# Lecture 2 Astron 1 Instructor: Dr. Babar Ali Fall 2014Astron 1Dr. Ali.

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Lecture 2 Astron 1 Instructor: Dr. Babar Ali Fall 2014Astron 1Dr. Ali

Announcements Add decision will be done after today. No MOODLE access yet No e-mail still Fall 2014Astron 12

Major Change Exam will no longer be 50% of the grade. Exam will be 25% of the grade. The remaining 25% to go to homework assignments. Fall 2014Astron 13

Observing The Sky The previous Lecture showed you our current understanding of the Universe. –Built on scientific research. –Perspective: We “flew” through the Universe. Now, we will switch the perspective and describe how we view and experience phenomenon while grounded on Earth. Fall 2014Astron 14

The Celestial Sphere Fall 2014Astron 1April, 2009 Celestial Sphere: Apparent sky sphere, centered on the observer –Useful to denote positions of objects Zenith: The point directly above your head opposite to direction of gravity) Horizon: Great circle on celestial sphere (90° from zenith)

The Celestial Sphere Celestial equator = projection of Earth’s equator onto the c.s. Zenith = Point on the celestial sphere directly overhead Nadir = Point on the c.s. directly underneath (not visible!) North celestial pole = projection of Earth’s north pole onto the c.s. Fall 2014Astron 16

The Celestial Sphere (Example) The Celestial South Pole is not visible from the northern hemisphere. Horizon Looking North Celestial North Pole 40.7 0 Looking South 49.3 0 Celestial Equator Horizon New York City: l ≈ 40.7º Fall 2014Astron 17

4 different locations on Earth Fall 2014Astron 18

Motion of Earth The Earth Rotates. That is … it spins about an axis that goes through the North and South Poles. –The axis is an imaginary line about which the Earth spins. The spin axis helps define cardinal points (North, South, East, West) not just on Earth, but on the sky as well. Earth also Revolves around the Sun. –Orbits the Sun. Fall 2014Astron 19

Apparent Motion due to rotation Looking north, you will see stars apparently circling counterclockwise around the Celestial North Pole. Fall 2014Astron 110

Apparent Motion Captured in long (several hour) pictures of the sky Fall 2014Astron 1April, 2009 Standing near the Earth’s equator Standing near the Earth’s North pole

Apparent Motion due to rotation Looking east, you see stars rising and moving to the upper right (south) Looking south, you see stars moving to the right (west) Fall 2014Astron 112

Circumpolar Constellations Some constellations never set. Fall 2014Astron 113

Review Topics What is the difference between rotation and revolution? How are the North & South Celestial poles defined? Terminology: –Zenith & Nadir –North & South celestial poles –Celestial Equator –Meridian Where on Earth is the Zenith in the same position on the sky as the North Celestial pole? What is the apparent motion of star? What causes this apparent motion? Fall 2014Astron 114

1-5: Astronomers use angles to denote the positions and apparent sizes of objects in the sky Fall 2014Astron 115

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Estimating Angles with Your Hand Fall 2014Astron 119

The Small Angle Formula Fall 2014Astron 120

A Telescope in Space Fall 2014Astron 121

Three Views of Orion Eighty-eight constellations cover the entire celestial sphere Fall 2014Astron 122

Constellations In ancient times, constellations only referred to the brightest stars that appeared to form groups. Fall 2014Astron 123

Constellations (2) They were believed to represent great heroes and mythological figures. Each culture has its own set of constellations, usually pertaining to local beliefs. Fall 2014Astron 124

Constellations (3) Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions. Fall 2014Astron 125

Constellations (4) The stars of a constellation only appear to be close to one another. Usually, this is only a projection effect: The stars of a constellation may be located at very different distances from us. Fall 2014Astron 126

Constellations (5) Stars are named by a Greek letter (  ) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Orion Betelgeuse Rigel Rigel =  Orionis Betelgeuse =  Orionis Fall 2014Astron 127

Constellations (6) Some examples of easily recognizable constellations and their brightest stars Fall 2014Astron 128

Apparent Motion of The Celestial Sphere Some constellations around the Celestial North Pole never set. These are called “circumpolar”. The circle on the celestial sphere containing the circumpolar constellations is called the “circumpolar circle”. Fall 2014Astron 129

Review Topics How are constellations used in modern astronomy? How many constellations are recognized by modern astronomers? Name three constellations. What is the concept of ‘projection’? And, how does it relate to constellations? What are circumpolar constellations? Fall 2014Astron 1April, 2009

Day and Night on Earth Fall 2014Astron 131

Why Diurnal Motion Happens 2-3: The appearance of the sky changes during the course of the night and from one night to the next Fall 2014Astron 132

Time Keeping: First use of astronomy Fall 2014Astron 133 USNO is the official time keeper for the US government

Time Keeping Based on Earth’s rotation. 1 day = 1 Earth rotation But, there is more than 1 way to define a full rotation. –Solar Day: Rotation w.r.t. Sun –Sidereal Day: Rotation w.r.t. “fixed” stars. Fall 2014Astron 134

Time on Earth Earth’s rotation is causing the day/night cycle Fall 2014Astron 135

Earth’s Motion The time it takes to complete on revolution is called a year. At any given time, only half the sky is visible at night. Fall 2014Astron 1April, 2009 You can see the constellations on this side only.

Conversely The sun appears in a constellation on the “other side” –Zodiac: Belt around sky, ~18° wide, centered on ecliptic, w/in which we find  & planets Root of Zodiac same as Zoo, means collection of animals Pattern of  ’s w/in zodiac belt reminded ancients of animals Fall 2014Astron 137

Precession (1) The Sun’s gravity is doing the same to Earth. The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession. At left, gravity is pulling on a slanted top. => Wobbling around the vertical. Fall 2014Astron 138

Precession (2) As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. It will be closest to Polaris ~ A.D. 2100. There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) ~ 12,000 years from now, the celestial north pole will be close to Vega in the constellation Lyra. Fall 2014Astron 139

Fall 2014Astron 140 Noon T=0 h T=23 h 56 m 04 s (Sidereal Day) T=24 h (Solar day) Noon Not to Scale Solar & Sidereal Days

Solar Time Apparent Solar Time: by position of Sun on sky –Starting point = midnight. –Morning, A.M. = Ante Meridiem –Afternoon, P.M. = Post Meridiem Mean Solar Time: Average Day over course of 1 yr. –Noon = When Sun is directly overhead at the meridian. –Since 1884, Standard Time used around Earth by international agreement –Divided into ~24 time zones + nonstandard ones. Daylight Saving Time: local Std. T + 1 hr adopted for Spring & Summer to prolong sunlight into evening hours Fall 2014Astron 141

Fall 2014Astron 142 http://timeanddate.com

Time Keeping 1 year = 1 Earth revolution around the Sun. –Measured against “fixed” stars. 1 year is not an integer multiple of a day. –We do not use fractional days. –Therefore, calendars require constant modifications to keep the time of year synched with seasons, solstices and equinoxes. –E.g. Leap year, leap second Fall 2014Astron 143

Calendars CalendarRulesAverage Length (days) Early Roman365 days each year365 JulianEvery year divisible by 4 is a leap year 365.25 GregorianCentury years not divisible by 400 are ordinary years 365.2425 Modified GregorianThe years 4000, 8000, 12,000, … are ordinary years 365.24225 Tropical year365.242199 Fall 2014Astron 144

Review Topics Why are there two ways to measure the day? –Which definition is used to keep time on a clock or watch? –What are the two methods called? What is a year? Which calendar do we use today? Why does the calendar need modifications/corrections? Fall 2014Astron 145

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