Our Place in the Cosmos and Introduction to Astrophysics Lecture 3 Patterns in the Sky - The Earth’s Rotation

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

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

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

View from the North Pole horizon

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

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!

Celestial poles and equator are directions in space not locations

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 1/144 th way round the Earth’s circumference, which is thus 144  290 km = 42,000 km Actual value just over 40,000 km (radius 6,400 km)

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

ItalyTanzania Sahara Dan Heller Photography

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

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

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

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

Viewed from space Viewed from Earth Equatorial pendulum - no rotation

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

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

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

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