Isaac Netwon's System of Astronomy

September 22, 2022

Keplers’ laws were cemented by the observations of Cassini. But there were still constant irregularities in the motions of heavenly bodies.

Sir Isaac Newton first attempted to give a physical account of the motions of the Planets to solve such irregularities.

The physical connection, by which

Descartes tried to bind the movements of the Planets through the laws of impulse. These flow from the inertness of matter and are the easiest for the mind to undertand.

Aside from this, we do not have any other natural concept such as that of gravity. This is because we never gravitate matter. Instead, we just observe how gravity affects matter.

Sir Isaac Newton had the superior genius and sagacity to discover that he could join the movements of the Planets by a familiar principle of connection.

He demonstrated that=

  • the Planets gravitated towards the Sun and to one another, amd
  • had a projecting force originally impressed on them.

The primary ones might all described ellipses in one of the foci of the sun.

The secondary ones might describe figures of the same kind round their respective primaries, without being disturbed by the continual motion of the centers of their revolutions.

If the force which kept the planets in their orbits was like that of gravity and directed towards the Sun they individually would describe equal areas in equal times.

This attractive power of the Sun*=

  • is diffused in rays from a center and
  • is diminished in the same proportion as the squares of the distances increased

*Superphysics note: This is Kepler’s inverse square law

The motions of the planets would be swiftest when nearest the Sun, and slowest when farthest in the same proportion.

This gradual reduction of their respective gravities, their periodic times would bear the same proportion to their distances, which Kepler and Cassini had established between them.

He then proved that gravity was the connecting principle which joined the movements of the Planets by showing that the gravity on Earth is stronger* near the surface of the Earth.

*Superphysics note= Descartes also proves this

Gravity can make a body fall by around 15 Parisian feet in the first 1 second of its descent.

The Moon’s Irregular Orbit

The Moon is around 60 radii of the Earth distant from its surface.

Gravity diminished as the squares of the distance increase. Therefore, an object on the Moon would fall towards the Earth in 60 seconds.

But the arch which the Moon describes in 60 seconds falls, by observation, around 15 Parisian feet below the tangent drawn at the beginning of it.

Thus, the Moon is constantly falling towards the Earth.

The system of Sir Isaac Newton explained the many other irregularities which Astronomers had observed in the Heavens.

  • It explained that the centers of the planets’ revolutions were not precisely in the Sun’s center because the planets were also attracting* each other.

*Superphysics note= Kepler proved this earlier as Kepler’s Third Law

This explained the irregularities with Jupiter and Saturn, whenever they are nearly in conjunction with one another.

But the Moon’s irregularities had perplexed Astronomers the most. It was solved by Newton’s system.

The Moon=

  • appears furthest from the Earth when the Moon is nearest to the Sun. At this point, the Moon is more attracted to the sun and more separated from the Earth
  • appears nearest to the Earth when the Moon is in her quarters. At these points, both the Earth and the Moon are at equal distance from the Sun. The Earth and Moon are not attracted to the sun in parallel lines, but in lines which meet in his center.

Sir Isaac Newton computed the difference of the forces, with which the Moon and the Earth should, in all those different situations, according to his theory, to be impelled towards one another.

He found, that the different degrees of their approaches, as they had been observed by Astronomers, corresponded exactly to his computations.

The attraction of the Sun, in the conjunctions and oppositions, reduces the gravity of the Moon towards the Earth. This makes her extend her orbit and require a longer time to revolve around the Earth.

But, when the Moon and the Earth are in that part of the orbit which is nearest the Sun, this attraction of the Sun will be the greatest. Consequently, the gravity of the Moon towards the Earth, will there be most diminished. The moon’s orbit becomes most extended. This cases her orbit time to be the longest.

The Moon’s orbit is not precisely in the same plane with that of the Earth, but is at an angle.

  • The intersection of those two planes are called the Nodes of the Moon.
  • These Nodes of the Moon are in continual motion. In 18 or 19 years, these revolve backwards from east to west, through all the different points of the Ecliptic.

For the Moon, after having finished her revolution, generally intersects the orbit of the Earth somewhat behind the point where she had intersected it before.

Generally, the motion of the Nodes is retrograde. But not always. It is sometimes direct, and sometimes even stationary.

The Moon generally intersects the Plane of the Earth’s orbit, behind the point where she had intersected it in her former revolution. But she sometimes intersects it before that point, and sometimes in the very same point.

It is the situation of those Nodes which determines the times of Eclipses. Their motions had always been attended to by Astronomers who were perplexed by their inconsistencies because they assumed that the Moon’s orbin was perfectly regular and equable.

Astronomy, therefore, has been focused the most on theories for connecting the Moon’s motions than for all the other heavenly bodies taken together.

The theory of gravity connected most accurately all those irregular motions by the different actions of the Sun and the Earth.

The time, quantity, and duration of those direct and retrograde motions of the Nodes, as well as of their stationary appearances, could then be predicted by calculation.

The attraction of the Sun thus accounts for:

  • the motions of the Nodes
  • the perpetual variation in the inclination of her orbit to that of the Earth

The Moon revolves in an ellipse, which has the centre of the Earth in one of its foci.

  • The longer axis of its orbit is called the Line of its Apsides
    • This line is not to be always directed towards the same points of outer space. Instead, it revolves forwards, from west to east, so as to pass through all the points of the Ecliptic, and to complete its period in about 9 years.

*Superphysics Note: Lateral precession?

This wasanother irregularity that very much perplexed Astronomers. This was also solved by the theory of gravity.