Transcript The Night Sky

Astronomy 202
Stars, Galaxies, and the Universe
Dr. Jeff Kriessler
What is science?
Discussion
Science is a collection of facts
Discussion
Scientists are looking for the truth
My view
Science is a collection of explanations that are
useful in scientists attempts to predict the
future.
Newton’s law of gravity
M 1M 2
F G
2
d
It is inconceivable that inanimate brute matter
should, without mediation of something else which
is not matter, operate on and affect other matter
without mutual contact. ... That gravity should be
innate, inherent and essential to matter, so that
one body may act upon another at-a-distance,
through a vacuum, without the mediation of
anything else by and through which their action
may be conveyed from one to another, is to me so
great an absurdity that I believe no man, who has
in philosophical matters a competent faculty of
thinking, can ever fall into it. (Newton. Letter to
Richard Bentley 25 Feb. 1693)
Discussion
Scientists always need to keep an open mind
Models in science
A scientific model needs to:
1) Explain past observations
2) Predict accurately future observations
3) Should be the simplest possible explanation
Occam’s Razor
If two or more models encompass the
observations equally well, the simplest model is
preferred.
The Night Sky
Discussion
Even though there are 6000 stars that are
bright enough to see without a telescope,
we can only ever see about 3000 at any
given clear night. Why is that?
Discussion
Why can’t we see stars during the day?
Light Pollution
Stellar magnitudes
We specify the apparent brightness of stars by
the magnitude scale. Brighter stars have a
smaller magnitude while fainter stars have a
larger magnitude.
The brightest star, Sirius has a magnitude –1.4
The faintest stars visible to the unaided eye
have a magnitude of about 6.
Discussion
The magnitude system that astronomers use
is based on the way our eyes scale the
intensity of light, which is not linear but
logarithmic. What do you think would be the
advantage of having eyes which scale intensity
in such a fashion?
The celestial sphere
We can picture the sky as the inside of a
hemisphere.
Discussion
On Earth’s surface, also a sphere, we specify
positions using latitude and longitude. How
does this system work?
Positions in the sky
Any position in the sky can be specified by
two angles, just like latitude and longitude
on the Earth’s surface.
Some terminology
Zenith – the point in the sky directly over
head.
Meridian – an arc drawn from due north on
the horizon to the point due south on the
horizon which passes through the zenith.
Diurnal (daily) motion of the
stars
Like the Sun, the stars generally rise in the
east and set in the west. This daily motion of
the stars is caused by the rotation of the
Earth.
All the stars appear to circle a point in the
sky called the celestial pole.
In the north this point lies near the star
Polaris, the north star.
In the south this point lies near the
Southern Cross.
Circumpolar stars
Not all stars rise and set. Some stars, called
circumpolar, always appear above the horizon.
Star trails on the celestial
equator
The celestial poles and equator
north celestial pole – point in the sky directly
above the Earth’s north pole
south celestial pole – point in sky directly
above the Earth’s south pole.
celestial equator – midway between the
celestial poles (90 degrees away), lies directly
above the Earth’s equator.
Discussion
This there any place on Earth where all the
visible stars are circumpolar, never falling
below the horizon? Explain.
Discussion
Is there any place on Earth where none of
the visible stars are circumpolar? Explain.
Discussion
If we were at the North pole, how far above
the horizon would the North celestial pole
be?
Discussion
How high above the north horizon is the
north celestial pole from Earth’s equator?
Discussion
How high above the northern horizon
would the north celestial pole appear in
Cleveland which has a latitude of about
42 degrees?
Discussion
The biggest problem in navigating using the
stars was in determining one’s longitude.
Explain why this is so difficult?
Celestial Coordinates
Declination (Dec) – measured in degrees
minutes and seconds, from the celestial
equator.
Right Ascension (RA) – measured in hour
minutes and seconds from the vernal equinox.
Annual motion of the stars
The same stars are not visible all year long.
Any given non-circumpolar star will set 4
minutes early each day until it becomes lost in
the glare of the setting Sun.
This motion is caused by Earth’s yearly motion
around the Sun.
In other words
While the Sun takes 24 hours to traverse the
sky, the stars take only 23 hours and 56
minutes.
The Sun moves 4 minutes eastward each day
relative to the stars.
Thus different stars are visible at different
times of the year.
Solar and sidereal days
Solar day – 24 hours
Sidereal day – 23 hours 56 minutes is the
actual rotation period of the Earth
The ecliptic
The ecliptic is the annual path through the
sky that the Sun appears to take.
In other words, the ecliptic is the plane of
Earth’s orbit projected onto the stars.