Sun: rises in the east sets in the west travels on an arc across the sky 24 hours Daily Motions Solar Day = 24 hours Stars: stars travel on arcs in the sky moving from east to west. some stars rise and set others do not and are circumpolar due to Rotation of Earth 23 hour 56 min Daily Motions Sidereal Day = 23 hours 56 min Solar and Sidereal Days Why is the Solar Day longer than the Sidereal Day? Annual Motions of the Sun The altitude of the Sun changes with season It reaches a maximum on the summer solstice (June 21 in the northern hemisphere) Its minimum altitude is reached on the winter solstice (December 21 in the northern hemisphere) 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 south of east in the winter Annual Motion of the Stars Coordinate system on Earth Every day a given star or constellation rises and sets 4 minutes earlier with respect to the Sun Therefore after one year it returns to the same position relative to the Sun Gainesville: 29.6510N, 82.3250W 1
Celestial Sphere You can imagine that the stars and sun are attached to the surface of a great sphere The earth appears to be at the center of the sphere The sphere rotates from east to west on an axis that points to the north celestial pole To explain the daily motions of the sky you can imagine the sphere rotating once in 23 hours 56 minutes Parts of the Celestial Sphere Celestial pole - extends from the rotation axis of the earth all stars appear to rotate about this point some stars are circumpolar some stars rise & set Celestial equator - extends from the earth s equator Your latitude on earth determines the angle at which stars will rise & set with respect to your local horizon Where are we? The position of a celestial pole near the horizon, tells us we must be near the Equator. Also consider the angles the star trails make with the horizon: 90 degrees. Question You are kidnapped, drugged and placed in a dark room for an indefinite period of time! You escape and find a radio to call for help but you don t know where you are! Luckily, it s a clear night. You watch the stars for while and you see the stars rising at a 20 degree angle with respect to your horizon. What can you tell your rescuers about your location? 2
Annual Motion of Sun The sun moves along a repeatable path on the celestial sphere throughout the year. This path is called the ECLIPTIC Sun moves east ward relative to stars on celestial sphere It moves 360 degrees in a year about 1 degree per day The constellations through which we see the sun move are the constellations of the ZODIAC This apparent motion is due to the Earth orbiting the Sun Annual Motions 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 Annual Motions of the Stars Coordinate system on Earth Gainesville: 29.6510N, 82.3250W Coordinate Systems Altitude : angular distance above horizon Azimuth : angular distance measured along horizon to location of object By convention azimuth is measured from the north going eastward Advantages Disadvantages Equatorial Coordinate System Declination the angular distance of a star along a circle north or south of celestial equator (degrees:arcmin:arcsec) Right Ascension the angular distance of a object measured eastward from the vernal equinox along the celestial equator (H:M:S) 3
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 Reasons for the Seasons? What causes this pattern? Two Possible Models: Distance of the Earth from the Sun? Tilt of the Earth s axis with respect to the Earth s orbital plane (ecliptic) 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 every where on Earth since the Earth is tiny compared to the Earth- Sun distance Reason for Seasons: Distance? Observations: 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? 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 4
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 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) 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 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 Demonstration 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 Question 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 it s orbital plane (ecliptic), what daily and annual effects would this have? 5
Temperature in Ecuador Gainesville: 29.6510N, 82.3250W What is special about the Tropics (i.e. between +23.5degrees and -23.5 degrees latitude)? Assignment Reference Reading Astronomy & Astrophysics by Zelik and Gregory - chapter 1, chapter 3, chapter 4-1 & 4-2 The Physical Universe, An Introduction to Astronomy by Shu 6