Download CyclesOfTheSky

Astronomy Rough Notes – Cycles of the sky
BRING:
Earth globe
Meter stick
Sunlamp
Draco hat
Little dipper
Box for orrery
Orrery
Celestial sphere/ geocentric model
Zodiac signs
Precession wheel
DISCLAIMER: These notes do NOT cover everything you need to know. You may need to look up some
item or concept online or in a text. Test questions are not exact copies of the OBJECTIVES but if you
know the OBJECTIVES thoroughly, you should do well on the exams.
HANDOUTS
None
OBJECTIVES
Explain why the Earth undergoes seasons. Explain why it is hot in the summer. Explain why the Sun
appears high in the sky during the summer but low in winter?
Define equinox and solstice? Sketch the Sun, Earth, and Earth’s orbit. On the sketch, draw Earth and
label the location of Earth at the solstices and equinoxes. Show the tilt of Earth’s axis.
What is meant by the plane of the solar system? Where is the Sun in this plane? the Moon? the planets?
the stars?
What is meant by “the Sun is in Leo”?
What is the zodiac? Why is it significant? How many constellations are in the zodiac? Why is it called
the zodiac? Explain why you only see some of the constellations of the zodiac at a given time.
Explain why the Sun is not in the constellation horoscopes say it is in when you are born.
What distinguishes science from non-science? Give an example.
Name the five North Circumpolar Constellations. Why are these important to us in MN?
Explain why Polaris is the North Star? Explain why the stars appear to revolve around the North Star?
What is meant by the Earth’s precession? Is Earth’s precession a long or a short time?
SEASONS
Why is it hot in the summer and cool in the winter?
Why is the Sun high in the sky during summer?
Why are the days longer in summer?
Earth is tilted to plane of solar system. The tilt essentially remains that way over a year.
Image from: www.faculty.edfac.usyd.edu.au
When the northern hemisphere is tilted toward the sun, it receives more direct rays from the sun and is
warmer. The southern hemisphere receives more oblique rays and is cooler.
Show Earth and sunlamp (Note where overhead is on Earth with Sun height)
Sun low in sky for N hemisphere.
Oblique rays from Sun. Winter.
Sun high in sky for N hemisphere.
Direct rays from Sun. Summer.
Image from: www.windows2universe.org
When the northern hemisphere is tilted toward the sun, it also experiences longer days.
Show longer daylight with globe
So seasons are also due to longer daylight in summer
Image from: www.astro.washington.edu
Note how someone in US in diagram would experience more daylight as Earth rotates then would
someone at the same latitude in the southern hemisphere.
App for how much daylight you get for given date and latitude
http://astro.unl.edu/classaction/animations/coordsmotion/daylighthoursexplorer.html
Mention equinox, solstice
Note: Seasons are not due to distance of Earth and Sun – Earth is closest to Sun in January
Decent/short explanation here: http://www.windows2universe.org/earth/climate/cli_seasons.html
Do a write. Show examples.
Click here for a sample of a good write.
SOME OTHER CYCLES IN THE SKY (You can observe these in the real sky)
 Last time we talked about:
Sun, Moon, stars, planets rising in the east and setting in the west due to Earth’s rotation.
Moon exhibits phases
Moon moves ~120 east in a day
 Most stars move in an east to west direction nightly.
Polaris does not seem to move.
Some stars/constellations appear to move around Polaris
North Circumpolar constellations = Ursa Major, Ursa Minor, Cassiopeia, Draco, Cepheus
Demo with Starry Night
Why?
Demo with globe
Demo with orrery

Over a year, the Sun, Moon, and planets appear to move through the same 13 constellations, called
the zodiac.
When the stars of Leo for example are in the background of the Sun, we say the Sun is “in” Leo.
Good diagram here: http://spaceplace.nasa.gov/en/kids/st6starfinder/st6starfinder2.shtml
Demo with orrery
Find the constellation that the Sun was in when you were born…
http://spaceplace.nasa.gov/en/kids/st6starfinder/st6starfinder3.shtml
Note that the constellations have different sizes.
So the horoscopes are off by about one whole constellation compared to the real sky.
Astrologers now tell you that they use a different zodiac that is not aligned with the constellations. That
zodiac has 12 “houses” of equal size.
I personally give no credence to astrology, think it is wrong and another form of bigotry. No valid
scientific study shows any correlation between the positions of the stars and human affairs.
For an excellent article on why astronomers are convinced that astrology is wrong, see Phil Plait’s
http://www.badastronomy.com/bad/misc/astrology.html
Let us stick with science, “the science of astronomy, and the methodological naturalism it uses to derive
facts from detailed observation and reliable mathematical models, can’t say anything about who should
marry whom or what lottery numbers they should play.” Jason Goodbody
PRECESSION - Example of a LONGER CYCLE
The Earth spins on its axis.
But that axis moves around in the sky like the axis of a spinning top
Image and description at: http://www-istp.gsfc.nasa.gov/stargaze/Sprecess.htm
The Earth’s North pole currently points to Polaris so we call Polaris our North Star
3000 years ago – North star = Thuban
12 000 AD
- North star = Vega
26000 year cycle
This is why the sun is about one whole constellation off from horoscopes. In 2000 years, it will be two
constellations off.
HOMEWORK:
Write out why the Earth has seasons, why it is warmer in summer than winter.
Explain that to someone.
Make a flash card for each objective or write out your answers to them. Have someone else ask you the
objectives and see if you can answer.
Review previous notes/flash cards.
Read text or web.
Revised 1 January 2016