Astronomy 1001 Lecture 1 5/30/07

The Moon Project Goal: understand how the moon “works” –Measure Lunar month, explain phases, connect actual observations with scientific model Requirements: 10 observations (and final project) –Time/date, phase, location How to make an observation –Finding due south very important

What is Astronomy About? The Universe is comprehensible Use physics to explain WHY things are as they are NOT just a collection of facts Use the Scientific Method

Basic Subjects in Astronomy Naked eye and deep sky objects “Near-by” objects –Sun, Earth, Moon, planets, asteroids, comets Galactic objects –Stars, clouds of gas, black holes, ISM Extra-Galactic objects –Other galaxies, clusters of galaxies, the Universe as a whole

Basic Astronomical Units SI units in general –Giga, mega, kilo, milli, nano Time –Year, second Mass –Gram, kilogram Units of Length –Parsec, Light-year, AU, meter, nanometer, Angstrom

Light and Seeing the Past Light travels at a finite speed, thus it takes time to travel distances Light from Sirius takes 8 years to reach us Light from the Orion Nebula takes 1500 years to reach us Light from Andromeda takes 3 million years to reach us

Where are We? Earth is a planet in solar system Our solar system is about halfway out in Milky Way Galaxy Milky Way is part of the Local Group Local Group is part of the Local Supercluster

Scale of the Universe Even nearby planets are distant compared to normal “human” scales Stars are very far away –Would take Voyager 1 100,000 years to reach Alpha Centauri Galaxy even larger –100 billion stars Universe contains 100 billion galaxies

Where Did We Come From? Universe began with the Big Bang Gas collapsed to from early stars Early stars “burned” for a few million years and blew up (supernova) Gas from supernovae is recycled into new generation of stars Left over material forms planets

Timekeeping and Navigation Four really important motions Earth orbits Sun The tilt of the Earth changes Earth rotates Moon orbits the Earth

Days There are 2 kinds of days –Sidereal (star) day –Solar day

Months Synodic Month Sidereal Month –Because sidereal months are shorter than synodic months, you can have “Blue Moons” You will measure the synodic month in the Moon Project

Years Sidereal year Tropical Year –20 minutes shorter than sidereal year –Thus, your year would be off by a day every 72 years

Keeping Time Apparent Solar time –What you would get with a Sundial –Noon is when the Sun is directly overhead Mean Solar time –Day isn’t exactly 24 hours long –Account for variations by taking an average

Keeping Time cont. Standard time –Solar times have major flaws –In the late 19 th Century, railroads started using standard times –In principle, standard time and solar time at the center of a time zone are identical –Universal time (UT, UTC, Zulu time) is used for global purposes

Time Zones

Leap Years Calendars are historically complicated –Egyptian calendar had 365 days, resulting in a shift of equinoxes by 1 day every 4 years –Julius Caesar introduced leap years in 50 BC –Equinoxes still shifting over periods of centuries –Pope Gregory XIII modified the leap years to account for this

Celestial Coordinates Several different ways –Altitude and direction (azimuth) –Right Ascension and Declination Declination is how high something is in the sky –Can be positive or negative RA is how far something is from Meridian –Measured in units of time

RA and Dec

The Movement of Stars Stars are at fixed locations The Earth rotates every ~24 hours This causes the stars to appear to move

Celestial Navigation Need to find 2 things: latitude and longitude Historically an important motivation for astronomy Similar methods were used for precise timekeeping until atomic clocks were invented

Latitude Simply need to determine the altitude of any star as it crosses the meridian Polaris is the simplest star to use Your latitude is simply the altitude of the star above your horizon

Longitude More difficult since you need transit times If you have fast communications you can use a sundial and communication device If not, you need transit times and a very good clock