astronomy 1001 lecture 1 5/30/07. the moon project goal: understand how the moon “works”...
TRANSCRIPT
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