Proxima b A very nearby Earth like Planet Alpha Centauri A and B Proxima Centauri Proxima b Discovery announced a few hours ago Artists Conception of System Credit ESO M Kornmesser Proxima b In the Habitable Zone Proxima Centauri Distance from Earth 4 2 light years Our closest Neighboring Star Habitable Zone Proxima Centauri Relative size of Mercury s orbit Proxima b Alpha Centauri System Proxima Centauri orbits two Sun like stars called Alpha Centauri A and B Discovery announced a few hours ago Credit ESO M Kornmesser Course Announcements Quiz 1 on Wednesday 31 August Hints today and in coming lectures Register for Mastering Astronomy as soon as possible Course ID LINDSAYA151F16 Assignments Reading 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 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 365 25 days to go around the Ecliptic 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 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 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 Christmas 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 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 Seasons Hint 3 Well I didn t see that one coming The Earth is closest to the Sun in January The Reason for the Seasons The Reason Earth s 23 5o Axial Tilt Induces Two Main Effects 1 Sunlight is more less concentrated in Summer Winter respectively Due to altitude of Sun 2 Days are longer shorter in Summer Winter respectively Univ of Nebraska Lincoln Seasons Simulator Altitude of Sun To Zenith S Solstice 50 23 5 73 5 At Equinox at 40o N RO TA TE 40o S Solstice 50 23 5 73 5 50o W Solstice 50 23 5 26 5 To Zenith NP 40 o Horizon Rotated View Sun The Reason for the Seasons Earth s 23 5o Axial Tilt More energy per square meter Less energy per square meter The Reason for the Seasons Earth s 23 5o Axial Tilt Autumnal Equinox Summer Solstice Definition Solstice literal translation is Sun Stands Ecliptic at northern most point in sky Summer Soltice Sunlight most direct in N Hemisphere Ecliptic at southern most point in sky Winter Soltice Sunlight most direct in Southern Hemisphere Winter Solstice Spring Equinox The Reason for the Seasons Earth s 23 5o Axial Tilt Equinox Equal Night Axial Tilt pointed in a different direction from Earth Sun line Ecliptic at southern most point in sky perpendicular to Sun moving above Celestial equator Vernal Spring Equinox Sun moving below Celestial Equator Autumnal Fall Equinox Autumnal Equinox Summer Solstice Winter Solstice Spring Equinox Earth s Motions Axial Tilt What it looks like from the ground 40o N In 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 Summer Solstice Equinox Spring Fall Winter Solstice Credit Stuart Robbins 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 Earth s Motions Precession A 26 000 year cycle The Earth s axis also rotates Earth s Motions Precession A 26 000 year cycle Different North Stars The yellow circle represents the precession path or rather where in the sky Earth s North Pole is pointed Currently Pointed at Polaris Egyptian Era Thuban Year 13 000 Vega Earth s Motions Precession What 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 EarthSun line Earth s Motions Precession The 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 Moon Earth Moon system to scale Orbital Period or Sidereal Period of the Moon is 27 3 days Again that means the amount of time to complete one 360 rotation Synodic versus Sidereal Month Again we have to take Earth s revolution around the Sun into account While the Moon orbit s the Earth the Earth still continues to orbit the Sun The Moon needs to continue it s revolution around the Sun to get back to a New Moon hase Full cycle of phases is called a synodic month and is 29 5 days long The synodic month is longer than sidereal month due to motion of Earth similar to solar day vs sidereal day Phases of the Moon Due to relative position of Sun Earth and Moon Quarter 3 Weeks Later Incoming Sunlight 4 Weeks Day 0 New 2 Weeks Later Full 1 Week Later Quarter Key Concepts 1 Moon is bright because of reflected sunlight 2 Half of the Moon is always illuminated 3 The phase we see is due to our view of the illuminated portion 4 Takes about 4 weeks to complete synodic period Moon Phases If you learn how to make the diagram to the right then you will be well on your way to Phases of the Moon mastery Advanced Skills
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