Analemma with a Total Solar EclipseCourse AnnouncementsAssignmentsEarth’s Motions - GeneralEarth’s Rotation: Diurnal MotionsEarth’s Rotation: Defining DaysEarth’s Revolution- Orbital MotionEarth’s Motions - RevolutionUnderstanding the Night SkyUnderstanding the Night SkyThe Celestial SphereThe Celestial SphereThe Celestial SphereA Earthly ViewSky Coordinate SystemsEarth’s Axial TiltEarth’s Motions – Axial TiltThe Reason for the SeasonsThe Reason for the SeasonsThe Reason for the SeasonsThe Reason for the SeasonsThe Reason for the SeasonsEarth’s Motions – Axial TiltThe Reason for the SeasonsThe Reason for the SeasonsEarth’s Motions - PrecessionEarth’s Motions - PrecessionEarth’s Motions - PrecessionEarth’s Motions - PrecessionAnalemma with a Total Solar EclipseCredit: Cenk E. Tezel & Tunç Tezel (TWAN)Course Announcements•Astronomy Labs Start This Week•Register for Mastering Astronomy as soon as possible!Course ID: LINDSAYA151F16AssignmentsReading 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 - GeneralWe have to keep track of several simultaneous motionsMotions 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 MotionsYou spin me right round.•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)Credit: Miguel ClaroEarth’s Rotation: Defining Days•Daily, or diurnal, noon-to-noon 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-Lincoln Astronomy SimulationThe Solar and Sidereal DayEarth’s Revolution- Orbital MotionWhat 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 - RevolutionEarth’s constant motion in its orbit changes what we see night to night (360o / 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 nightySouthern horizon @(Northern) SummerSouthern horizon @(Northern) WinterUnderstanding the Night SkyThe 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 on1. your location on Earth (latitude) 2. the time of year•For the Northern hemisphere observerthe depicted Southern sky stars are forever below the horizonUnderstanding the Night SkyThe 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 on1. 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 isin its orbit.The Celestial SphereProjecting 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–North and South Celestial Poles and The Celestial EquatorNorth Celestial PoleSouth Celestial PoleCelestial EquatorThe Celestial SphereRight Ascension and DeclinationA fixed coordinate systemMeasured in anglesRight 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 clockDeclination (Dec.) is analogous to latitude- For the curious: Traditional angular measurement, but with additional sub-degree units introduced laterThe Celestial SphereRight Ascension and DeclinationA fixed coordinate systemRight 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 clockDeclination is analogous to latitude- For the curious: Traditional angular measurement, but with additional sub-degree units introduced laterEvery star can now have a unique RA and Dec assigned to it.E.g., The star Vega is atR.A. = 18h 36m 56.33635sDec. = +38° 47’ 1.3”A Earthly ViewAnother common system:Altitude-AzimuthA not-fixed systemUseful terms to know:•Zenith: The point directly overhead•Meridian: Line going through South, zenith, and NorthAltitude – Azimuth (Alt.-Az) •Altitude: angular measure of how far above horizon•Azimuth: Direction in number of degrees away from NorthSky Coordinate SystemsCelestial 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
View Full Document
Unlocking...