Download Day 1: How to Describe the Sky The Motions of the Stars

The Motion of the
Sun, Earth, and Stars
Day 1:
How to Describe the Sky
The Motions of the Stars
We know...
• ... that the Earth rotates once every day (in what direction?)
• ... that the Earth orbits around the sun once every year (in what direction?)
• What does that mean for our view of the sky?
• First, some terminology.
The Celestial Sphere
• “Imaginary”
• North Celestial Pole
• South Celestial Pole
• Celestial Equator
• Ecliptic: path of sun
• Is it aligned?
The Local Sky
• Horizon
• Zenith
• Meridian
Identify an object by:
• Direction (Azimuth)
• Altitude
Angular Size
• The angle an object appears to span in your field of view.
• Depends on:
• Actual size of object
• Distance from us to the object
Angular Size
• The Sun and the Moon have about the same angular size. How can that be?
Not to scale!
Card Question
• When an astronomer describes the altitude of something in the local sky, he or she means
• A: how high something is in the sky, in units of miles or kilometers.
• B: how high something is in the sky, in units of degrees.
• C: the direction toward something ­ North, South, East, or West.
Do the stars that we can
see change with...
• ... the time of night?
• ... your latitude on Earth?
• ... your longitude on Earth?
• ... the time of year (the season)?
Stars and the Time of Night
Lecture Tutorial:
Position
Groups of 3­4
Try to take ~ 10 minutes
Stars and the Time of Night
Lecture Tutorial:
Position
Groups of 3­4
Try to take ~ 10 minutes
Stars and the Time of Night
• Circumpolar stars: always up
• Others: rise East, set West
• Near South Celestial pole: never rise
Circumpolar Stars
Card Question
• How much time is there between when a star rises and when it sets?
• A: Less than 12 hours
• B: About 12 hours
• C: More than 12 hours
• D: It depends on the star.
Card Question
• How much time is there between when a star rises and when it sets?
• A: Less than 12 hours
• B: About 12 hours
• C: More than 12 hours
• D: It depends on the star.
Card Question
Figure 2.13 animated
The North Celestial Pole points towards Polaris, and stars appear to rotate counterclockwise.
• Which way do stars rotate around the South celestial pole?
• A: Clockwise
• B: Counterclockwise
Card Question
Figure 2.13 animated
The North Celestial Pole points towards Polaris, and stars appear to rotate counterclockwise.
• Which way do stars rotate around the South celestial pole?
• A: Clockwise
• B: Counterclockwise
Stars and Your
Location on Earth
•
•
Latitude (North/South)
•
Affects location of horizon and zenith.
•
Altitude of celestial pole = your latitude.
Longitude: no change in local time (explain). Why?
Card Question
• Where on Earth would you see the most circumpolar stars?
• A: Oslo, Norway (latitude 60N)
• B: Boulder, Colorado (latitude 40N)
• C: Bangalore, India (latitude 12N)
Card Question
• Where on Earth would you see the most circumpolar stars?
• A: Oslo, Norway (latitude 60N)
• B: Boulder, Colorado (latitude 40N)
• C: Bangalore, India (latitude 12N)
Stars and the
Time of Year
• What does your “Zodiac sign” mean?
Figure 2.14
Lecture tutorial:
Seasonal Stars
~ 15 minutes
Stars and the Time of Year
Stars rise 4 minutes earlier each night due to the orbit of the Earth around the Sun.
Clicker Question
• You go out tonight and see the brightest star in the constellation Orion rise at about 10PM. One week from now this star will rise at about
• A: 9:30PM
• B: 10:00PM
• C: 10:30PM
• D: 10:00AM
Clicker Question
• You go out tonight and see the brightest star in the constellation Orion rise at about 10PM. One week from now this star will rise at about
• A: 9:30PM
• B: 10:00PM
• C: 10:30PM
• D: 10:00AM
Clicker Question
• One evening at midnight, you observe Leo high in the Southern sky. Virgo is to the East of Leo and Cancer is to the West. One month earlier, which of these constellations was high in the Southern sky at midnight?
• A: Leo
• B: Virgo
• C: Cancer
Clicker Question
• One evening at midnight, you observe Leo high in the Southern sky. Virgo is to the East of Leo and Cancer is to the West. One month earlier, which of these constellations was high in the Southern sky at midnight?
• A: Leo
• B: Virgo
• C: Cancer
Do the stars that we can
see change with...
• ... the time of night?
• Yes, due to the rotation of the Earth. Many stars rise and set at different times of night. Some are circumpolar, some we don’t see at all.
• ... your latitude on Earth?
• Yes. Your latitude affects your horizon and zenith.
• ... your longitude on Earth?
• No.
• ... the time of year (the season)?
• Yes, due to our orbit around the Sun.
Precession
Gradual change in the orientation of Earth’s axis.
Clicker Question
• Why are different stars seen in different seasons?
• A: Because of Earth’s axis tilt.
• B: Because stars move during the year.
• C: Because as Earth orbits the Sun, we see the Sun in front of different constellations.
• D: Because of precession.
Clicker Question
• Why are different stars seen in different seasons?
• A: Because of Earth’s axis tilt.
• B: Because stars move during the year.
• C: Because as Earth orbits the Sun, we see the Sun in front of different constellations.
• D: Because of precession.
Skills learned:
Celestial Sphere: definitions
Why is the ecliptic different from the celestial equator?
Local Sky: definitions
Angular vs. Physical Size
Stars: rise and set; direction of travel; circumpolar stars
Latitude and Longitude: altitude of polaris = latitude
Motion through the Zodiac: why does it happen?
Stars rise 4 min earlier each day; why?
Precession: wobble of the Earth's axis over LONG periods of time