Our Place in the Cosmos and Introduction to Astrophysics

1. Our Place in the Cosmos andIntroduction to Astrophysics Lecture 3 Patterns in the Sky - The Earth’s Rotation

2. Rotation of the Earth • The most familiar, literally “everyday”, astronomical phenomenon, the passage of night and day, is due to the fact that the Earth spins on its axis [not that the Sun orbits around the Earth, as Ptolemey and others thought] • As viewed from above the North pole, Earth rotates in a counterclockwise direction, completing one rotation in 24 hours

4. Rotation of the Earth • As the rotating Earth carries us from west to east, objects in the sky appear to move in the opposite direction, from east to west • The meridian is an imaginary line running directly north-south, passing through the point directly overhead, the zenith • True local noon occurs when the Sun appears to cross the meridian at our location • Midnight is 12 hours later when we face the opposite direction

5. Location, location, location • What we see depends strongly on our latitude (degrees north or south of the equator) • At the North Pole, you are standing on the Earth’s rotation axis • The point directly overhead remains stationary while everything else appears to rotate counterclockwise around this point, which is called the north celestial pole • Stars at greater angular distance from the pole appear to follow larger circular paths

6. View from the North Pole horizon

7. Location, location, location • Whatever your location, you can only see one half of the sky at any given time, that half above the horizon • The half below the horizon is blocked by the Earth • For most locations the visible half of the sky is constantly changing as Earth rotates • However at the poles, one always sees the same half of the sky: nothing rises or sets • Objects near the horizon will follow a circular path that keeps them the same distance above the horizon • At the South Pole, one observes the opposite hemisphere, and stars appear to move clockwise

8. Latitude • Latitude measures how far north or south of the equator a point is • Any point on the equator is at 0 latitude, the North Pole is at +90, South Pole -90 [we are at 50 52’] • As one travels south from the North Pole, horizon tilts and zenith moves away from north celestial pole • At 60 latitude, north celestial pole (NCP) is at 60 above the horizon, the altitude of the NCP • NB pole stays fixed, it is the horizon which changes • Altitude of NCP is same as northern latitude - useful navigational aid!

10. Celestial poles and equator are directions in space not locations

11. Size of the Earth • Location of north celestial pole can be used to measure the size of the Earth! • Measure altitude of NCP at current location • Head north and repeat measurement • 290 km north NCP will have risen by 2.5 • This is 1/144 of a complete circle (360) • We have therefore travelled 1/144th wayround the Earth’s circumference, which is thus 144  290 km = 42,000 km • Actual value just over 40,000 km (radius 6,400 km)

12. Changing Night Sky • Apart for an observer at the poles, the visible part of the night sky is constantly changing as Earth rotates • For observer at latitude b, stars within b degrees of the pole are always above the horizon - they are described as circumpolar • Stars within b degrees of opposite pole are never visible - they are always below the horizon • The remaining stars are visible for part of each night • From the equator, one can observe the entire sky over a 24 hour period

14. Sahara Italy Tanzania Dan Heller Photography

15. Celestial Sphere • An imaginary sphere centred on the Earth, with the stars on its surface • Real stars are at varying distances, so a point on the celestial sphere represents a direction in space • The celestial sphere is divided into northern and southern halves by the celestial equator - the projection of the Earth’s equator onto the sky • The celestial equator intersects the horizon at points due east and due west

17. Measuring Earth’s Spin • A point on the equator moves at 1,674 km/h due to Earth’s spin (circumference of earth divided by rotation period) • Not normally noticeable, apart from two measurable effects: • Foucault Pendulum • Coriolis effect

18. Foucault Pendulum • Jean-Bernard-Léon Foucault made a 67 metre long pendulum in the Panthéon dome in Paris in 1851 • A pendulum normally swings within a fixed plane, but Foucault noticed that the plane of motion rotated in a clockwise direction (viewed from above) • Foucault realised this is because the Earth is rotating beneath the pendulum

19. Foucault Pendulum • At the North Pole, a Foucault pendulum will make a complete rotation in 24 hours • On the equator, the Earth is no longer spinning under a pendulum and no rotation is seen • At intermediate latitudes, a pendulum will take longer than one day to complete a rotation • Foucault’s pendulum in Paris, at a latitude of 49, took about 32 hours to complete one period

20. Viewed from space Viewed from Earth Equatorial pendulum - no rotation

21. Coriolis Effect • Due to different speeds between different latitudes • Fire a cannon directly north from a point in northern hemisphere • Ball has west-east motion due to movement of cannon • Because rotation slows away from the poles, ball is moving eastward faster than its target • For a ground-based observer, ball appears to curve eastward more and more the further north it travels • Opposite effect if you fire to the south

23. Coriolis Effect • Coriolis effect causes counterclockwise rotation of northern hurricanes, clockwise rotation of southern ones • Imagine a low-pressure system in the north • Air flowing up from lower latitudes will be deflected to the right of the system • Air flowing down from higher latitudes will be deflcted to the left • Result is counterclockwise rotating wind pattern

25. Summary • Night and day, and the apparent motion of the Sun and stars are due to the Earth’s rotation • Latitude may be determined from the altitude of the celestial poles • Changing altitude of pole with latitude provides estimate of Earth’s radius • Earth’s rotation demonstrated directly by two observable effects: Foucault pendulum and Coriolis effect

26. Next Friday, 12 October • Peter Thomas will give the 9am lecture and hold the 10am workshop for Intro to Astrophysics • The 10am workshop for Our Place in the Cosmos is postponed until later in the term for a multiple-choice quiz • Instead, I will pose a few questions for discussion on the Study Direct Open Forum