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

A1 F2015 Seasonal Redux copy.keySpring F2015
Seasons Planetary Motion
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
• due next Tuesday 9/22
Last Class
• LT Position
• Annual Motion
This Class
• Annual Motion
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?
Visible Night Sky Changes
• Which stars are visible at night changes over the orbit.
Like a merry-go-round that takes a year to make one rotation
Constellation Transiting at Midnight Changes
• Midnight — direction opposite Sun
“The Sun is In”
• Constellation “behind” the Sun
• 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
• 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
Seasonal Constellations
• This is ~ Dec 1
• actually Ophiucus
• This is ~ Jan 1
• The Sun is in ?
Cancer
Leo
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
One night, you see the star Sirius rise at exactly 7:36 PM. The following night it will rise
A. slightly earlier.
C. slightly later.
You go out tonight and see the brightest star in the 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.
D. setting on your western horizon.
Name:____________________! ! Date:__________ ICE 2! ASTRO 110/120! Levine Spring 2013
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
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
• 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
Axial Tilt
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
Why does axial tilt drive the seasons?
• It’s about the transfer of Energy
• Specifically energy from Sun to Earth
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.
• gravitational potential energy — boulder at top of hill
• thermal energy — motion of atoms and molecules
• chemical energy
• light energy
• etc.
Energy from the Sun
• Light or electromagnetic radiation
• Net absorbtion of energy = heating
• Sun puts out same energy summer & winter, but in summer energy transfer is locally more efficient
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
The Length of the Day
Northern summer = southern winter
Northern winter = southern summer
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
Terminology: Solstice & Equinox
• “sun stop”
• “Equal night”
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
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?
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
Astronomy 1 — Elementary Astronomy LA Mission College
Spring F2015
Planets... the wanderers
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
Mars and Retrograde Motion
QuickTime movie from Dr. Ted Snow's web site at The University of Colorado, Boulder.)
For how many days was this planet in retrograde motion?
A. 10 days
B. 12 days
C. 15 days
D. 17 days
E. 32 days
On which date would the planet appear to rise in the west?
A. March 1
B. March 31
C. April 5
D. April 12
WRAP-UP
Topic for Next Class
Reading Assignment
Homework

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

Documents

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