Our Place in the Cosmos Our Place in the Cosmos Rotation of the - PDF document

Our Place in the Cosmos Our Place in the Cosmos Rotation of the Earth and and � The most familiar, literally “everyday”, Introduction to Introduction to astronomical phenomenon, the passage of night and day, is due to the fact that the Earth Astrophysics Astrophysics spins on its axis [ not that the Sun orbits around the Earth, as Ptolemey and others thought] Lecture 3 � As viewed from above the North pole, Earth Patterns in the Sky - rotates in a counterclockwise direction, The Earth’s Rotation 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 View from the North Pole � What we see depends strongly on our latitude (degrees north or south of the equator) � At the North Pole, you are standing on the horizon 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

Location, location, location Latitude � Whatever your location, you can only see one half of � Latitude measures how far north or south of the the sky at any given time, that half above the horizon equator a point is � The half below the horizon is blocked by the Earth � Any point on the equator is at 0 ° latitude, the North Pole is at +90 ° , South Pole -90 ° [we are at 50 ° 52’] � For most locations the visible half of the sky is constantly changing as Earth rotates � As one travels south from the North Pole, horizon tilts and zenith moves away from north celestial pole � However at the poles, one always sees the same half � At 60 ° latitude, north celestial pole (NCP) is at 60 ° of the sky: nothing rises or sets above the horizon, the altitude of the NCP � Objects near the horizon will follow a circular path � NB pole stays fixed, it is the horizon which changes that keeps them the same distance above the horizon � At the South Pole, one observes the opposite � Altitude of NCP is same as northern latitude - useful hemisphere, and stars appear to move clockwise navigational aid! Celestial poles and equator are directions in space not locations Size of the Earth Changing Night Sky � Location of north celestial pole can be used to � Apart for an observer at the poles, the visible part of the night sky is constantly changing as Earth rotates measure the size of the Earth! � For observer at latitude b , stars within b degrees of • Measure altitude of NCP at current location the pole are always above the horizon - they are • Head north and repeat measurement described as circumpolar • 290 km north NCP will have risen by 2.5 ° � Stars within b degrees of opposite pole are never • This is 1/144 of a complete circle (360 ° ) visible - they are always below the horizon • We have therefore travelled 1/144 th way round the � The remaining stars are visible for part of each night Earth’s circumference, which is thus 144 � 290 km � From the equator, one can observe the entire sky over = 42,000 km a 24 hour period • Actual value just over 40,000 km (radius 6,400 km)

Sahara Italy Tanzania 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 Foucault Pendulum � Jean-Bernard-Léon Foucault made a 67 � A point on the equator moves at 1,674 metre long pendulum in the Panthéon km/h due to Earth’s spin (circumference dome in Paris in 1851 of earth divided by rotation period) � A pendulum normally swings within a � Not normally noticeable, apart from two fixed plane, but Foucault noticed that measurable effects: the plane of motion rotated in a • Foucault Pendulum clockwise direction (viewed from above) • Coriolis effect � Foucault realised this is because the Earth is rotating beneath the pendulum

Foucault Pendulum Viewed from � At the North Pole, a Foucault pendulum will space 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 Viewed from Earth rotation � Foucault’s pendulum in Paris, at a latitude of 49 ° , took about 32 hours to complete one Equatorial period 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 Next Friday, 12 October � Night and day, and the apparent motion of the � Peter Thomas will give the 9am lecture and Sun and stars are due to the Earth’s rotation hold the 10am workshop for Intro to Astrophysics � Latitude may be determined from the altitude � The 10am workshop for Our Place in the of the celestial poles Cosmos is postponed until later in the term for � Changing altitude of pole with latitude a multiple-choice quiz provides estimate of Earth’s radius � Instead, I will pose a few questions for � Earth’s rotation demonstrated directly by two discussion on the Study Direct Open Forum observable effects: Foucault pendulum and Coriolis effect