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Astronomy Picture of the Day

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What properties of physical objects would you expect to be important in Astronomy? Mass Size Distance from Earth Temperature Speed and Velocity Brightness (Luminosity)‏ Age Chemical composition

Question Which is longer a Solar Day or a Sidreal Day? Why? A. Solar, Earth both rotates on it's axis and orbits the sun B. Sidreal,Earth both rotates on it's axis and orbits the sun C. Solar, the Earth slows down as it rotates D. Sidreal, the Earth speeds up as it rotates

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!

Question The “wobble” of the Earth on its axis is called _______. It is caused by _______. A. precession, the Earths rotation on it's axis B. rotation, conservation of angular momentum C. precession, the gravitational pull of the moon D. Earth's orbit around the sun, the seasons

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!

Question Why is looking out into space like looking back in time? A. ancient civilizations named the constellations B. the speed of light is infinite C. the speed of light is finite

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. Reflected Light

(DEMO)‏

Why doesn't the earth prevent the sunlight from reaching the moon during its “Full phase”?

Moon's orbit tilted compared to Earth-Sun orbital plane: Sun Earth Moon Side view 5.2 o

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

Varieties of Solar Eclipses

During which phase(s) can a lunar eclipse occur? What about a solar eclipse? Why don't eclipses occur every month?

Regions of Earth that saw or will see total (red) or annular (blue) solar eclipses between the years 2000 and Each track represents the path of the Moon’s umbra across Earth’s surface during an eclipse. High-latitude tracks are broader because sunlight strikes Earth’s surface at an oblique angle near the poles and because of the projection of the map.

Triangulation - Using Geometry to Measure Distances Measure:  Angle at A  Angle at B  Length of Baseline Calculate:  Distance to object distance =baseline*tan(B)‏ (SOH CAH TOA )‏

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 The apparent displacement (shift) of a foreground object relative to the background as the observer’s location changes is known as parallax.parallax

Parallax Geometry Parallax angle is inversely proportional to distance of object Close object causes large parallax Distant object causes small parallax The closer an object is to the observer, the larger the parallax.