1. Observing and the Sky
Astronomy 311
Professor Lee Carkner
Lecture 3

2. Our Point of View
Think about the motions of the heavens from different points of view
Remember that everything is moving at once

3. The Changing Sky
Sky makes one complete rotation per 24 hours (Diurnal motion)
Sky also makes one complete rotation per year (Annual motion)
Caused by the motion of the Earth around the Sun

4. Diurnal Motion

5. Annual Motion

6. The Observer’s View

7. Observing
Can measure distance on the sky in degrees (360 degrees = complete circle)
Horizon --
Zenith --
Meridian -- line running from north to south through zenith

8. The Solar Year

9. The Seasons
When the northern hemisphere is pointed at the Sun, it is summer here
Seasons are not caused by Earth being more or less distant from the Sun!!!

10. Direct and Indirect Sunlight

11. Solstice Equinox When the Sun is highest or lowest in the sky
Longest or shortest day of year
When the Sun is overhead at the equator
Day and night equal in length

12. Changing Day Length

13. Lines on a Globe Equator --
Tropic of Capricorn /2 degrees south of the equator, Sun overhead on Winter solstice
Tropic of Cancer --
Arctic and Antarctic Circles /2 degrees south or north of the pole, Sun never sets or rises at solstice time

14. Equal to the angle of Polaris above horizon
Longitude (degrees East or West of the Prime Meridian running through Greenwich England)
Navigation

15. The Celestial Sphere
Project the lines on a globe into space to form a coordinate system
North Celestial Pole --
Celestial Equator -- Project the Earth’s equator into space

16. Celestial Coordinates
Right Ascension -- degrees east from where the sun is on the Vernal Equinox (first day of spring)
Declination --
The coordinates do not move or vary with location on the Earth, they are fixed to the stars

17. The Ecliptic
Most of the objects in the solar system have their orbits in the same plane, called the ecliptic
The ecliptic passes through 12 constellations known as the zodiac

18. Constellations of the Zodiac

19. Precession
The Earth “wobbles” as it spins, causing the Earth’s axis to point at different parts of the sky
This changes where the equinoxes are in the sky

20. Astrology
Your sign is supposed to be where the Sun was on the zodiac when you were born
For example: Sept 10th = Virgo, but Sun is in Leo. All star signs are about 1 month off

21. Science and Pseudoscience
Astronomy is a science, it tries to form a picture of the universe based on observation and reason. It is subject to proof
Astrology is a pseudoscience, it uses some of the terminology of science, but its basic tenets are not subject to proof

22. Next Time
Meet back in Science 102 on Monday
Read Chapter 1.6