Our Place in the Cosmos Our Place in the Cosmos Summary of Previous - PDF document

Our Place in the Cosmos Our Place in the Cosmos Summary of Previous Lecture and and • Night and day, and the apparent motion of the Introduction to Introduction to Sun and stars are due to the Earth’s rotation • Latitude may be determined from the altitude Astrophysics Astrophysics of the celestial poles • Changing altitude of pole with latitude provides estimate of Earth’s radius Lecture 4 • Earth’s rotation demonstrated directly by two Patterns in the Sky - observable effects: Foucault pendulum and Coriolis effect Earth’s Motion about the Sun Earth’s Orbit around Sun Spring Summer • Earth orbits the Sun in same direction as its spin (counterclockwise as viewed from above North Pole) taking 1 year for a complete orbit • Responsible for seasons and changing patterns of stars through the year • Overhead at midnight, one is looking away from the Sun • This direction changes throughout the year and so we see different stars - six months 40 deg from now we will be looking in the opposite north Autumn direction at midnight Winter The Ecliptic • The Sun traces out a great circle against the background stars along a path known as the ecliptic • The constellations along the ecliptic are the signs of the zodiac • Nothing significant about these constellations - just random patterns of distant stars that happen to lie near the plane of the Earth’s orbit about the Sun September 1 apparent path of Sun Sun in Leo

Measuring Earth’s Motion • Rain falling vertically will appear to be falling at angle from a moving car • By the time the raindrop has fallen from the top of the window to the bottom, the car has moved forward, and so raindrop appears to be moving backwards • In the same way, light from a distant star appears to be coming from a slightly different direction due to Earth’s motion through space • Over the course of a year stars appear to trace out a loop - aberration of starlight Aberration of Starlight Distance to the Sun • Cannot be detected by human eye but easily • For a nearly-circular orbit, radius is detectable with a telescope circumference divided by 2 � : 150 million km • Aberration of starlight first detected in 1720s • This distance, the average distance between by Samuel Molyneux and James Bradley the Earth and Sun is known as one astronomical unit (AU) • They showed that Earth moves on roughly (but not exactly) circular orbit about the Sun with • Modern measurements use radar signals average speed 29.8 km/s bounced off Venus • Since distance = speed � time, one may • The AU provides the basis for the astronomical multiply this speed times the time for one distance scale orbit to find the circumference of the Earth’s orbit: 942 million km The Seasons The Seasons • The seasons are not a result of the • If Earth’s rotation axis were ellipticity of Earth’s orbit (it is too perpendicular to the ecliptic plane, the small) Sun would always lie in the celestial equator, and above the horizon for • They result from the combined effects exactly 12 hours per day, every day of • There would be no seasons • Earth’s spin on its axis • In fact, rotation axis is tilted by 23.5 ° • Earth’s orbit about the Sun from perpendicular, pointing in a fixed • Misalignment of the axes of these two motions direction as Earth orbits the Sun

The Seasons • Thus for parts of the year the North Pole is tilted towards the Sun and hence the northern hemisphere has more than 12 hours of sunshine per day (northern summer), while it is winter in the south • Six months later, the North Pole is tilted away from the Sun and northern days are shorter than 12 hours (northern winter, southern summer) • As well as longer days in summer, Sun is more directly overhead and hence sunlight more intense Animation Equinoxes Solstices • At the (northern) summer solstice (“Sun • Equinoxes are the times when the Sun’s standing still”), around 21 June, the Sun apparent orbit crosses the celestial equator reaches it’s most northerly point, when North and the Earth’s rotation axis is perpendicular Pole points most directly towards Sun - to the direction to the Sun midsummer’s day • Twice each year: • At winter solstice, around 22 December, North • Spring (or vernal) equinox around 21 March Pole tipped most directly away from Sun - • Autumn equinox around 23 September midwinter, the shortest day of the year • Equinox = “equal night” - day and night both • Northern summer solstice is southern winter, last for 12 hours everywhere and vice versa Land of the Midnight Sun • Above the Arctic Circle (66.5 deg north) Sun is circumpolar around midsummer - 24 hour daylight (“land of the midnight sun”) • Same true below Antarctic Circle • Temperatures remain cool as Sun is never very high in the sky • There are equally long periods in winter when the Sun never rises Anim

The Equator The Tropics • On the equator all stars, the Sun included, are • Between latitudes 23.5 deg south and north, above the horizon 12 hours per day twice during the year, the Sun will be directly • Days and nights are each 12 hours long overhead at noon throughout the year • This band is known as the tropics and is • Sun passes directly overhead on the equinoxes bounded by the tropic of Cancer to the north • Sun is furthest from overhead at the summer and the tropic of Capricorn to the south and winter solstices • Where do these names come from? Summer • The equator thus experiences eight seasons a solstice is located in Taurus, winter in year: two summers, two winters, two springs Sagittarius and two autumns, although no season is very different from another Summer Sun Precession of the Equinoxes in Taurus • Earth’s axis wobbles slightly (like a spinning top) taking 26,000 years to complete one orbit of precession • North celestial pole tracks out a large circle amongst the stars • Polaris is currently close to NCP, but it has not always been! • Celestial equator remains perpendicular to rotation axis, so the locations where it crosses the ecliptic - the equinoxes - also change Winter Sun in Sagittarius apparent path of Sun Precession of the Equinoxes Leap years • This is known as precession of the equinoxes • Modern calendar based on a tropical year of 365.242199 days, the time from one spring equinox to • Tropics so-named due to Sun’s location at the next equinoxes in Ptolemey’s time, not today • Leap years prevent extra fraction of a day causing • This shift of the seasons through the year the seasons to “drift” through the year from century to century played havoc with • Leap years have 366 days (extra day is 29 Feb) construction of reliable calendars instead of 365 • Leap year every four years gives an average of • In 1752 England and colonies dropped 11 days 365.25 days/year, so not every 4th year is a leap from calendar year to fall into line with rest year of Europe. Mass rioting ensued from these 11 • Century years apart from those divisible by 400 (eg. days being “stolen” from people’s lives 2000) remain as 365 day years

Summary Seminar Quiz Q1 • Our view of the stars changes during the year • Draw a picture of the Earth showing the • The ecliptic is the apparent yearly path of the location of an observer at our latitude Sun against the stars and his or her horizon. • From simple observations of the aberration of • Mark the direction of the north and starlight one may infer the orbit of the Earth about the Sun and the distance to the Sun south celestial poles and the celestial • The seasons are explained by the rotation of equator. the Earth, the orbit of the Earth about the • For how many hours per day is a star on Sun and the fact that the Earth’s rotation the celestial equator visible? axis is at an angle relative to the orbital axis Seminar Quiz Q2 Seminar Quiz Q3 • A soldier fires a cannon directly at a • You are half way between the Carolinas distant target toward the east and and Bermuda heading due east, and you makes a perfect hit. She then fires a know there is a hurricane nearby. A shot directly at another target to the strong wing is blowing straight out of north, but the shot lands west of this the south. target. Would you continue on to Bermuda or • Was the soldier in Australia or Canada? head back? • Would she have fared better on the equator? Seminar Quiz Q4 Seminar Quiz Q5 • In which direction does the Sun rise on • Suppose the Earth’s axis were tilted at the morning of the spring equinox? an angle of 35 instead of 23.5 degrees to its orbit. • In the summer, does the Sun rise further north or further south? • At what latitudes would the Arctic and Antarctic circles and the Tropics be • And in the winter? located? • What effect would this have on our seasons?