Observing Project Remember that for one of your observing projects you can go to a star party (stargazing). This is available at the Lawrence Hall of Science every 1st and 3rd Saturday of the month. For more information (be sure to call ahead) see: http://lawrencehallofscience.org/visit/exhibits/stargazing Also: The Chabot Space & Science Center has stargazing every Friday and Saturday 7:30-10:30. This can also count for your star party observing project. This Saturday, there will be a special lecture (James Allen Telescope Array - ATA) and star party, starting at 7:30 pm. Chabot Space & Science Center: http://www.chabotspace.org/visit/observatories.aspx East Bay Astronomical Society: http://www.eastbayastro.org/
Celestial Phenomena Today s Lecture: Celestial Phenomena (chapter 4, pages 68-89) Solar and lunar eclipses Positions of the constellations Day/night and seasons The Solar System (chapter 5, pages 90-94) Early history of the Solar System Copernicus and Galileo
Phases of the Moon The position of the Moon in the sky is closely related to the phases! Can you see a Full Moon at 12 o clock noon? What would someone see for the phases of the Earth from the Moon?
Total Solar Eclipse Sun Moon Earth The Sun s diameter is 390 times bigger than the Moon s, but the Sun is also 390 times farther away! This means that they subtend (or cover) the same angle in the sky (1/2 ). This is an amazing coincidence!
Annular Eclipse Sun Moon Earth The Moon s orbit is NOT circular. It s elliptical. This means sometimes the Moon closer or further from the Earth. Because of this, sometimes the Moon does not completely cover the Sun, making an annular eclipse.
Annular eclipses from 2001 to 20025
General Relativity Important historical use of total solar eclipses: provided the first test of Einstein s General Theory of Relativity (1916). Showed that mass warps space and time, giving rise to what we call gravity. Apparent position Earth Moon Sun True positions Apparent position
Arthur Eddington measured the apparent positions of stars during a solar eclipse in 1919. apparent true Eclipsed Sun Results roughly agreed with Einstein s predictions for his strange and new theory, making him an instant celebrity. The closer starlight passes to the Sun, the greater the apparent-true separation.
Total Lunar Eclipse Sun Earth Moon Sometimes the Earth passes between the Sun and the Moon, causing a lunar eclispe. When this occurs, the Moon looks very red. Why would this be?
Total Lunar Eclipses Look Reddish The Earth s atmosphere refracts the red light, but the blue and green light is preferentially absorbed and scattered. Green and blue light scattered Sunlight Red light is refracted Earth Moon
Total Lunar Eclipse (cont.) If lots of dust is in the atmosphere, then not much light gets through; get a dark eclipse instead. A total lunar eclipse is visible from the entire dark hemisphere of the Earth (unlike a total solar eclipse) Eclipses don t occur every month because of a 5 angle between the Moon/Earth and the Earth/Sun orbital planes. So instead, there are only a few per year (mostly partial).
Why isn t every new Moon an eclipse?
Constellations: 88 groups of stars (mostly of Greek origin) Stars are actually at different distances, and not clustered as they appear! Cassiopeia
Our view of the sky Stars appear fixed to a large very distant celestial sphere. From any point on Earth, you can see half of the celestial sphere at any given time. As the Earth rotates, stars move across the sky with circles centered on the North Celestial Pole and the South Celestial Pole.
Star circles
Changing In Dec, we see Orion overhead at midnight. view of the night sky In Mar, Orion is overhead at sunset (6 hours earlier) Thus each successive night, we see Orion overhead (or rising) about 4 minutes earlier. 6 hrs = 360 minutes 360 min / 90 days = 4 min / day
Seasons: because Earth tilts The seasons are a consequence of the tilt of the Earth s axis of rotation (23.5 from perpendicular to the Earth s orbital plane).
Seasons: because Earth tilts In the spring/summer, the tilt causes the Sun to be higher in the sky: there are more daylight hours and the sunlight is more direct (more heating). Differences in the distance of the Earth to the Sun (either orbital or due to tilt) are small. (Earth is actually closest to the Sun during the northern hemisphere s winter.)
There is less heating per unit area of the ground if the Sun is low (the sunlight is less direct) At places where the Earth is more tilted from the Sun, the sunlight is less direct.
The Solar System Early history of Solar System Studies (Greece) Planets wander slowly (over many weeks) among the fixed stars. (This is NOT to be confused with the daily east-towest movement due to the Earth s rotation.) Earth Usually the planets drift from west to east. It is natural (but incorrect) to think of the Earth as the center of the Universe Spheres for each planet, the Moon, the Sun, and the stars seem to rotate around it. S Stars embedded in the celestial sphere
Retrograde Motion Retrograde motion is a problem for the simplest geocentric models. Each year, planets drift from east to west for a while! E W Ptolemy (~140 AD) devised a new geocentric theory to explain this. E W
From Ptolemy to Copernicus Ptolemy s geocentric system was very complicated, but also very accurate. It lasted for nearly 1500 years! But most people still thought that the perfect reality was a bunch of nested spheres as Aristotle originally suggested. Copernicus wrote about heliocentric theory, first published in 1543 (after death) Galileo s used a telescope to see that Venus goes through an entire set of phases (like the Moon) confirming the heliocentric hypothesis (1610)
The Phases of Venus Venus according to Copernicus: all phases Venus according to Ptolemy: only crescent and new Since Venus always appear close to the Sun in the sky, it could not go through a complete set of phases in Ptolemy s system, but it should in Copernicus system.
Galileo made many other discoveries. Galileo Galilei 4 moons of Jupiter (The Earth is not the only center of orbiting bodies! Also, these moons are not left behind as Jupiter moves.) The Roman Catholic Church was extremely uptight about this: they put Galileo under perpetual house arrest. Galileo also published his earlier studies of the motion of falling bodies (that he had found that the time it takes an object to fall from a given height is independent of its mass).