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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)
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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?
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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.
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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
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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?
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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?)
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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)
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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
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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.
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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...
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Precession : Tops vs. PlanetsPrecession : Tops vs. Planets
The Earth's spin pole is very slowly (26,000
years) precessing like a top.
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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
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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.
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The Seasons (Fig 2.15)The Seasons (Fig 2.15)
Careful, orbit is a circle seen edge on here...
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Motion of Earth around the Sun
animation
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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.
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Tilt causes TWO effects.Tilt causes TWO effects.2. Different solar intensity at groundDemo of intensity in class, and on web site
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Reason for the Seasons
animation
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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
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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
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The Sun's movement: Final roundThe Sun's movement: Final roundThe daily arc of the Sun changes each day. Why?
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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
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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.
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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
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Measuring the universe with a stickMeasuring the universe with a stick
Step 1. Eratosthenes measures the diameter of the
Earth.
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Finding the Earth's diameterFinding the Earth's diameterWhen no shadow in Syene, 7° shadow in from
a gnomon in Alexandria
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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
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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?
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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
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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!