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Astronomy 101: 9/18/2008
Announcements
• Pick up a golf ball at the front of the
class or get one from Alex; you will
need it for an in-class activity today.
You will also need the question sheet from
Alex.
• My office hours will be on Friday from 1-3
PM this week
• The first real OWL homework is
now available. Called
“Homework 1”
• Due 9/24 before midnight
An astronomer specifies an object’s location
using two angles. This method of specifying
an address is familiar:
Coordinates on the Earth
• Latitude: angle to position north or south of equator
• Longitude: angle to position east or west of prime
meridian (runs through Greenwich, England)
Orchard Hill
Observatory
Open every Thursday
evening if skies clear!
Located at the base of the cell
phone tower, go from Orchard
Hill Dorms following the gravel
road to the left.
Call 577-4166 after 6pm on
Thursdays to check whether the
telescope will be open.
Show your ID and tell the TA that
you are in this section of Astro
101.
Bring a flashlight and go with
friends—the path is dark!
Dress warmly!
Observing the sky:
The Earth Gets in the Way...
We can’t see below the horizon (we can’t see
through the Earth!). So, we need to have telescopes
in different locations, and we have to think about
the timing of the Earth’s rotation when planning
observations.
The Earth is constantly
rotating, so a telescope
has to constantly move
to follow a star or galaxy
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The Celestial Sphere -- a model for
For mapping, we treat
mapping the sky
the stars as if they are
all at the same distance
on the surface of an
imaginary sphere
This is a quite
useful model, but
it is an awful theory
PRS Question:
Most stars rise and set, but
some never rise or set (the
circumpolar stars).
Is this statement universally
true for any observer on
Earth?
1. Yes. At any place, some stars will be
circumpolar, and some will rise/set.
2. No.
Consider an observer at
the North or South Pole:
At midnight, the
star is here.
The observer
can still easily
see the star.
At noon, a star is
here.
The way stars move also differs
from place to place
Rotate into
local orientation:
The observer
can easily see
it at noon.
At the North or South Pole, all stars are circumpolar.
Nothing rises or sets!!
Stars don’t necessarily rise straight up from the
horizon. A telescope has to know the angle at which
objects rise so that it can follow the star as time passes.
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The way stars move also differs
from place to place
Rotate into
local orientation:
Rotate into
local orientation:
• Non-circumpolar stars, (& Sun, Moon, planets) rise in
East and set in West at an angle = [90° − your latitude].
Hey! Hey,
wait a sec…
This angle is the
observer’s latitude
on the Earth.
This angle is the
altitude of the star
Polaris (which is
directly above the
north celestial pole).
Using geometry, we can
show that these angles
are the same!
Navigation by the stars
The altitude of
Polaris above the
horizon is
approximately the
same as the
observer's latitude
in the Northern
Hemisphere.
Hey! By measuring the altitude of Polaris, one can determine
one’s latitude on the surface of the Earth.
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The Sun gets in the way too?
Stars don’t disappear in
the daytime. Where
would they go?
Stars can’t be seen with the
naked eye during the day
because they are lost in the
glare of light from the Sun.
Annual Motion
Seasons are due to the tilt of the earth’s axis.
• As the Earth orbits the Sun, the Sun appears to move eastward with
respect to the stars.
• The Sun circles the celestial sphere once every year.
• The path that the Sun follows through the celestial sphere is the
ecliptic.
• The constellations along the ecliptic are the zodiac.
The Sun is just another star, and we can
think about its daily motion through the sky
the same way.
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Seasons are due to the tilt
Imagine
of thethis
earth’s
axis of
axis.
rotation instead.
Day length
= 24 hours
PRS QUESTION: Suppose that the tilt of the Earth’s axis of
rotation were 90 degrees instead of 23.5 degrees, and the North
Pole faces the Sun in June. HOW MANY HOURS OF SUNLIGHT
WOULD WE HAVE IN AMHERST IN DECEMBER?
The land of midnight Sun
Position of the Sun over the course of a day north of the Arctic
Circle in the summertime
At this location and time, the Sun is circumpolar.
Day length
= 0 hours
Day length
= 12 hours
In this imagined Solar System, the amount of daylight in
Amherst at different times of year would be as indicated…
Consider the angle between the zenith
and the Sun at noon:
In the summer, the
angle between the
Sun and the zenith is
small
In the winter, the
angle between the
Sun and the zenith is
much larger.
So what?
Midnight
6:00 AM
Noon
6:00 PM
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Consider the angle between the zenith
and the Sun at noon:
Going from winter into summer,
the Sun rises to higher altitudes
and stays in the sky longer
Reasons the Earth’s tilt
causes the seasons
Seasonal Change in Sun’s
Altitude
• The days are longer in the summer; the Sun spends
more time heating things up.
• Sunlight is more direct and more concentrated in the
summer.
Beginning of the seasons:
• The “Figure 8” shows Sun at same time each
day over a year.
• Summer solstice: the North Pole is tilted toward the
Sun; longest day of the year in the Northern
Hemisphere (shortest in the south)
• Winter solstice: the North Pole is tilted away from the
Sun
• Spring and Fall equinox: sunlight shines equally on
both the Northern and Southern Hemispheres.
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To pinpoint an object in the local
sky, we simply specify two angles:
altitude
Problem: the local sky
depends on time and place.
the angle above the horizon
azimuth
An angle between a
defined point (say, due
West) and the place on the
horizon from which we
measure the altitude.
More often than not, astronomers measure azimuth with
respect to due North, but it doesn’t matter  the reference
point is arbitrary as long as it is specified.
Celestial motions define
the cycles of our lives
• Daily/diurnal motions due to spin of
the Earth: day
• Annual motions due to the orbit of the
Earth around the Sun: year
• What defines the month ?
Astronomers are all over the world and measure coordinates
at all sorts of times. In order to talk to each other, astronomers
measure coordinates with respect to defined reference points
on the celestial sphere, not the local sky.
• Declination: angle
from the celestial
equator up to the star
(analogous to altitude)
• Spring (or “vernal”)
equinox: point where
the ecliptic and celestial
equator intercept
• Right Ascension:
east-west angle between
the spring equinox and
the position of the star
on the celestial equator.
Answer: The Moon
Phases of the Moon’s 29.5 day cycle
• Why does the Moon’s appearance
change this way over the course of the
month?
Moon → Moonth → Month
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