ASTR 1010 Astronomy of the Solar System. Course Info. Course Info. Fall 2006 Mon/Wed/Fri 11:00-11:50AM 430 Aderhold Learning Center

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1 ASTR 1010 Astronomy of the Solar System Fall 2006 Mon/Wed/Fri 11:00-11:50AM 430 Aderhold Learning Center Instructor: Wes Ryle Course Info In-class Activities/Quizzes (10%) 4 Homework Assignments (15%) 3 Exams - Best 2 scores count toward final grade, lowest score is dropped (30%) Final Exam on December 13th (20%) Lab is part of your grade! (25%) Don't forget about lab! Use it to your advantage. It's worth a quarter of your grade! Course Info Ask lots of questions!!! Feel free to interrupt at any time Lecture notes will be posted online before class On occasional Fridays, we will have in-class activities/demonstrations (who wants to listen to a lecture everyday anyway?) 1

2 Why Astronomy? Most of you are fulfilling a science requirement and are hoping for an easy course Astronomy is basically math and physics in space I will try to challenge you while still having fun. Astronomy is very interesting! The Main Problem with Astronomy Astronomy deals with numbers that are either too large or too small to imagine We get a better understanding by comparing relative sizes Remember the basics: One thousand = 1,000 = 10 3 One million = 1,000 X 1,000 = 1,000,000 = 10 6 One billion = 1,000 X 1,000,000 = 1,000,000,000 = 10 9 Size and Scale in Astronomy 1.7 meters 2

3 Human X 14,000 = Atlanta Perimeter ~ 24,000 meters = 24 kilometers Atlanta X 530 = Diameter of Earth Earth's Diameter is 12,756 km Earth X 109 = The Sun The Sun's Diameter is 1,392,000 km or x 10 6 km 3

4 The Sun X 107 = One Astonomical Unit (1 A.U.) NEW UNIT!!! The average distance between the Earth and the Sun is called an Astronomical Unit, or A.U. for short. 1 A.U. = 149,600,000 km or 1 A.U. = x 10 8 km Orbit of Neptune = 30 A.U. Neptune is 30 A.U. from the Sun. 30 A.U. = 4.49 x 10 9 km The Nearest Star System, Alpha Centauri, is ~268,000 A.U. Away 4

5 New Unit!!! - The Light Year Once we move outside the realm of our Solar System, distances become so large we need a new unit to describe them Light travels at a constant speed through space, usually written as the letter c The speed of light is: c 300,000 km = 3 10 s km s A light year (ly) is the distance light will travel in one year: 12 1ly = km 6 trillion miles! The Alpha Centauri system is approximately 4.3 light years away 5 Our Stellar Neighborhood All known stars within ~13 ly The Milky Way Galaxy ~ 100,000 light years across 5

6 Our Galactic Neighborhood ~ 2,000,000 light years And beyond... Observable universe ~ 14,000,000,000 light years Time Keeping in Astronomy The universe is constantly changing around us on timescales from fractions of second to millions of years Most of the ways we keep track of time (days, months, years) are directly linked to astronomy 6

7 Earth's Rotation - The Solar Day The solar day is the length of time it takes for the Sun to return to the same position in the sky (noon to noon, for example) The motion of the Sun is caused by Earth's rotation about it's axis The solar day is 24 hours long, and is the traditional 'day' we all know and love Earth's Rotation & Revolution The Sidereal Day In addition to rotating about it's axis, the Earth is also revolving around the sun A simple 360 o rotation of the Earth is a sidereal day and is about 23 hours and 56 minutes long To return the Sun to the same position in the sky, an extra four minutes is needed Sidereal day: 23 hours 56 minutes Solar or Civil Day: 24 hours Earth's Revolution - The Year Note: Image horribly not to scale The average time the Earth takes to travel around the Sun (yes, it does change) is the mean tropical year Currently, the mean tropical year is days Since this isn't a whole number, the calendar needs to be adjusted occasionally through the use of leap years Keeping track of the calendar can get 1 tropical year = days quite complicated - On Friday, we'll do an exercise on calendar creation 7

8 Earth's Tilt - The Seasons The Earth's axis of rotation is tilted by 23.5 o from the plane of it's revolution around the Sun Earth's Wobble - Precession Currently, the rotation axis of the Earth points toward Polaris, the North Star The direction of this axis changes very slowly over time, just like a spinning top In about 15,000 years, the rotation axis will point toward the star Vega 1 precession cycle = 26,000 years A complete precession cycle takes about 26,000 years More Astronomical Timekeeping When taking astronomical measurements, precise timing is very important since observations are made by many different astronomers at different locations In order to make sure everyone's information can be used, astronomers use Universal Time (UT) instead of traditional time Universal Time is simply the time at the Earth's prime meridian (0 o longitude) in Greenwich, England Daylights saving time is an adjustment in order to better use the daylight hours during the summer Daylight savings time last from the 1st Sunday in April to the last Sunday in October During daylight savings, add 4 hours to our time to find UT Outside of daylight savings, add 5 hours to our time to find UT For example, this class starts at 13:00 UT 8

9 More Astronomical Timekeeping Another (somewhat odd) way of keeping astronomical time is called the Julian Date (JD) Instead of worrying about seconds, minutes, hours, weeks, months, years, this method simply just keeps a running count of days The start point was chosen to be Monday, January 1, 4713BC This happens to be a point when three ancient calendars agreed with each other Fractional days are used with this system For example, 0.5 days = 12 hours, 0.25 days = 6 hours Monday, August 21st, 12 noon EDT corresponds to: JD 9