# Chapter 2 : A Brief History of Astronomy

### Topics covered in this snack-sized chapter:

In the past, humans observed following (without telescope):
- The Sun: It rises and sets.

- The Moon: It has a monthly cycle of phases.

- The Fixed Stars: The patterns stay fixed and the appearance of different constellations marks the different seasons.

In our solar system, the planets circle around our Sun.
The planets travel in the same direction (counter- clockwise looking down from the Sun's North Pole).
A model can explain existing observations and be able to predict the outcome of future experiments.

Aristotle (an influential Greek philosopher) favored a geocentric model for the Universe:
- The Earth is at the center of the Universe.

- The perfect shape of Earth is a sphere, and the natural motion is rotation.

###### Aristotle’s Geocentric Model

The motion of the Sun around the Earth accounts for the rising and setting of the Sun.
The motion of the Moon around the Earth accounts for the rising and setting of the Moon.
The fixed stars were on the “Celestial Sphere” whose rotation causes the rising and setting of the stars.
Although Aristotle’s ideas were commonly accepted, there was a need for a more accurate way to predict planetary motions.
Ptolemy known as a mathematician, astronomer, geographer and astrologer presented a detailed model of the Universe.
- It explained retrograde motion by using placement of circles.

###### Ptolemy’s Epicycles based Model

By adding epicycles, complicated motion could be explained.

Nicolas Copernicus proposed a heliocentric model of the Universe.
The Sun was at the center, and the planets moved around it in perfect circles.
The Sun was the center of the Universe.
Each planet moved in a circle, and the speed of the planet’s motion decreased with increasing distance from the Sun.
Retrograde motion of the planets could be explained as a projection effect.
The philosophical shift was important i.e. the Earth is not at the center of the Universe.
Kepler was a mathematician.
He believed in the Copernican view:
- The motions of the planets on perfect circles.

*“The orbit of every planet is an ellipse with the Sun at one of the two foci.”
*

Planets move around the sun in elliptical paths with the sun at one focus of the ellipse.

An ellipse has two foci, F1 and F2
For any point P on the ellipse, F1 P + F2 P is a constant.
The orbits of the planets are nearly circular (F1 and F2 are close together), but not perfect circles.
A circle is an ellipse with both foci at the same point-the center.

"*A line joining a planet and the Sun sweeps out equal areas during equal intervals of time."
*

While orbiting, a planet sweeps out equal areas in equal times.

The green shaded sector has the same area as the red shaded sector.
Thus, a planet moves from C to D in the same amount of time as it moves from A to B.
This means a planet must move faster when it is closer to the sun.
For planets this affect is small, but for comets it is quite noticeable, since a comet’s orbit has much greater eccentricity.

*"The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit."
*

T^{2
}R^{3
}

Assuming that a planet’s orbit is circular, then the mean distance from the sun is a constant-the radius (R).
F is the force of gravity on the planet.
F is also the centripetal force.
If the orbit is circular, the planet’s speed is constant, and v=2pR/T

Cancel m and simplify:

Rearrange:

Since G, M, and 4π^{2
} are constants,

∴ T^{2
}R^{3
}

^{
}

Where,

- G is the Gravitational constant.

- M is the mass of the larger body.

- R is the distance between the centers of mass of the two bodies.

Galileo was the first to use a telescope to study astronomical objects.
His observations of the moons of Jupiter and the phases of Venus provided strong support for the heliocentric model.
Isaac Newton gave the law of gravity and three general laws of motion that apply to all objects:
- Newton’s first law of motion states that an object remains at rest or continues in a state of uniform motion if no external force acts upon the object.

- Newton’s second law of motion states that if a net force acts on an object, it will cause an acceleration of that object.

Earlier astronomers incorrectly identified Earth and Sun as the center of the universe.
There is no center of the universe because there is no edge of the universe.
- In a finite universe, space is curved so that if you could travel billions of light years in a straight line you would eventually finish back where you started.

- It is also possible that our universe is infinite.

In both examples, groups of galaxies completely fill the universe and are moving apart at all points making the universe expand.