Astronomy 101 Planetarium Lab
Instructor: Brian Pohl
ConOps: Craig Zdanowicz
www.physics.unc.edu/~bpohl/
YOU MAY SIT WHERE YOU WISH
Planetary Motions
• How long does Mars take to orbit the Sun?
• How about Venus?
• Why do the planets appear to move
backwards … sometimes??
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3
The Original Planets!
The Sun
The Moon
Mercury
Venus
Mars
Jupiter
Saturn
NO EARTH!!
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5
6
7
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Periodic Orbital Motion
Synodic period - the period as seen from earth
•Tied to the motion of the Earth
Sidereal period - the true period as seen from
“outside”, relative to the background stars
•Independent of the motion of the Earth
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Lab Instructions
• Find synodic periods - synodic period is complete when planet appears at same
place in sky with respect to the sun
• We will measure the angle between the sun, us, and the planet.
– Mark position of sun and planet on ecliptic
– Use the ecliptic as a ruler so that the date is a measurement. The current
date is the number that the sun is on.
– Measure the difference between the two positions (the units will be days,
later convert to angles)
– This gives you the angle.
• When the angle repeats, one period has been completed.
– Write down how many days elapsed between period
– Hint: Use difference in DOY for sun (col. 2).
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Oct-15,2007 – Dec-15, 2008
Things to watch out for!
Write this Down!!
• Add 365 days to DOY after each year has
passed
• The period for Mars will be is in between the
measurements, so you might not see the exact angle
repeat. Need to interpolate!
– Guess the DOY of Sun where period would repeat
Use fractions or graph it
• Keep track of negative angles:
-225 degrees = 135 degrees
135-360 = -225
+135 deg
-225 deg
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Calculations
• Find synodic period using your measurements
– Show any relevant calculations (e.g. interpolation)
• Convert this to sidereal period using equation
• Calculate theoretical sidereal period using Kepler’s law
• Find % error for: sidereal of Venus and Mars from
measurements compared to value you look up & sidereal of
Venus and Mars from measurements compared to those
you calculate from Kepler’s law (4 total).
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Lab Write Up
• Separate page for calculations
• Include sample calculation for: each calculation in the table,
sidereal, Kepler’s, & % error. Include equation for each.
• Explain how you found the synodic period
– Include your interpolation method (if any)
• Two sources of error: questions to consider:
– Is this a ‘significant’ source of error?
– What does it mean to make a ‘good’ measurement?
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All Power Points are on my website:
http://www.physics.unc.edu/~bpohl
Visit office hours or email me if you have questions!
bpohl@physics.unc.edu
Morehead room 404
Mondays (week-after-lab) 3:00 pm – 5:00 pm
Tuesdays (week-after-lab) 5:00 pm - 7:00 pm