# How is the distance of a superior planet measured from the sun

Deepak Kumar Shringi
4 years ago
Kepler was able to find the distance of the
superior
planets (
Mars, Jupiter,
and
Saturn
) from the Sun by using geometric drawings. That is, he made a scale drawing of
the orbits of Earth and
superior
planets in which he called the distance between Earth and
Sun 1 Astronomical Unit (A.U.).
From a study of the synodic periods the superior planets, Kepler was
able to determine their sidereal periods.
(See
Synodic and Sidereal
Periods of the Planets
.)
Once this was known for each planet, he would
be able to determine
the
average
daily motion
s
of these planets. (Recall
that Earth’s
average
daily motion is 360º/365.2
5
d = 0.9856º/d.)
Knowing how many degrees
Earth
along
in its orbit
since the tim
e of opposition
(angle E
1
SE
2
), how far the superior planet had moved since the time of opposition (angle
P
1
SP
2
)
,
determ
in
in
g the difference between them (θ
=
E
1
S
E
2
-
P
1
SP
2
) and measuring the
angular distance of the superior planet from the sun (ε),
he was able to construct a scale
triangle from which he was able to determine the distance of the superior planet from the
sun in terms of Astronomical Units.