Orbital Properties of the Solar System
The positions of the Inner planets on June 14, 2002
- Mars is "behind" the sun from the earth's point of view
and is therefore not visible in the nighttime sky (i.e. its up
in the daytime).
- Venus it at greatest elongation and appears in the western
sky as a bright object for 2-3 hours after sunset
Properites of the Solar System
Summary From Before:
- All orbits can be understood by Newtons Gravitational
- Orbital characteristics depend only on the distance from the
sun, not the mass of the object
- Relation between orbital Period and Distance from The Sun
- Mercury: A = 0.39 AU P = 0.24 yr
- Venus: A = 0.72 P = 0.61
- Mars: A = 1.52 P = 1.88
- Jupiter: A = 5.2 P = 11.87
- Saturn: A = 9.55 P = 29.48
- Verification: Multiply A by itself 3 times (e.g. Jupiter:
5.2 x 5.2 x 5.2 = 140.61; take the square root of this quantity
and it should be very close to P; square root of 140.61 = 11.86)
Overview of Solar System Orbits:
The problem of the orbit of Mercury Einstein
to the rescue:
- All planetary orbits lie very nearly in the same plane. The
two biggest exceptions (apart from Mercury which orbits in
curved spacetime) are Pluto with an
inclination of 17 degrees and Venus with an inclination of 3.4
- All planets revolve around the sun in the same direction
(appear to move eastward with respect to the stars)
- In general, planets rotate in the same direction they
are revolving around the sun (exceptions are Venus and Uranus)
- Most orbits are very nearly circular. Pluto has a large
departure from circularity of 25% and Mars has a departure of 9%.
Moving Towards General Relativity
In the late 19th century, precision observations of the position of Mercury showed that it did not agree with the predictions from Newtonian theory
This is similar to the case of Mars, where Kepler used positional discrepancies to show that planetary orbits had to be elliptical in shape.
The resolution of the positional discrepancy of Mercury requires that space be "curved" in the vicinity of Mercury so that Mercury orbits inside this curvature. Such an orbit will differ slightly from an orbit in purely flat space.
Newton had implicitly assumed that space was flat.
General Theory of Relativity
Space communicates with matter and instructs it how to move and, in turn,
matter communicates with space and instructs it how to curve.
Mercury orbits in Curved Space
because it is a near a very large mass (the Sun).
The Visibility of the Planets:
Depends on the the planet-earth-sun angle. For the Outer Planets:
When this angle is 180 degrees , planet is overhead at
When this angle is 0 degrees , the planet is up in the day
When this angle is 90 degrees the planet is overhead at
When this angle is 270 degrees the planet is overhead at
For the Inner Planets the situation is much different as they
are always relatively near the Sun. There is a time in the orbit
of Mercury and Venus called greatest elongation in which Venus
appears at its maximum angular separation from the Sun. This is
illustrated below. Note that since there are 15 degrees in one hour
of time, then the angle of 46 degrees shown below corresponds to
about 3 hours of time. When Venus is at the position in its orbit
shown below, it will appear in the sky either 3 hours before or
after sunrise. At any other position in its orbit, Venus will appear
closer in time and angular separation to the sun.
For further experimentation please us
Real Examples from the
System Orbital Simulator