– Precession

• Coordinate Systems– Local Horizon and Celestial Coordinate

• Time– Sidereal and Synodic

• Phases of the Moon• Solar and Lunar Eclipses• Tides

Shape and Size of the Earth

• Sphere vs. Oblate Spheroid

Motions of the Earth• Revolution

– Around the Sun• Equinoxes, Solstices

• Rotation– On its axis– Tilt of the Earth determines the seasons !!!– Coriolis Effect

• Remember the Sun is also moving– Revolution around the center of the Milky Way Galaxy– Rotation on its axis

Rotation, Revolution, Precession

• Rotation– On its axis

• Revolution– Around

the Sun• Precession

– Of the axis, like a top

Coordinate Systems• Celestial Coordinates vs. Local Horizon System

– Box Figure 14.1 CONTAINS ERRORS– Remember what was shown in lecture previously

iClicker Question

• What are the coordinates needed to locate your position on Earth?A Altitude and azimuthB Right ascension and declinationC Latitude and longitudeD More than one of the above are needed

Time• There are different ways

to track the passage of time– Solar Time is time

measured with respect to the Sun.

– Sidereal Time is time measured with respect to the celestial sphere.

• The Hour Angle (HA) of an object is its position, measured around the celestial equator, westward from the observer's meridian.– Local Sidereal Time (LST)

= RA + HA

More Time• Sun Time

– Mean Solar Time is defined as the Hour Angle of the Mean Sun (HAMS) + 12h.

– Solar Time is the time given by a sundial. It differs from Mean Solar Time by the Equation of Time (E). E can be as great as 16 minutes.

• E = Solar Time - Mean Solar Time – The observable effect of this is that the position of the

midday sun at 12h clock time will describe an irregular lemniscate, called the analemma, throughout the year.

• Universal Time– Universal Time (UT). For most everyday purposes, this can

be taken as the Mean Solar Time on the Greenwich Meridian (Greenwich Mean Time [GMT]). UT is actually calculated from sidereal time.

– Co-ordinated Universal Time (UTC) is used for time-signal broadcasts. It is derived from International Atomic Time (TAI), from which it differs from a whole number of seconds.

Analemma

Thought Question

• The constant inclination and constant orientation of the Earth’s axis producesA Variation in the number of daylight hours

throughout a yearB Variation in the seasons throughout a yearC Variation in the rotation rate in a dayD Both A and B aboveE A, B and C above

iClickerQuestion

• Which point is closest to the location of this classroom on the Earth?

A

B

C

D

E

• Where do you think the Tropic of Cancer got its name from?A An explorer named Cancer.B An explorer named Tropic.C The Sun’s position in the constellation of Cancer.D The Sun’s position in the tropics.E The Moon’s position in the constellation of Cancer.

Thoughtful Answer• The Tropic of Cancer marks the latitude 23.5 degrees north,

where the sun is directly overhead at noon on June 21. The Tropic of Capricorn marks the latitude 23.5 degrees south where the sun is directly overhead at noon on December 21. When named 2000 years ago, the Sun was in the constellation of Capricorn during the winter solstice and Cancer during the summer solstice, hence the names. However, due to the precession of the equinoxes the sun is no longer in these constellations during these times, but the names remain.

• At the equator, the sun is directly overhead at noon on the equinoxes.

• The Arctic and Antarctic Circles are located at +/-66.5 degrees latitude. Note that 66.5 + 23.5 equals 90 degrees. On December 21, when the sun is directly over the Tropic of Capricorn at noon, it will not be visible from the Arctic Circle. So above the Arctic Circle, there is a period during the winter when the sun remains below the horizon. Similarly for the Antarctic Circle. On June 21st, when the sun is directly over the Tropic of Cancer at noon, it is not visible from below the Antarctic Circle.

Time Zones

iClicker Question

It is Noon at Dallas, Texas. What time is it at Jacksonville, Florida?A 1 PMB 12 NoonC 2 PMD 11 AME 10 AM

Thought Question

Why is it that the time shown by a sundial does not agree with the time shown by a clock (assuming standard time)?A Time zones have nothing to do with time.B Your location doesn’t match the middle of the

time zone you are within.C Time zones never agree with sundial time.D Your latitude effects the time.E All of the above are contributors.

iClicker Question

It is Noon at Dallas, Texas. What time is it at Bakersfield, California?A 1 PMB 12 NoonC 2 PMD 11 AME 10 AM

iClicker Question

It is Noon at Dallas, Texas. What time is it at Denver, Colorado?A 1 PMB 12 NoonC 2 PMD 11 AME 10 AM

Phases of the Moon

Solar and Lunar Eclipses

Tides

iClicker Question

What is the phase of the Moon at point 3 in the diagram?A first quarterB fullC newD third quarterE waning crescent

iClicker Question

On a particular day, you note that the Moon rises at sunset. What is the phase of the Moon?

• A First Quarter• B Full Moon• C Last Quarter• D New Moon• E It cannot be determined.

Thought QuestionWhat is the

position of the Moon during a total eclipse of the Sun?

• A 1• B 3• C 5• D 7• E 8

iClicker Question• What locations

of the Moon are possible for Spring Tide?

• A 1 and 3• B 3 and 7• C 7 only• D 5 only• E 1 and 5

iClicker Question

• What locations of the Moon are possible for Neap Tide?

• A 1 only• B 1 and 5• C 5 only• D 7 and 3• E 7 only

Thought QuestionWhy don’t we have lunar eclipses every month?• A Because the Sun and Moon are not in the

same location with respect to the stars.• B Because the Sun, Earth and Moon are not

always aligned in a linear formation.• C Because the Sun, Earth and Moon are

different sizes.• D Because the Earth and Moon don’t have the

same angular diameter all the time.• E That’s not so, we have lunar eclipses every

month.

Thought Question

• Where are you on Earth if you observe the shadow from a vertical stick points northward at local noon on 22 September?A North of the EquatorB On the EquatorC South of the EquatorD Any of the above is possible

Thought Question

• Where are you on Earth if there is no shadow from a vertical stick on a clear day at local noon on 22 SeptemberA North of the EquatorB On the EquatorC South of the EquatorD Any of the above is possible

Thought Question

• Where are you on Earth if the shadow from a vertical stick on a clear day points southward at local noon on 22 SeptemberA North of the EquatorB On the EquatorC South of the EquatorD Any of the above is possible