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Unformatted text preview: Class Logistics: • Problem Set 1 AS 101 Lecture 3 AS
• Labs: Building a Star Catalog Due Tue • Pick up Timeouttothink card off and running! Last Class • Nature of Science & Astronomy • Time and Distance in Astronomy Today’s Class • Time and Distance in the Universe • The Celestial Sphere • Observing Coordinates • Seasons on Earth • Apparent Solar Motions What’s within 100 light years? What’s
Estimating the number of stars within 100 light years Know: • 20 stars within 25 light years • VBox3D=r3 Technique: • Ratios r is the length of the side of the box • N100ly=N25ly 64 =~ 1280 =~ 1000 Kasey Elliott: When we look at an object Kasey that is 5 billion light-years away, how far back in time are we looking? Observing the night Sky Observing The Celestial Sphere The The sky above looks like a dome…a hemisphere.. Earth lies in the center of this sphere. Star Positions do not change on the celestial sphere Distance to stars not represented Locations in the local sky Locations To pinpoint a spot in the local sky: Specify altitude and direction along the horizon Measuring Angles in the Sky Measuring
Angular Measurements Full circle = 360º 1º = 60 arcmin 1 arcmin = 60 arcsec Units: 360º = 2 Π radians radians Angular Size Sizes calculated (Small Angle Formula) D 2p =a =f d 360
α in degrees, φ in radians Derivation: (Trigonometry) φ /2 D/2 D /2 = tan(f / 2) d æ2p ö @f / 2 = a ç ÷/ 2 è 360 ø Requires: • • φ in radians φ < < Π (a few degrees) << (a few degrees) Apparent Motions of the Celestial Sphere Sphere Apparent Star Motions Apparent Circumpolar Stars Circumpolar Definition: circumpolar stars are those that never set Observing Questions Observing Anthony Kinnis: Can any stars be observed all night from the N. Pole? Thanh Vo : Can any objects be observed all night from the equator? Can stars in the zodiac be observed all night from Boston?
• No zodiac stars are near the equator – not circumpolar stars • No • All visible stars Circumpolar Stars Time out to Think Question Time 1. 2. 3. 4. 5. From Boston (420 ), how many degrees above the celestial equator are circumpolar stars?
00 420 480 560 900 Answer: 900 – 420 =480 Seasons Question Seasons
Why is it cold in January in Boston? 1) The Earth lies farther from the Sun in January 2) The Northern Hemisphere is farther from the Sun in January 3) The Sun is lower in the sky and provides fewer hours of daylight in January 4) The Sun gives off less heat in January. The Earth orbits around the Sun once every year! The Earth’s axis is tilted by 23.5º! Reason for the Seasons Reason Small variation for Earth — about 3% Why doesn’t distance matter to seasons? seasons?
notably Mars and Pluto. • distance does matter for some other planets: Seasons are more extreme in N. hemisphere, • • Despite Earth being closer to Sun in S. hemisphere summer and farther in S. hemisphere winter. The Earth orbits the Sun every 365.2424 days Sun-Earth Distance: • ~150,000,000km Nevline Nnaji: If a star lies to the right, is it visible • 1AU during the northern hemisphere’s summer? • 8 light Minutes The Earth’s axis is tilted by 23.5º! The sky varies as Earth orbits the Sun The ...
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This note was uploaded on 04/14/2011 for the course ASTRO 101 taught by Professor Oppenheim during the Spring '11 term at BU.
- Spring '11