Announcements

1
More cycles in the sky

• (no, not that kind)

• 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.

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

• A gnomon casts a shadow straight north in
  Vancouver at apparent Vancouver noon
• But in Victoria and Kelowna they are not straight
  north.

SUN

• 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

• 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.

• Sect S1.3

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.

• 1. Differing solar input due to varying duration
  of daytime.

• Here, days of winter in north are shorter than
  those days in the south.

17
Tilt causes TWO effects.

• 2. Different solar intensity at ground

• Demo of intensity in class, and on web site

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

• Because it moves N and S in declination.

Arc of the Sun's path changes each day, within
limits
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!