Transcript Linking Asteroids and Meteorites through Reflectance

Astronomy 100
Tuesday, Thursday 2:30 - 3:45 pm
Tom Burbine
[email protected]
Homework #4
• Its on OWL.
• The first homework assignment has 10 questions. For
each question, you will get two attempts to answer it
correctly.
• After your last attempt, the correct answer will be shown.
• If you get all 10 questions correct, you will get one
homework point.
• If you get less than 10 questions correct, I will divide the
number of questions you get correct by 10 to determine
the fraction of a homework point that you will receive.
• This is due by Thursday evening (2/10) at 11:59 pm
2 Rooms for Test
• Hasbrouk 20 (For people whose last names begin
with letters between A and S)
• Morrill Science Center (Bldg IV) room N201
(For people whose last names begin with letters
between T and Z)
Test on Tuesday
• What you should do:
• Go over the lecture notes
• Read the book
Test
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40 questions
Choices A, B, C, D, E
1 hour and 15 minutes to complete it
No calculator
No books
No notes
Test Philosophy
• People who have gone to class should be
rewarded
• People who have not gone to class should be
punished
What the test covers
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Chapter 1
Chapter 2
Chapter 3
Supplemental Chapter (what I cover in class
today)
Formulas you need to know
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velocity = distance/time
Distance = velocity * time
Time = distance/velocity
If you are given two of the quantities, you can
calculate the third
• This equation will primarily be used for
calculating the distance light travels or the time it
takes for light to travel a particular distance
• Speed of light = 3 x 108 m/s
For example
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How far will light travel in an hour?
Distance = velocity * time
Distance = 3 x 108 m/s * 60 s/minute * 60 minute/hr
Distance = 10800 x 108 m = 1.08 x 1012 m
Kepler’s
rd
3
Law
• More distant planets orbit the Sun at slower
average speeds, obeying the precise mathematical
relationship
p2 = a3
where p is a planet’s orbital period in years and
a is the average distance from the Sun in
astronomical units.
Calculations
• The period for the Earth to go around the Sun is
1 year
• The distance of the Earth to the Sun is
1 Astronomical Unit
How long does it take Jupiter to go
around the Sun
• If Jupiter is 5.2 Astronomical Units from the Sun,
how long does it take Jupiter to go orbit the Sun
once
• p2 = a3 = 5.23 = 140.6
• p = √140.6 = 11.9 years
Another example
• Mercury is 0.4 Astronomical Units from the Sun.
• How long does it take Mercury to orbit the sun
once?
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A) 1 year
B) 3 months
C) 9 months
D) 5 years
The calculation
• p2 = a3 = 0.43 = 0.064
• p = √0.064 = 0.25 years
Things you need to know
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Ecliptic
Zodiac
Order of the planets
Umbra, penumbra
Famous people – what did they do
Kepler’s 3 laws
Lightyear
Everybody okay?
Keeping Time
• If you want to predict the positions of the planets
and stars, you need to keep accurate time
• What’s a day?
One thing that you need to know
• 360 degrees in a circle
Types of Day
• Solar Day – time it takes the Sun to make one
circuit around our local sky
• An average of 24 hours over the course of a year
Types of Day
• Sidereal day – the time it takes between successive
appearances of any particular star on the meridian
23 hours 56 minutes 4.09 seconds
•Sidereal
means
“related to the
stars”
Why is this number not 24 hours?
• Because the Earth is also moving in its orbit
• The Earth moves about 1 degree a day in its orbit
around the sun
• So the Earth must rotate 361 degrees around its
axis
• Each 4 minutes the Earth rotates around 1 degree
Months
• Synodic month – there is a 29.5 day cycle of phases of the
Moon
Sidereal Month
• Sidereal Month – the time it takes for the Moon to
orbit the Earth once (relative to the stars)
= 27.33 days
Year
• Sidereal Year – time it takes for the Earth to
complete one orbit around the stars
• Tropical year- time it takes for the Earth to go
from one spring equinox to the next spring
equinox
• The length of the Tropical Year is the basis of the
modern calendar
Time Difference
• Tropical year is 20 minutes shorter than a sidereal
year
• This would cause the year to be out of sync with
the seasons by 1 day every 72 years
• The length of the Tropical Year is the basis of the
modern calendar
PRS question
PRS Question
• Why is there a time difference between the
sidereal year and the tropical year?
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A) Conjuction
B) Precession
C) Ecliptic
D) Horizon
Precession
• Precession changes the orientation of the axis in
space but also changes the location in Earth’s
orbit when seasons occur
• Each year the location of the equinoxes and
solstices relative to the stars shifts about 1/26,000
of the way around its orbit
• (1/26000) * one year = 20 minute
PRS question #2
Why are there leap years
(years with 366 days)?
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A) Tropical Year is ~365.25 days long
B) Tropical Year is ~366 days long
C) Tropical Year is ~365.1 days long
D) Tropical Year is ~365.5 days long
Answer
• A) Tropical Year is ~365.25 days
Spring Equinox
Without a leap year
• The dates of the spring equinox would move
• Egyptians came up with the idea of a year having 365
days
• In 46 BC, Julius Caesar decreed the adoption of a leap
year (his spring equinox was March 24)
• However, a tropical year is about 11 minutes short of
exactly 365.25 days
• So by the 1500s, the spring equinox had changed by 13
days
Pope Gregory
• Wanted to return the spring equinox to the same
date
• So in 1582, the Pope decreed that the day after
Oct. 4th would be Oct. 15th
• This made the spring equinox March 21
• Made exceptions to the leap year rules
– Leap year is skipped when a century changes unless
the leap year is divisible by 400
• A year in the Gregorian calendar is almost exactly
the same as a tropical year
Celestial Coordinates
• Declination is like latitude
• Right Ascencion is like longitude
Questions?