More cycles in the sky

1. Announcements LABS: You've all done one now... Reading for Friday: Section 2.3 Assignment 1 distributed today Friday Sept 15 Due Sept 22nd, 5PM, in your lab slot. More cycles in the sky • (no, not that kind)

2. So, that's the Calendar. I can tell you what day it is, but what about what time it is? • Apparent solar time can be told with a sundial (or a stick!) • Relies on Sun's motion • Rate can vary (see below) • Is only LOCAL; why? • Effect of longitude. • When it's noon Vancouver, what apparent solar time is it in Kelowna? Or Victoria?

3. Local (or apparent) solar time varies with longitude SUN • A gnomon casts a shadow straight north in Vancouver at apparent Vancouver noon • But in Victoria and Kelowna they are not straight north. So local noon is different for everyone; good enough if you can't travel fast. EAST WEST Kelowna Vancouver Victoria

4. Modern solution is TIME ZONES • If politics didn't intervene, there would be 15 degrees of longitude/zone

5. Where in a time zone are you in sync with apparent solar time? • That is, at which longitude is the time-zone time equal to the apparent solar time? • For example, at only one longitude in the time zone will the Sun be right on the local meridian at noon. Where?

6. Where in a time zone are you in sync with apparent solar time? • That is, at which longitude is the time-zone time equal to the apparent solar time? • For example, at only one longitude in the time zone will the Sun be right on the local meridian at noon. Where? • Somewhere in the middle, at the multiples of 15 degrees. • Daylight savings messes this up! (Why?)

7. What about Universal time (UT?) • Time at Greenwich England (on the prime meridian). Useful in astronomy and navigation. Vancouver is UT - 8 hours (except for daylight savings)

8. Navigation : Using the stars Sect S1.3 • We must understand how to measure and use the heavens to determine latitude (easy) and longitude (very hard usually). • An understanding of how to find longitude is why Britain became a world power. It's fleet was the most skilled at navigation.

9. Longitude and Greenwich • The British realized you could determine longitude if you knew the RA/DEC of the Sun and the time in Greenwhich • Thus you MUST have an accurate clock that keeps UT.

10. I rock your world: precession • The position of the NCP is not fixed. • It slowly drifts, meaning that the sidereal year is 20 minutes longer than the tropical year (why? Because the vernal equinox moves). • This DOESN'T mess up the calendar...

11. Precession : Tops vs. Planets • The Earth's spin pole is very slowly (26,000 years) precessing like a top.

12. Precession : Tops vs. Planets • DON'T get confused; spin axis is very stable (gyroscope effect). But this is visible over the course of human history

13. The Seasons • The stability of the Earth's spin-pole direction (over times much shorter than 26,000 years) is the reason for the seasons. • As the Earth goes around the Sun, the spin axis of the Earth does NOT wobble, and always points towards the SAME distant stars • Otherwise NCP would not be constant over a year.

14. The Seasons (Fig 2.15) • Careful, orbit is a circle seen edge on here...

15. Motion of Earth around the Sun • animation

16. Tilt causes TWO effects. Here, days of winter in north are shorter than those days in the south. 1. Differing solar input due to varying duration of daytime.

17. Tilt causes TWO effects. Demo of intensity in class, and on web site 2. Different solar intensity at ground

18. Reason for the Seasons • animation

19. The Tropics and arctic circles • Note that the winter (northern) solstice Sun is directly over the Tropic of Capricorn (all longitudes rotate 'under the Sun' at local noon). • Directly over Tropic of Cancer in (N) summer • 'Circles' are seasonal regions of total darkness

20. So the tropics bound the region around the equator where seasonal climate variations are smaller

21. The Sun's movement: Final round • The daily arc of the Sun changes each day. Why?

22. The Sun's movement Arc of the Sun's path changes each day, within limits • Because it moves N and S in declination.

23. Think about the geometry on the celestial sphere here • How does this relate to the previous slide? • Make sure you understand.

24. Local solar time is not perfect, even at the `middle' of the time zone • Sun appears to speed up and slow down slightly in its motion along the ecliptic (to be discussed later) • So we invent MEAN solar time: a clock ticking at a constant rate

25. Measuring the universe with a stick • Step 1. Eratosthenes measures the diameter of the Earth.

26. Finding the Earth's diameter • When no shadow in Syene, 7° shadow in from a gnomon in Alexandria

27. The Eratosthenes experiment • Relies that Sun is far away. 7.2/360 must be distance from Syene-Alexandria divided by circumuference (D) of the Earth

28. Measuring the universe with a stick • Step 1. Eratosthenes measures the diameter of the Earth. • Step 2. Aristarchus estimates the distance to (and hence size of the Moon). Assignment 1. • Step 3. How about the distance to the Sun?

29. INTERESTING Factoid.... • The Sun and the Moon have nearly the same apparent size! • How do we know what their relative sizes and distances are??? • Could the Moon be halfway to the Sun, and thus half as big? • See Assignment 1

30. The ghost of Lecture 2: measuring the Vancouver-Seattle distance • Knowing the distance to the Earth, we can now calculate the distance from Vancouver to Seattle. • For a radius of the Earth of 6378 km, one gets: • distance = 183.9 km • CONFUSED? Get help here!