Observing the Heavens Flashcards
Why is astronomy considered a science?
It tests theoretical predictions with observations
Examples of historical astronomical observations
•Mayan long count Callander
•Egyptian 365-day calendar
•Khoisan used shape, orientation of moon, Milky Way to mark when to hunt, harvest
•Chinese predicted eclipses, meticulously detailed comets, supernovae
The ecliptic
•path that planets and sun follow
•made up from 12 zodiacal constellations
•takes 12 months for sun to move through
Greeks philosophers understanding 500BCE
•spheres considered perfect so heavens made of spheres
•Heliocentric model vs Aristotle’s geocentric model
Which model was favored by Ptolemy?
Geocentric
Accepted by western religions
What could geocentric model explain?
•nightly rotation of the heavens around earth
•phases of the moon
•solar and lunar eclipses
•simple planetary motion
•lack of motion of stars relative to each other
Problems with Aristotle model
Failed to explain why mercury and Venus stay close to the sun, retrograde motion, why planets moved at different rates at different times
How many epicycles did Ptolemy require to match observations?
> 40
What did Copernicus argue?
•sun is at centre of universe
•earth-sun distance ting compared to distance to stars
•rotation of earth makes stars go around, and annual motion of sun caused by earth going around it
•retrograde motion caused by motion of our vantage point on earth
•mercury and Venus closer to sun than earth
What is the Copernican Principle?
There is nothing special about the Earth, the sun, our galaxy
What did Galileo discover with the telescope?
•Jupiter had its own moons
•moon had craters and imperfection (not perfect sphere)
•saw more stars and deduced Milky Way was many stars blended together
What did Kepler propose about planetary motion?
They move in ellipses not circles
What were keplers laws?
- Each planets orbit is an ellipse with the sun at one focus
- Planets sweep out equal areas in equal times
- For each planet, its orbital period squared is proportional to its semimajor axis cubed
When does parallax arise?
When you change viewpoint
Why can’t we see positions of stars change relative to each other
Stars are so far away that changes aren’t measurable
Why do planets farther out have longer orbits?
•higher up things have more potential energy, lower down have more kinetic energy
•more kinetic energy so more speed so moves around faster so smaller period
When are suns, moon, planets at highest point in sky? (North hemisphere)
When due south
When does sun rise furthest north?
Summer solstice
When does sun rise furthest south?
Winter solstice
When does sun rise due east?
Spring and autumn equinoxes
What is declination?
•like latitude
•+90 at north celestial pole, -90 at south
What is right ascension?
•like longitude
•ranges 0-360, or 0-24h around equator
What is coordinates at spring equinox?
0,0
New moon
Moon in same direction as sun
•rises at dawn, sets at sunset
Waxing moon
•inverted C in northern hemisphere
•visible in evening, sun illuminates west side of moon
Full moon
Moon in opposite direction to sun
Rises at sunset, sets at dawn
Waning moon
C shape, visible in morning
Sidereal period
•moon goes round earth every 27.3 days
•moon rises 50 mins later each day
•sets things like tides
Synodic period
•moon faces same direction to sun every 29.5 days
•sets moons phases
Lunar eclipses
•occur when moon passes through shadow cast by earth
•only occur at full moon
Solar eclipse
•occur when moon blocks light from sun
•only occur at new moon
What is the lunar oribit tilt?
Orbit of moon round earth inclined at 5 degrees relative to ecliptic
