Astronomy 101: 9/25/2008 Announcements Exam study materials will be posted on the course web page tomorrow. Homework 2 is now available on the OWL Due 10/01/08 before midnight 1st midterm: 10/2/08 Midterms will be pyramid exams: 75% of grade from inclass exam, 25% from takehome exam (same exam) If you don t do the take-home part, then 100% of the grade will PRS is on channel 9 be based on the in-class exam score The Greek Model of the Solar System Geocentric model (because the Earth is at the center) Astronomical perspective Surprising aspects of our current understanding of the universe Nuts and bolts of observing the sky A bit of history: accomplishments (and misteps) of ancient astronomers The peculiar motion of Mars... MARS URANUS Serious flaw: had difficulty explaining retrograde motion! By and large, the planets move EAST with respect to the background stars. But once in a while, they move westward briefly... Westward movement = Retrograde motion. 1
Why does retrograde motion occur? The different planets have different orbital periods. For example, while the Earth takes one year to orbit the Sun, Mars requires 1.88 years. Jupiter requires 11.9 years. Pluto takes 248 years to complete one orbit around the Sun! More distant planets orbit the Sun at slower average speeds. Most sophisticated geocentric model developed by Claudius Ptolemy Epicycle idea: each planet follows a smaller circular orbit that, in turn, follows the bigger circular orbit. Like a wheel attached to a wheel. Predicted future positions of planets relatively accurately Model remained favored and in use for 1500 years! Aristarchus deduced the relative size of the Moon and the Earth by observing the Earth s shadow on the Moon: Aristarchus used easily observed lunar eclipses (when the Moon moves through the Earth s shadow) to estimate the relative sizes of the Earth and Moon. Aristarchus deduced the relative size of the Moon and the Earth by observing the Earth s shadow on the Moon: 2
Aristarchus deduced the relative size of the Moon and the Earth by observing the Earth s shadow on the Moon: Phases of the Moon PRS question. This month, the new moon was on September 29th. When will the next full moon occur? 1. September 29th 2. October 1st 3. October 7th 4. October 15th 5. Halloween Review: Phases of the Moon Exercise Review: Phases of the Moon Exercise Viewed from above the North Pole, the Earth rotates in the counterclockwise direction. Noon: front Sunset: left Midnight: back Sunrise: right 3
Why do we see phases? Half the Moon illuminated by Sun and half dark We see some combination of the bright and dark faces Why do we see phases? Half the Moon illuminated by Sun and half dark We see some combination of the bright and dark faces Why do we see phases? Half the Moon illuminated by Sun and half dark We see some combination of the bright and dark faces Moon phases in-class activity Any answer: 91% of points Graded question: 5. What time does the 1st quarter Moon rise and set? When is it on the meridian? Rise: noon Set: ~12 hours later, i.e., midnight Meridian crossing: halfway between rise and set, i.e., around 6 PM or sunset 4
What is the dark side of the Moon? What is the dark side of the Moon? The dark side of the Moon is really the far side of the Moon. We always see the same face of the Moon! Why do we see the same face? If you complete exactly one rotation while you complete one orbit, you will always face the ball. Important term: period = time required to complete one cycle PRS Question. Have you ever seen an eclipse? What type of eclipse did you see? 1. Partial lunar eclipse 2. Partial solar eclipse 3. Total lunar eclipse 4. Total solar eclipse 5. None. I ve never seen one. Enter all answers that apply, e.g., if 1 and 3, then enter 13 Rotation period = orbital period 5
Solar Eclipses Lunar Eclipses A solar eclipse occurs when the Moon casts a shadow on the Earth This can only occur at new Moon. New moon occurs once every month. Why don t solar eclipses occur once per month too? A lunar eclipse occurs when the Earth casts a shadow on the Moon This can only occur at full Moon. Full moon occurs once every month. Why don t lunar eclipses occur once per month too? Lunar eclipses have a surprising aspect At first, the Earth s shadow is seen on the Moon The next total lunar eclipse observable in Amherst will occur on Dec. 21, 2010 However, when the Moon is fully engulfed by the Earth s shadow, it doesn t disappear! Instead, it turns red. When the sunlight passes through the Earth s atmosphere, the blue light is scattered in many directions. Very little blue light illuminates the Moon. The red light, however, is bent by refraction, and much of the red light does illuminate the Moon. Above: the Moon at various stages during a lunar eclipse 6
When the sunlight passes through the Earth s atmosphere, the blue light is scattered in many directions. Very little blue light illuminates the Moon. Partial solar eclipse The red light, however, is bent by refraction, and much of the red light does illuminate the Moon. Solar eclipses come in three flavors: Annular solar eclipse Total solar eclipse An eclipse casts two types of shadow The anatomy of an eclipse Umbra: region where light is entirely blocked Penumbra: region where light is only partially blocked ALL POSSIBLE RAYS ARE BLOCKED IN THE UMBRA This light ray not blocked in penumbra Penumbra: the region of the shadow where light is only partially blocked by the eclipsing object Umbra: the region of the shadow where all light rays are blocked by the eclipse This light ray blocked (by Earth) in penumbra 7
For an object with a fixed size (a fixed linear diameter), its angular diameter decreases when the object s distance is increased. Full circle = 360 Full circle = 2π radians (π = 3.141592654) 360 = 2π radians Angle (in radians) = linear diameter distance The Moon s orbit around the Earth is not a perfect circle -- at some times, the Moon is a small amount closer to the Earth, and sometimes it is a bit farther. Therefore, the angular size of the Moon changes depending on where it is in the orbit. PRS Question If the Moon is relatively far from the Earth so that its umbra does not reach the Earth s surface, someone directly below the umbra will observe: 1. a penumbral lunar eclipse 2. a partial lunar eclipse 3. a partial solar eclipse 4. an annular eclipse 5. no eclipse 8