Night Sky II Annual Motion Seasons Planetary Motion (Take 2)

Astronomy 1 — Elementary Astronomy LA Mission College Spring F2015 Quotes & Cartoon of the Day

“Astronomy is useful because it raises us above ourselves; it is useful because it is grand; …. It shows us how small is man's body, how great his mind, since his intelligence can embrace the whole of this dazzling immensity, where his body is only an obscure point, and enjoy its silent harmony.”

— Henri Poincaré, The Value of Science, George Halsted translation, 1907.

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Announcements

• First HW, through part of today’s material, posted • due next Tuesday 9/22

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Last Class

• LT Position • Annual Motion • Zodiac • Seasons • Planetary Motion (time permitting)

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 This Class

• Annual Motion • Zodiac • Seasons • Planetary Motion

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Zodiac and Apparent Solar Motion

Astronomy 1 — Elementary Astronomy LA Mission College Spring F2015 Apparent Solar Motion

• Due to Earth’s rotation, the sun & stars rise & set • The Sun also appears to move against the background stars due to the Earth’s orbit around it • When do we see stars? What does that mean? Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Visible Night Sky Changes

• We see stars in the direction away from Sun • Which stars are visible at night changes over the orbit.

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Like a merry-go-round that takes a year to make one rotation

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Constellation Transiting at Midnight Changes

• Midnight — direction opposite Sun

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 “The Sun is In”

• Constellation “behind” the Sun

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Apparent Solar Motion

• The path the Sun appears to take is called the Ecliptic • The Ecliptic Plane is the plane in which the Earth orbits the Sun • Planets appear near the ecliptic • The zodiacal constellations lie near the Ecliptic

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Apparent Solar Motion

• Zodiac = imaginary belt about 8° on each side of the ecliptic • within which are the apparent paths of the sun, moon, and principal planets. • There are actually 13 classical constellations in the Zodiac • Ophiucus is between Scorpius and Sagittarius Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Seasonal Constellations

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Seasonal Constellations

• Sun moves ~1 constellation per month. • This is ~ Dec 1 • The Sun is in Scorpius • actually Ophiucus • Taurus transits near midnight

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Seasonal Constellations

• Sun moves ~1 constellation per month. • This is ~ Jan 1 • The Sun is in ? • ? transits near midnight • Did Taurus rise earlier Cancer or later? Leo

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Let’s Practice If The Sun appears in Virgo in the Fall (mid Sep to late Oct), approximately what time of year will it appear in Sagittarius?

A. Fall B. Spring C. Summer D. Winter

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Name:______!! Date:______One night, you see theICE 2 !star SiriusASTRO rise 110/120 at! exactly Levine Spring 7:36 2013 PM. The following night it will rise

A. slightly earlier. B. at the same time. C. slightly later.

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Name:______!! Date:______You go out tonight and seeICE 2! the brightestASTRO 110/120 star! in Levine the Spring 2013 constellation Orion just rising above your eastern horizon at 10 PM. One week later at 10 PM this same star will be _____.

A. slightly higher in the sky. B. below your horizon. C. at the same height as before. D. setting on your western horizon.

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 What component of Earth’s motion causes the stars to rise earlier on successive nights?

A. its rotation about its axis B. its orbit around the Sun C. the tilt of its rotation axis

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 SO... WHY DO WE HAVE SEASONS? We have Seasons because we are tipped!

• The Earth’s axis is tipped 23.5° from the plane of it’s orbit. • Axial Tilt • Axial tilt is the cause of seasonal variations • It is NOT!!! Because the Earth’s orbit is elliptical! • I bet you know something that tells you it can’t be this! • The Earth’s orbit is almost circular AND the point of closest approach, perihelion, is in January

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Axial Tilt

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 The Reasons for the Seasons

• Like a gyroscope, the Earth “holds” it’s tilt as it travels around the Sun. The tilt does not rotate as the Earth revolves

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Why does axial tilt drive the seasons?

• It’s about the transfer of Energy • Specifically energy from Sun to Earth

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Energy • Energy=capacity to start, stop, accelerate, turn, raise etc. something • microscopically or macroscopically • capacity to “do work” — change the energy state of something • E.g. • kinetic energy — energy of motion • gravitational potential energy — boulder at top of hill • thermal energy — motion of atoms and molecules • chemical energy • light energy • electric potential energy — capacity to move electrons • etc. Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Energy from the Sun

• Light or electromagnetic radiation • visible light • infrared light • ultraviolet light • Net absorbtion of energy = heating • Sun puts out same energy summer & winter, but in summer energy transfer is locally more efficient

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Why does the tilt affect the Solar Energy Transfer?

• The length of the day • Summer days are longer than winter days • The angle at which sunlight falls on the surface • The sun appears higher in the sky in summer and lower in winter • Sunlight is more direct in summer

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 The Length of the Day

Northern summer = Northern winter = southern winter southern summer longer days mean more total exposure

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 The Angle of Sunlight Winter vs. Summer

• Sun appears higher in the sky in summer &l ower in winter • lower= light hits surface at steeper angle. • Steeper angles = less concentrated light; dimmer

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Terminology: Solstice & Equinox

• Tilt toward or away from Sun — solstice • “sun stop” • start of winter or summer • Tilt parallel to Sun — equinox • “Equal night” • start of spring or fall Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Direction of Sunrise and Sunset are also affected

• At summer solstice, the Sun rises N of due E (in the northern hemisphere) • At winter solstice the Sun rises S of due E • This is the same reason the day length varies

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Let’s Practice Which lettered position (A-E), in the image below, best represents the location on Earth that is experiencing winter in the Northern Hemisphere?

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 If Earth were tilted more (60° rather than 23.5°), then during winter at your location you would _____ compared to what you experience during winter now.

A. experience cooler temperatures B. experience warmer temperatures C. not experience any significant change in temperature

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Planets... the Wanderers

Astronomy 1 — Elementary Astronomy LA Mission College Spring F2015 Planets... the wanderers

• It was a warm summer evening in ancient Greece....

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Apparent Motion of Planets

• Indeed originally named for their “wandering” movement • “Late Old English planete, from Old French planete (Modern French planète), from Late Latin planeta, from Greek planetes, from (asteres) planetai "wandering (stars),..." • Most of the time rise a bit later each night • move eastward against the background stars • Sometimes appear to go “backwards” for a while • i.e. rise earlier each night/go westward • This is called retrograde motion • We will explain how this works later in the course

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Mars and Retrograde Motion

QuickTime movie from Dr. Ted Snow's web site at The University of Colorado, Boulder.)

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Let’s Practice For how many days was this planet in retrograde motion?

May

A. 10 days B. 12 days C. 15 days D. 17 days E. 32 days

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 On which date would the planet appear to rise in the west?

May

A. March 1 B. March 31 C. April 5 D. April 12 Never,E. Never Ever, Never, Ever, EVER!!!!!!! Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 WRAP-UP Topic for Next Class

• Night Sky III • Lunar Phases • Eclipses

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Reading Assignment

• Ch 2 in either text

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015 Homework

• Sci, Annual, Diurnal due 9/22

Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015