Analemma with a Total Solar Eclipse Credit Cenk E Tezel Tun Tezel TWAN Course Announcements Astronomy Labs Start This Week Register for Mastering Astronomy as soon as possible Course ID LINDSAYA151F16 Assignments Reading Assignments Chapter 1 Sections 1 5 1 6 Read by Wednesday 24 Aug Parallel Lectures CC Astronomy Episode 3 Cycles in the Sky Watch by Wednesday 24 Aug Mastering Astronomy Introduction to Mastering Astronomy Due Thursday 25 Aug at 11 59 PM EDT Earth s Motions General We have to keep track of several simultaneous motions Motions in the Night Sky as observed from Earth Daily or Diurnal motions Rise in the East set in the West Because of Earth s rotation about its spin axis Yearly or Annual motions What stars constellations are viewable at night changes from month to month over the course of a year and then repeats Because of Earth s revolution or orbital motion around the Sun 26 000 year precession cycle Earth s rotational access wobbles around like a top over a 26 000 year cycle slightly changing our view of the night sky from Earth Changes what star if any is the North Star and South Star currently no South Star Earth s Rotation Diurnal Motions You spin me right round Credit Miguel Claro Celestial objects rise in the East and set in the West Due to Earth s rotation We all know that our day has something to do with Earth s rotation But what defines a day Is it Noon to Noon Is it one complete rotation of 360o That is the time it takes for a star in the sky to return to the same place in the sky a k a a sidereal rotation Earth s Rotation Defining Days The Solar and Sidereal Day Daily or diurnal noon tonoon defines the Solar Day 24 hours Single 360o rotation is slightly shorter see figure Defined as the Sidereal Day 23 hrs 56 min Reason Earth is not only rotating but also revolving University of Nebraska Lin Earth s Revolution Orbital Motion What stars can be seen change throughout the year 365 25 days Great way to play around with this concept is using Stellarium it s free Recommendation Open the Date Time Window and change the date one day at a time one week at a time by pressing or one month at a time Earth s Motions Revolution Earth s constant motion in its orbit changes what we see night to night 360 o 360 days 0 986o per day The result of this motion is a slow 12 month progression through the constellations night sky objects such that the winter sky is entirely different from the Summer Sky Each star will appear 1o to the west each nighty Southern horizon Northern Summer Southern horizon Northern Winter Understanding the Night Sky The viewable stars From any vantage point on Earth you can see roughly 3 000 stars Only half of the Celestial Sphere indicating that about 6 000 stars are visible from Earth with the naked eye The constellations that are visible depend on 1 your location on Earth latitude 2 the time of year For the Northern hemisphere observer the depicted Southern sky stars are forever below the horizon Understanding the Night Sky The viewable stars From any vantage point on Earth you can see roughly 3 000 stars Only half of the Celestial Sphere indicating that about 6 000 stars are visible from Earth with the naked eye The constellations that are visible depend on 1 your location on Earth latitude 2 the time of year For the Northern hemisphere observerthe depicted Southern sky stars are forever below the horizon E g A person at the North Pole can t see any of the Southern sky regardless of where Earth is in its orbit The Celestial Sphere North Celestial Pole Projecting Earth s Coordinate System With time of year and latitude dependencies it would be useful to have a concept that allows us to place the stars in our sky Stars seem to be held at a fixed distance on the inner surface of a sphere We call this imaginary sphere the Celestial Sphere Projects Earth s coordinate system Poles Equator latitude and longitude onto this imaginary sphere Celestial Equator North and South Celestial Poles and The Celestial Equator South Celestial Pole The Celestial Sphere Right Ascension and Declination A fixed coordinate system Measured in angles Right Ascension RA is analogous to longitude For the curious An angular measurement given as if it was a measurement of the hour hand around a 24 hour clock Declination Dec is analogous to latitude For the curious Traditional angular measurement but with additional sub degree units introduced later The Celestial Sphere Right Ascension and Declination A fixed coordinate system Right Ascension is analogous to longitude For the curious An angular measurement given as if it was a measurement of the hour hand around a 24 hour clock Declination is analogous to latitude For the curious Traditional angular measurement but with additional sub degree units introduced later Every star can now have a unique RA and Dec assigned to it E g The star Vega is at R A 18h 36m 56 33635s Dec 38 47 1 3 A Earthly View Another common system Altitude Azimuth A not fixed system Useful terms to know Zenith The point directly overhead Meridian Line going through South zenith and North Altitude Azimuth Alt Az Altitude angular measure of how far above horizon Azimuth Direction in number of degrees away from North Sky Coordinate Systems Celestial vs Horizontal Celestial Coordinates use Right Ascension and Declination Fixed coordinate system such that every star and deep space object has a specific RA and Dec Does not change Horizontal Coordinates use Altitude and Azimuth Alt Az Convenient to point out a point of the local sky Because everything is moving in the sky objects Alt Az Coordinates are constantly changing Earth s Axial Tilt The Earth s Rotation Axis is tilted 23 50 with respect to it s orbital path The 23 5 Tilt always points in the same direction while the Earth revolves around the Sun To Polaris To Polaris Earth s Motions Axial Tilt Just have to tilt our Celestial Sphere by 23 5 and consider where the Sun is Definition The Ecliptic is the path of the Sun on the celestial sphere over the course of a year Because of Earth s axial tilt of 23 5o the Ecliptic is also inclined 23 5o with respect to The Sun Earth orbital plane Defines the Plane of the Solar System The Reason for the Seasons Addressing a common misconception We are now prepped to address what seems to be a simple question What is the reason that Earth has seasons Summer Autumn Winter Spring Spend the next minute convincing your neighbor what the reason for the seasons is Hint It s not
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