Transcript Astronomy 1001

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
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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