ORBIT OF MERCURY
ASTR 110G Section M09
September 30th, 2010
http://www.nso.edu/press/archive/merc_transit/mercury_transit_composite03.jpg
PLANETARY ORBITS
All planets orbit the Sun on an ellipse
 Semi-major axis: a, half of longer axis length
 Period: P, the time it takes a planet to go around
the Sun once
 Eccentricity: e, measure of how “stretched” the
orbit is from a circle

e = 0: orbit is a circle
 0 < e < 1: orbit is an ellipse with 1 being very
stretched and 0 being circular
 e = (ra – rp)/(ra + rp)

ra is the farthest a planet is from the Sun
 rp is the closest a planet is from the Sun

MERCURY’S ORBITAL PARAMETERS

WRITE THESE DOWN!

You need them for your summary
Period: 87.969 days
 Semi-major axis: 0.387 AU
 Eccentricity: 0.2056


http://www.astro.lsa.umich.edu/undergrad/Labs/planetMotions/elongation.gif
Greatest Eastern
Elongation
http://www.uwgb.edu/dutchs/AstronNotes/HowSolSysWorks.HTM
GREATEST ELONGATION
WHAT YOU HAVE TO DO

Work in pairs (or three people if you have an odd
number in your group) to create Mercury’s orbit
Do calculations, then find all Earth positions, then all
Mercury positions. LABEL EVERYTHING!
 Each person should find some of the Earth and
Mercury positions – 2 people: 6-7 positions each, 3
people: 4-5 positions each
 Check with me to be sure your first couple points are
correct

Once done with that, compare orbits to rest of
group and pick best orbit. Ask me if you are
unsure
 Work on the rest of the lab in groups using the
best orbit and attach it to group lab

EARTH POSITIONS
•
•
•
Large circle is
orbit of Earth
Arrows on orbit
represent
direction Earth
is moving
around Sun
Sun is the
center dot
EARTH POSITIONS
1.
2.
3.
42°
Draw dashed
line between
Earth (X) and
Sun (center
dot)
Line up bottom
edge of
protractor on
line with
center on Sun
Measure out
calculated
angle (last
column of
Table 7.1) from
previous Earth
position to get
new Earth
position
EARTH POSITIONS
1.
2.
Draw a dotted
line between
the Sun and
your new Earth
position
Label this
position so you
know what line
this is later
EARTH POSITIONS
1.
2.
3.
72°
4.
Again, line up
the bottom edge
of protractor on
Earth-Sun line
with Sun at
center and
Earth at 0°
Measure out
calculated angle
and mark it
Draw dashed
line between
Sun and new
position
Label position
EARTH POSITIONS
•
•
This is what all
13 positions
should look like
All should be
labeled with
dotted lines
between Earth
and Sun
MERCURY POSITIONS
27.2°
Line bottom
edge of
protractor
with EarthSun line
2. Earth at
center of
bottom of
protractor
3. Measure out
given angle
(column 3 in
Table 7.1) –
position #1 has
east angle
East angle: left of
the Sun
West angle: right
of the Sun
1.
MERCURY POSITIONS
1.
2.
3.
Draw solid line
between Earth
and mark from
measured angle
Be sure the line
goes past Earth
– necessary for
the next step
Mercury is
located
somewhere on
this line
MERCURY POSITIONS
1.
2.
3.
Place the
protractor so
that the solid
line goes
between the
center of the
bottom and the
90° tick
Keeping that
alignment, line
up the bottom
of the
protractor with
the Sun
Where the
protractor
bottom
intersects the
solid line, place
a dot

http://www.astro.lsa.umich.edu/undergrad/Labs/planetMotions/elongation.gif
Greatest Eastern
Elongation
http://www.uwgb.edu/dutchs/AstronNotes/HowSolSysWorks.HTM
GREATEST ELONGATION
MERCURY POSITIONS
1.
2.
3.
Place the
protractor so
that the solid
line goes
between the
center of the
bottom and the
90° tick
Keeping that
alignment, line
up the bottom
of the
protractor with
the Sun
Where the
protractor
bottom
intersects the
solid line, place
a dot
MERCURY POSITIONS
18.1°
1.
2.
3.
Position #2
has a west
angle so angle
must be
measured to
the right of the
Sun
Line up
protractor and
measure angle
to the left of
Sun
Draw solid line
between Earth
position 2 and
new angle
mark
MERCURY POSITIONS
1.
2.
3.
4.
Line up
protractor at
90° on
Mercury line
Line bottom of
protractor on
Sun
Mark where
bottom of
protractor
crosses
Mercury line
Label Mercury
locations based
on Earth
position #’s
MERCURY POSITIONS
•
•
This is what all
13 Mercury
positions
should look like
Each position
should be
labeled same
number as
corresponding
Earth position
(needed for
later question
in lab!)