Admin. 8/29/17 1. Class website http://www.astro.ufl.edu/~jt/teaching/ast1002/ 2. Optional Discussion sections (start Aug. 30th) (see me at end of lecture if you can t make one of these times)! Tuesday ~11.30am (period 5), Bryant 3 (Basement level)! Thursday ~12.35pm (period 6), Bryant 3 (Basement level)! Office hr: Tuesday 12.30-1pm; Wed. 12.30-1.00pm (by appt.), Bryant 302 3. Homework 1: due Mon. Sept. 4th 11.59pm via Canvas e-learning under Quizzes 4. Reading for weeks 1 and 2: Chaisson & McMillan Ch. 0, 1, 2.1, 2.2, 2.3. 5. Observing project deadline: Thursday Oct. 26th 2017, however, you are strongly advised to complete observing by Fri. Oct. 6th 6. Email me Astro-news, jokes, tunes, images: ast1002_tan-l@lists.ufl.edu 7. Copies of the Syllabus if you did not get one last time. 8. Printed class notes? Name tags? Celestial Sphere: extra notes Your horizon plane If you re at North Pole, you ll NEVER see stars that are below your horizon Key Concepts: Lecture 4 Annual Motions of Sun and Stars Your horizon plane The Seasons. Example of the Scientific Method But if you re at latitude nearer the equator, SOME stars that are below your horizon will rise and set.
Annual Motion of the Sun: The Ecliptic Every day the Sun rises and sets 4 minutes later with respect to the stars Therefore after one year it returns to the same position relative to the stars But if you re at latitude nearer the equator, SOME stars that are below your horizon will rise and set. Annual Visibility of the Stars The constellations that are visible on a given night are those opposite from the Sun on the celestial sphere Thus you see different stars at different times of the year as the Sun moves along the ecliptic relative to the stars The apparent motion is due to the Earth orbiting the Sun But, if not exactly on the equator, some stars are never visible to you!!
Annual Motion of Sun Annual Motions of the Sun The Sun appears to move along a repeatable path on the celestial sphere throughout the year. This path is called the ECLIPTIC Sun appears to move eastward relative to stars on celestial sphere It moves 360 degrees in a year, i.e. about 1 degree per day The constellations through which we see the sun move are the constellations of the ZODIAC The ecliptic is tilted by 23.5 degrees to the celestial equator This apparent motion of the Sun through the celestial sphere is due to the Earth orbiting the Sun Annual Motions of the Sun Earth s rotation axis is tilted by 23.5 degrees compared to the direction perpendicular to the Earth s orbital plane 23.5 The maximum daily altitude (angle above horizon) of the Sun occurs near midday (i.e. in the northern hemisphere: when the Sun is due south). This maximum daily altitude changes with season: This has its largest value on the summer solstice (June 21 in the northern hemisphere north of the Tropics) This has its smallest value on the winter solstice (December 21 in the northern hemisphere north of Tropics) The sunrise and sunset points along the horizon also change with season It rises and sets due east on the equinox (Sep 21, and March 21) It rises north of east in the summer, and south of east in the winter
Gainesville: 29.6510N, 82.3250W Tropic of Cancer Tropic of Capricorn Reasons for the Seasons? What causes this pattern? We can consider two possible theories or models: Is it caused by variation in the distance of the Earth from the Sun? Is it caused by the tilt of the Earth s rotation axis with respect to the Earth s orbital plane about the Sun (ecliptic) What is special about the Tropics (i.e. between +23.5degrees and -23.5 degrees latitude)? Seasons With the annual motion of the Earth around the Sun, we notice a cyclic pattern in the weather, known as the seasons. Observations: Seasons are reversed in the Northern and Southern Hemispheres For example, you re at the beach at Cape Cod in July, they re skiing in New Zealand! The region near the equator is hottest; poles are the coldest The Sun s position in the sky changes Reason for Seasons: Distance? Predictions: Relation between Earth-Sun distance and season for example, the Earth should be closest to the Sun in summer Same season everywhere on Earth since the Earth is tiny compared to the Earth-Sun distance
Reason for Seasons: Distance? Reason for the Seasons: Tilt? Observations: Distance in Millionskm 153 152 151 150 149 148 147 January March February April May July September November June August O ctober December Month The Earth s axis of rotation is tilted by 23.5 degrees with respect to the ecliptic pole As the Earth moves around the Sun, the Earth s north pole points to a fixed position in space Reason for Seasons: Distance? Predictions don t match Observations: Seasons are reversed in the Northern and Southern Hemispheres For example, you re at the beach at Cape Cod in July, they re skiing in New Zealand! The Earth is closest to the Sun during northern winter Reason for the Seasons: Tilt? Predictions: Summer occurs in hemisphere pointed toward the Sun Winter occurs in the hemisphere pointed away from the Sun Therefore: Seasons are reversed in northern and southern hemisphere
Reason for the Seasons: Tilt Reason for the Seasons It s warmer in the summer and colder in the winter for two reasons: the Sun is higher in the sky in the summer the days are longer The Sun reaches it s highest point in the sky on the summer solstice (June 21 in northern hemisphere) An area near the equator catches more rays than the same area near the poles. Same effect means that it is hotter at midday than in the morning. Reason for the Seasons: Tilt The angle at which the Sun s rays strike the surface of the Earth is closer to a right angle (90 degrees) in the summer Therefore a given area of the Earth s surface intercepts more rays of sunlight and hence more energy This is the main reason that it is warmer in the summer Reason for the Seasons: Tilt Predictions match observations: The northern hemisphere and southern hemisphere seasons are reversed The region near the equator is hottest; poles are coldest
Questions When is the hottest time of year in Quito, Ecuador? If the Earth s rotation axis was perpendicular to it s orbital plane (ecliptic), what would you expect the seasons to be like? If the Earth s rotation axis was parallel to its orbital plane (ecliptic), what daily and annual effects would this have? Example of the Scientific Method Identify Patterns Experiment or or observation Observation Make a Hypothesis or theory or a Theory There is always some possibility of error Predict Contrast with Astrology