Could there be a
problem with Newton's description of gravity?
Planetary Orbit Properties
Kepler and Newton determined the general orbital properties of bodies of
the solar system:
Although all of the orbits of the planets could be described by
Newtonian Mechanics, it was noticed in the mid 19th centure that
the observed position of Mercury did not seem to agree with what the Newtonian model predicted. Could there be a
problem with Newton's description of gravity?
Description of the Mercury Problem:
Einstein to the rescue!
Mercury's Motion through the curve spacetime near the Sun
In the late 19th century, precision observations of the position of Mercury s
howed that it did not agree with the predictions from Newtonian theoryThis 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.
This lead to a refinement of Newtonian Gravity known as Einstein's General Theory of Relativity:
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).
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.
