Announcements

LABS: You've all done one now... Reading for Friday: Section 2.3Assignment 1 distributed today Friday Sept 15

Due Sept 22nd, 5PM, in your lab slot.

More cycles in the skyMore cycles in the sky

(no, not that kind)

So, that's the Calendar. I can tell you what day So, that's the Calendar. I can tell you what day

it is, but what about what time it is?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?

Local (or apparent) solar timeLocal (or apparent) solar time

varies with longitudevaries with longitudeA gnomon casts a shadow

straight north in Vancouver at

apparent Vancouver noon

But in Victoria and Kelowna

they are not straight north.

SUN

Vancouver

Victoria

Kelowna

EASTWEST

So local noon is

different for everyone;

good enough if you

can't travel fast.

Modern solution is TIME ZONESModern solution is TIME ZONES

If politics didn't intervene, there would be If politics didn't intervene, there would be

15 degrees of longitude/zone15 degrees of longitude/zone

Where in a time zone are you in sync Where in a time zone are you in sync

with apparent solar time?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?

Where in a time zone are you in Where in a time zone are you in

sync with apparent solar time?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?)

What about Universal time (UT?)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)

Navigation : Using the starsNavigation : Using the starsWe 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

Longitude and GreenwichLongitude 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.

I rock your world: precessionI 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...

Precession : Tops vs. PlanetsPrecession : Tops vs. Planets

The Earth's spin pole is very slowly (26,000

years) precessing like a top.

Precession : Tops vs. PlanetsPrecession : Tops vs. PlanetsDON'T get confused; spin axis is very stable

(gyroscope effect). But this is visible over

the course of human history

The SeasonsThe SeasonsThe 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.

The Seasons (Fig 2.15)The Seasons (Fig 2.15)

Careful, orbit is a circle seen edge on here...

Motion of Earth around the Sun

animation

Tilt causes TWO effects.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.

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

Reason for the Seasons

animation

The Tropics and arctic circlesThe Tropics and arctic circlesNote 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

So the tropics bound the region around So the tropics bound the region around

the equator where seasonal climate the equator where seasonal climate

variations are smallervariations are smaller

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

The Sun's movementThe Sun's movementBecause it moves N and S in declination.

Arc of the Arc of the

Sun's path Sun's path

changes changes

each day, each day,

within within

limitslimits

Think about the geometry on Think about the geometry on

the celestial sphere herethe celestial sphere here

How does this relate

to the previous

slide?

Make sure you

understand.

Local solar time is not perfect, even Local solar time is not perfect, even

at the `middle' of the time zoneat the `middle' of the time zoneSun 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

Measuring the universe with a stickMeasuring the universe with a stick

Step 1. Eratosthenes measures the diameter of the

Earth.

Finding the Earth's diameterFinding the Earth's diameterWhen no shadow in Syene, 7° shadow in from

a gnomon in Alexandria

The Eratosthenes experimentThe Eratosthenes experiment

Relies that Sun is far away.

7.2/360 must be

distance from

Syene-Alexandria

divided by

circumuference

(D) of the Earth

Measuring the universe with a stickMeasuring 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?

INTERESTING Factoid....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

The ghost of Lecture 2: measuring the The ghost of Lecture 2: measuring the

Vancouver-Seattle distanceVancouver-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!