Download Observing the Planets Inferior (inner) planets

Observing the Planets
Inferior (inner) planets - Mercury and Venus
Superior (outer) planets - Mars, Jupiter, Saturn etc.
Rising and Setting Times
the time when an object intersects the horizon
Transit Time
the time when an object crosses the meridian
the meridian is the circle on the sky joining north to south
Opposition
a planet is in opposition when it lies on the opposite side
of the sky with respect to the sun (180 degrees away).
the full moon is in opposition
Conjunction
two objects are in conjunction when they have the same
value of right ascension
planets are in conjunction with the sun, moon and with
each other as well as with stars
the new moon is in conjunction with the sun
Maximum Elongation
the inferior (inner) planets are never at opposition
they reach a maximum angle from the sun called
maximum elongation
they undergo superior and inferior conjunctions
1
Conjunction
Superior Planet
Superior Conjunction
Inferior Planet
Maximum
Elongation
Inferior Conjunction
Earth
Opposition
AST101
Lecture 2
AST101
2
Measuring the Distance to the Planets
Triangulation
trigonometry - triangles of all sizes have the same
internal angles
you only need to know two angles and the length of a
side to solve for a triangle
to find the distance to an object observe it from two
positions at the ends of a baseline
tree
C
c
BC
AC
a
river
b
A
AB
B
baseline
1. measure length AB
2. measure angles a and b
3. determines AC and BC
AST101
Lecture 2
AST101
3
Astronomical distances are large
skinny triangles with two angles near 90 degrees and
one tiny angle
use the Parallax – apparent angular shift of a foreground
object with respect to a very distant background object
in practice we measure the parallactic angle
The Distance to the Planets
e.g. two observers on opposite sides of the earth
measure the position of Mars at the same time
baseline of the triangle is nearly the diameter of the earth
Mars’ parallax is about one arcminute at opposition
which is easily detectable with modern telescopes
The Diameter of the Earth
Method of Eratosthenes
measure length of shadows at different points on the
earth at the same time
length of shadow gives altitude angle of the sun
gives the angle on the earth’s surface between two
locations
knowing the distance gives the earth’s circumference
and diameter
Modern distance measurements come from radar ranging
send a radar signal to a planet
listen for echo
distance = time delay speed of light
AST101
Lecture 2
AST101
4
Parallax
background stars
Mars
parallactic angle
d
s/d
s
Earth
AST101
s - half of the baseline
d - distance
Lecture 2
AST101
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