Please turn off cell phones - thanks! panda.unm.edu/courses/sanfratello/astro101 Remember - Read syllabus and schedule (contains textbook reading assignments and suggested problems from the text.)
The Constellation Orion
The "Solar Day" and the "Sidereal Day" Solar Day How long it takes for the Sun to return to the same position in the sky (24 hours). Sidereal Day How long it takes for the Earth to rotate 360 o on its axis. These are not the same! Which is longer? Why? (Demo)
One solar day later, the Earth has rotated slightly more than 360 o. A solar day is longer than a sidereal day by 3.9 minutes (24 hours vs. 23 hours 56 minutes seconds). Difference due to rotation and revolution of Earth!
Q: What causes the seasons?
Seasons
Play Movie
Summer Winter In winter, sunlight is spread out more thinly across the ground => each bit of ground receives less radiation => cooler Seasons caused by tilt of Earth’s axis of rotation! Earth is farthest from sun during summer!
Precession What causes precession? How long does one cycle take?
Precession The Earth has a bulge. The Moon "pulls down" on the side of the bulge closest to it, causing the Earth to wobble on its axis. Earth Moon Spin axis * * Vega Polaris Precession Period 26,000 years!
Triangulation - Using Geometry to Measure Distances Know: Angle at A Angle at B Length of Baseline Calculate: Distance to object distance =baseline*tan(B)
Parallax Triangulation - Measure angles at points A and B Parallax - Know Baseline. Measure third angle in triangle made by A, B, and object in space Baseline problem
Parallax Geometry Ex. “Stationary” moon in moving automobile Parallax angle is inversely proportional to distance of object Close object causes large parallax Distant object causes small parallax
How are the Earth, Moon, Sun, and stars laid out in space? What lies at the “center”? Motion(s) of the Earth? Motion(s) of the Moon? What about the stars?
The Motion of the Moon The Moon has a cycle of "phases", which lasts about 29 days. Which way is the Sun here? Moon moves in synchronous motion – it rotates on its axis in exactly the same amount of time it takes to orbit earth.
Why doesn't the earth prevent the sunlight from reaching the moon during its “Full phase”?
Eclipses What sort of alignment of the Earth, Moon, and Sun is required for a lunar eclipse? How about for a solar eclipse?
Eclipses Lunar Eclipse (around 100 minutes) When the Earth passes directly between the Sun and the Moon. Sun Earth Moon Solar Eclipse (no more than about 8 minutes) When the Moon passes directly between the Sun and the Earth. Sun Earth Moon
Lunar Eclipse
Total Solar Eclipse
During which phase(s) can a lunar eclipse occur? What about a solar eclipse? Why don't eclipses occur every month?
Moon's orbit tilted compared to Earth-Sun orbital plane: Sun Earth Moon Side view 5.2 o