HNRS 227 Fall 2007 Chapter 14 Earth in Space presented 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 azimuth B Right ascension and declination C Latitude and longitude D More than one of the above are needed
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 Time
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 produces A Variation in the number of daylight hours throughout a year B Variation in the seasons throughout a year C Variation in the rotation rate in a day D Both A and B above E A, B and C above
iclicker Question 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 PM B 12 Noon C 2 PM D 11 AM E 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 PM B 12 Noon C 2 PM D 11 AM E 10 AM
iclicker Question It is Noon at Dallas, Texas. What time is it at Denver, Colorado? A 1 PM B 12 Noon C 2 PM D 11 AM E 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 quarter B full C new D third quarter E 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 Question What 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 Question Why 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 Equator B On the Equator C South of the Equator D 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 September A North of the Equator B On the Equator C South of the Equator D 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 September A North of the Equator B On the Equator C South of the Equator D Any of the above is possible