Solar+and+Cosmology

Curiosity touch down - a great moment for space exploration
(see below for the main solar and cosmo page)

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This section covers (1) the structure of space for our solar system and the wider Universe (below, with links to other topics), and (2) astrophysics, covering the historical development of our understanding of our place in the Universe (below, in section (2)) and our current understanding of the evolution of time and space (see The evolution of the Universe).

Overview
Our solar system (Our Solar System) is made up of a star (The Sun) about which eight planets orbit, as well as a number of dwarf planets (Dwarf Planets), and many millions of rocks and lumps of ice (Asteroids, Comets, the Kuiper belt and Ort cloud). The planets can be broken up into two distinct groups: (1) the Earth and our 3 rocky neighbours (The Four Inner Planets); and (2) the four gaseous outer planets (The Gas Planets). Many planets have moons, and Earth's only natural satellite, the Moon (The Moon and Phases of the Moon) is the largest object in our night sky.

Outside our solar system (The Wider Universe) we know that we sit in one arm of the Milk Way Galaxy which is made up of 100 billion (1x10 11 ) stars, some much like our own, some very different, as well as clouds of gas in which new stars are born (see picture below). Stars themselves can be categorised as falling into certain groups and a star's appearance will change over its lifetime (Stars and stellar evolution).

Our galaxy is then one of perhaps 1x10 11 galaxies in the universe. The scale of the planets, our solar system, the galaxy and the universe is difficult for us to understand as we, as humans, are more comfortable on the scale of meters and kilometers, rather than parsecs, astronomical units (AUs) and light years (ly) (link). The final part of this section, Thinking about astronomic measurements, tries to put come perspective these immense distances.



The "Pillars of creation" and "Butterfly" nebulae.

Summary index of section (1): Dwarf Planets Our Solar System Outside our solar system Rocky Neighbours

The early history of cosmology - The Greeks:
Aristotle - believed in a geocentric solar system (and indeed Universe) - i.e. the Earth was at the centre. Aristarchus - believed in a heliocentric solar system - i.e. the Sun was at the centre

The Aristotelian view won out, as Aristarchus was not able to explain (using the ideas of physics as understood at the time) (i) why things fell to the Earth if it was not at the centre; (ii) why things did not spin off the Earth if it was spinning; and (iii) why there was no stellar parallax.

The geocentric model and was believed for more than 18 centuries (until the work of Copernicus in the 15 century).

Eratosthenes was a contemporary of Aristarchos and used some basic geometry and clever thinking to work out the circumference of the Earth (starting from a measurement of the distance from the Alexandria to Aswan, roughly North-South, with Aswan having the Sun directly overhead at midday), the moon's diameter (using the circumference of the Earth and a lunar eclipse), the distance to the moon, and the distance from the Earth to the Sun.

For more information of these early ideas, download the embedded word doc below (particularly for details of Eratosthenes' methods)



From Copernicus to Newton
The period from X to X saw a complete shift in cosmological thinking as a run of scientists with a combination of excellent observational skills and/or free thinking minds turned over the views accepted for the last 1800 years. Copernicus suggested a simpler heliocentric system, Brahe observed that the "celestial sphere" was not unchanging (there were supernovae and comets), Kepler refined the Copernican system with elliptical orbits (and his resulting 3 laws of planetary motion, Galileo developed the telescope and provided observational evidence in support for the Copernican view, and developed the ideas of gravity being a force and of relative motion, and finally Newton created a new system of mathematics, mechanics, to explain planetary motion in terms of the law of gravity.



__**Key ideas**__


 * Copernicus' Universe**

Copernicus was aware of Aristarchus' (some write this Aristarchos - I'm going with the Encyclopaedia Britannica spelling) work, and was dissatisfied with how complex Ptolemy's geocentric model had become. Copernicus proposed a heliocentric model with the Sun at the centre, all the planets orbiting the Sun, and the moon orbiting the Earth. The stars remained on a sphere at a great distance. His Universe was therefore:
 * Heliocentric
 * Finite in size
 * Based on perfect mathematical forms (spheres)


 * Kepler's Laws of Planetary Motion**

1) The orbit of every planet is an ellipse with the Sun at one of the two foci. 2) A line joining a planet and the Sun sweeps out equal areas during equal intervals of time. 3) 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 =kr 3 ).

The third law needs some explaining: The semi-major axis of an ellipse is half of the longest diameter (i.e. a line through both foci), or, equivalently the longest radius. So the third law can equivalently be written: for all planets the ratio of the square of the orbital period to the cube of the radius is a constant:

__T__ 2 = constant. r 3


 * Galileo's 3 Laws of Motion**


 * 1) If there is no force acting on an object it will remain stationary, or if moving, continue moving at a steady speed in a straight line.
 * 2) Unbalanced forces cause acceleration.
 * 3) All forces are opposed by an equal and opposite force.


 * Newton's Law of Gravitation**

F = GMm/r 2

Newton was able to use his law of gravitation to predict the motion of the planets extremely accurately, and he was also able to derive Kepler's laws (which was based on empirical observations) from it, which gave people confidence that Newton had identified a fundamental law (although we now know that it only an approximation, although for large masses (bigger than an atom) and low energies (<<c) it is an extremely good approximation).

This allowed predictions to be made to guide astronomers' observations, and so the slight irregularities in Uranus' orbit lead to the discovery of Neptune (1845), and Pluto was discovered in a similar manner in 1930.

Newton also realised that as gravity was a purely attractive force, then in time all the mass should be attracted to the centre, so the Universe would collapse. However, there was an argument to avoid this conclusion: that the Universe was infinite, and therefore centreless. Furthermore, the motion of everything could, in principle, be measured and the mathematical laws would then allow prediction of the position of everything at any point in the future or past. This was a deterministic Universe. Newton's Universe was therefore:
 * Centreless
 * Infinite
 * Based on mathematical laws of motion and gravitation
 * Deterministic


 * Further material**

For more of these scientists, download the embedded word doc below:

For information on our current understanding of the Universe and its evolution see The evolution of the Universe.


 * Teaching aid**

Use the two attached handouts to think about which systems (geocentric and heliocentric) were more believable given the knowledge available at the time.




 * Our current understanding of our place in the Universe**

See: The evolution of the Universe


 * Circular motion**

For a teaching idea related to space and circular motion see Physics in space