Proxima b – A (“very”) nearby Earth-like PlanetProxima b – In the Habitable ZoneCourse AnnouncementsAssignmentsEarth’s Motions – Axial TiltEarth’s Axial TiltThe Reason for the SeasonsThe Reason for the SeasonsThe Reason for the SeasonsThe Reason for the SeasonsAltitude of SunThe Reason for the SeasonsThe Reason for the SeasonsThe Reason for the SeasonsEarth’s Motions – Axial TiltEarth’s Motions - PrecessionEarth’s Motions - PrecessionEarth’s Motions - PrecessionEarth’s Motions - PrecessionMotion of the MoonSynodic versus Sidereal MonthPhases of the MoonMoon PhasesExample Quiz QuestionExample Quiz QuestionEclipsesEclipses - GeneralA more true perspectiveLunar EclipsesMotion of the MoonMotion of the MoonSolar EclipsesTotal Solar EclipseSolar EclipsesQuiz 1 – Possible Questions21 August 2017 Total Solar Eclipse21 August 2017 Total Solar EclipseEclipse SeasonsEclipse SeasonsProxima b – A (“very”) nearby Earth-like PlanetDiscovery announced a few hours ago!Artists Conception of SystemCredit: ESO/M. KornmesserProxima CentauriAlpha Centauri A and BProxima bProxima b – In the Habitable ZoneDiscovery announced a few hours ago!Credit: ESO/M. KornmesserHabitable ZoneProxima bRelative size of Mercury’s orbitProxima CentauriProxima Centauri Distance from Earth:4.2 light-years(Our closest Neighboring StarAlpha Centauri System:Proxima Centauri orbits two Sun-like stars called Alpha Centauri A and BCourse Announcements•Quiz 1 on Wednesday, 31 August–Hints today and in coming lectures•Register for Mastering Astronomy as soon as possible!Course ID: LINDSAYA151F16AssignmentsReading Assignments•Chapter 1. Sections: 1.7, More Precisely 1-1, 1.2 [Read by Friday, 26 Aug.]Parallel Lectures•CC Astronomy Episodes 4 and 5[Watch by Monday, 29 Aug.]Mastering Astronomy•Introduction to Mastering Astronomy[Due Thursday, 25 Aug. at 11:59 PM EDT]•Chapter 1 Homework[Due Wednesday, 31 Aug. at 11:59 PM EDT]Earth’s Motions – Axial TiltJust 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)365.25 days to go around the EclipticEarth’s Axial TiltThe Earth’s Rotation Axis is tilted 23.50 with respect to it’s orbital pathThe 23.5° Tilt “always” points in the same direction while the Earth revolves around the SunTo PolarisTo PolarisThe Reason for the SeasonsAddressing a common misconceptionWe 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 ChristmasThe Reason for the SeasonsAddressing a common misconceptionWe 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 Christmas-Hint 2: It’s Summer here, but what season is it in Australia? Peru? Does your reason explain why the North and South Hemisphere have opposite seasons?The Reason for the SeasonsHint 3… Well, I didn’t see that one coming.The Earth is closest to the Sun in January!The Reason for the Seasons1. Sunlight is more/less concentrated in Summer/Winter, respectively.- Due to altitude of Sun2. Days are longer/shorter in Summer/Winter, respectivelyThe Reason: Earth’s 23.5o Axial Tilt!Induces Two Main EffectsUniv. of Nebraska, Lincoln Seasons SimulatorNPTo ZenithSun40oHorizonTo Zenith40o50oROTATEAltitude of SunAt Equinox at 40o N@ S. Solstice50+23.5° = 73.5°@ W. Solstice50-23.5° = 26.5°S. Solstice50+23.5° = 73.5°Rotated ViewThe Reason for the SeasonsEarth’s 23.5o Axial TiltMore energy per square meterLess energy per square meterThe Reason for the SeasonsEarth’s 23.5o Axial TiltSpring EquinoxAutumnal EquinoxWinter SolsticeSummer SolsticeSolstice (literal translation is “Sun Stands”)Ecliptic at northern- most point in sky – Summer Soltice•Sunlight most direct in N. HemisphereEcliptic at southern- most point in sky – Winter Soltice•Sunlight most direct in Southern HemisphereDefinitionThe Reason for the SeasonsEarth’s 23.5o Axial TiltSpring EquinoxAutumnal EquinoxWinter SolsticeSummer SolsticeEquinox (“Equal Night”)Axial Tilt pointed in a different direction from Earth-Sun lineEcliptic at southern- most point in sky [perpendicular to]-Sun moving above Celestial equatorVernal/Spring Equinox-Sun moving below Celestial EquatorAutumnal/Fall EquinoxEarth’s Motions – Axial TiltWhat it looks like from the ground (40o N)Credit: Stuart RobbinsIn summer, the Sun rises north of east, travels through the southern sky, and sets north of west.•travels a longer and higher path through the sky (Longer Days)In winter, the Sun rises South of East, travels through southern sky, and sets south of west•travels a shorter and lower path through the sky (Shorter Days)SummerSolsticeEquinox(Spring/Fall)WinterSolsticeEarth’s Motions - PrecessionA 26,000 year cycle. The Earth’s axis also rotates.Earth’s Motions - PrecessionA 26,000 year cycle. Different North StarsThe yellow circle represents the precession path, or rather, where in the sky Earth’s North Pole is pointed.Currently Pointed at PolarisEgyptian Era: ThubanYear ~13,000: VegaEarth’s Motions - PrecessionWhat’s a year again?Time for Earth to complete one orbit around the sun relative to the fixed stars is the Sidereal Year (365.256 days).Time between two Vernal (Spring) Equinoxes is defined as the Tropical Year (365.2422 days), and it happens slightly sooner because Earth’s axis has rotated toward the Earth-Sun line.Earth’s Motions - PrecessionThe Tropical Year is our Calendar Year•The Tropical Year (365.2422 days) is our calendar year.•Keeps seasons at same time of year•Very slow change of what season a constellations is.•In 13,000 years, Orion will be a summer constellation•If we used the Sidereal Year, in 13,000 years Orion would still be a Winter constellation, but Northern summer would be in December.–i.e., the Spring Equinox would drift through the the calendar, occurring about 20 minutes later each year.Motion of the MoonEarth – Moon system to
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