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 19th 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