ASTRO 101

1. ASTRO 101 Principles of Astronomy

2. Instructor: Jerome A. Orosz (rhymes with “boris”)Contact: • Telephone: 594-7118 • E-mail: orosz@sciences.sdsu.edu • WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ • Office: Physics 241, hours T TH 3:30-5:00

3. Text: “Discovering the Essential Universe, Fifth Edition”by Neil F. Comins

4. Course WWW Page http://mintaka.sdsu.edu/faculty/orosz/web/ast101_spring2013a.html Note the underline: … ast101_spring2013a.html … Also check out Nick Strobel’s Astronomy Notes: http://www.astronomynotes.com/

5. Homework due January 29: Question 15 from Chapter 1 (Why is it warmer in the summer than in winter?) • Write down the answer on a sheet of paper and hand it in before the end of class on January 29.

7. Speaking of infinity… • Which set is larger: • The set of positive integers (1,2,3,…) • The set of even integers (2,4,6,…)

8. Speaking of infinity… • Which set is larger: • The set of positive integers (1,2,3,…) • The set of even integers (2,4,6,…) • Both are the same size since there is a one-to-one mapping between the two: (1, 2, 3, 4, …) (2, 4, 6, 8, …)

9. Pop Quiz • What is the shape of the Earth? http://www.alaska.net/~clund/e_djublonskopf/Flatearthsociety.htm http://fixedearth.com

10. The Earth • How do we know the Earth is spherical?

11. The Earth • How do we know the Earth is spherical? • Curved shadow of the Earth on Moon during eclipse • The manner in which ships at sea disappear when they sail away • The fact that as you go north-south certain stars disappear

12. The Earth • How do we know the Earth is spherical? • Curved shadow of the Earth on Moon during eclipse • The manner in which ships at sea disappear when they sail away • The fact that as you go north-south certain stars disappear

13. Questions for Today • What causes the seasons? Let’s ask some Harvard students: http://www.learner.org/resources/series28.html • Does the Earth go around the Sun? Let’s ask… http://www.collegehumor.com/video:1773116

14. Next: Discovering the Night Sky

15. Coming Up: • Introduction to the Sky • Constellations • Stellar Brightness • Stellar coordinates and the Celestial Sphere • The “clockwork” of the sky • Day/night • Phases of the moon • The seasons

16. Looking at the Night Sky • On any given night, you can see about 3000 stars without a telescope, provided the sky is dark.

17. Constellations • People have long made up stories about groups of stars that appear close together on the sky. • Such groupings are called constellations. The sky was “officially” divided up into 88 constellations in 1930 so that a star is associated with only one constellation.

18. Constellations • The modern constellations have strictly defined boundaries by international agreement.

19. Constellations • Many constellation names are derived from characters in Greek or Roman mythology. • Here is Cassiopeia, with its distinctive “W”. • The stars are usually not physically associated with each other.

20. Constellations • Here is the “Big Dipper”, which is not an “official” constellation but part of a larger one. • Again, the stars are usually not physically associated with each other.

21. Constellations • Constellations can help you mark the seasons.

22. Stellar Brightness: Magnitudes • Historically (e.g. Hipparcos in the First Century), the brightness of stars as seen by the eye have been measured on a magnitude scale: • The brightest stars were “first magnitude”. • The faintest stars were “sixth magnitude”. • Brighter objects have smaller magnitudes.

23. Stellar Brightness: Magnitudes • In modern times, it was discovered that the human eye has a nonlinear response to light: if one source of light has twice the light as a second source, then the first source would not appear by eye to be twice as bright. • The response of the eye is logarithmic, so that differences of magnitudes correspond to ratios of brightness.

24. The Magnitude Scale • The modern of the magnitude scale is set up so that a difference of 5 magnitudes corresponds to a ratio of brightnesses of 100. • Bright objects can have negative apparent magnitudes.

25. The Celestial Sphere • Imagine the sky as a hollow sphere with the stars attached to it. This sphere rotates once every 24 hours. This imaginary sphere is called the celestial sphere. • Even though we know it is not the case, it is useful to imagine the Earth as being stationary while the celestial sphere rotates around it.

26. The Celestial Sphere • The north celestial pole is directly above the north pole on the Earth. • The south celestial pole is directly above the south pole on the Earth. • The celestial equator is an extension of the Earth’s equator on the sky. • The zenith is the point directly over your head. The horizon is the circle 90 degrees from the zenith.

27. The Celestial Sphere • The celestial poles and the celestial equator are the same for everyone. • The zenith and the horizon depend on where you stand. http://www.astronomynotes.com/nakedeye/s4.htm

28. Stellar Coordinates and Precession • There are a few ways to specify the location of a star (or planet) on the sky: • Altitude/Azimuth: • The altitude describes how many degrees the star is above the horizon, the azimuth describes how far the star is in the east-west direction from north. • The altitude and azimuth of a star is constantly changing owing to the motion of the star on the sky!

29. Stellar Coordinates and Precession • There are a few ways to specify the location of a star (or planet) on the sky: • Equatorial system: • Lines of longitude on the earth become right ascension, measured in units of time. The RA increases in the easterly direction. • Lines on latitude on the earth become declination, measured in units of degrees. DEC=90o at the north celestial pole, 0o at the equator, and -90o at the south celestial pole. • http://www.astronomynotes.com/nakedeye/s6.htm

30. Stellar Coordinates and Precession • The north celestial pole moves with respect to the stars very slowly with time, taking 26,000 years to complete one full circle.