1. Scientific Theories

2. Explaining the Origin of the Universe
Edwin Hubble was an American astronomer who was one of the first scientists to study galaxies.
First, he confirmed that many other galaxies existed beyond the Milky Way.
Second, he found that almost all galaxies are moving away from each other.
These observations helped to support the proposal made by Belgian priest and physicist Georges Lemaître that the universe is expanding.
Both of these ideas would lead astronomers and physicists to develop theories on how the universe may have formed.

3. Spectrum
Hubble closely examined each galaxy’s light spectrum. When white light passes through a prism, it is separated into a continuous rainbow known as the visible light spectrum. This is pluralized as spectra.
The measurements of different light spectra helped support Hubble’s theory. Evidence from thousands of observations of galaxies has confirmed his findings. Hubble’s work remains the foundation of modern understanding of the nature and origin of the universe.

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5. The Big Bang Theory
From the light spectrum of each galaxy he studied, Hubble was able to determine the speed at which each galaxy was moving away from our own. Then, for those same galaxies, he separately determined the distance between each one and the Milky Way. Plotting both the speed and the distance measurements together, Hubble discovered a clear relationship between the two. The farther away a galaxy was, the faster it was moving away.

6. The Big Bang Theory
These findings, along with observations of many other scientists of the time, gave further support to Lemaître’s idea that the universe is expanding. Out of this and the work of many other physicists, mathematicians, and other scientists of the day, came one of the most remarkable theories of the 20th century. The theory, which came to be called the Big Bang theory, states that the universe formed when an infinitely dense point suddenly and rapidly expanded in a single moment.

7. The Big Bang Theory
All the matter and energy that exists today was created during the early minutes of that hot, rapid expansion. It is now commonly accepted by scientists that the universe formed 13.7 billion years ago.
That moment marks the beginning of the universe and also the beginning of time. Today, at several research facilities around the world, scientists are trying to recreate various aspects of the conditions that might have existed in the early moments of the universe in hopes to gain a better understanding of the origin of everything within the universe.

9. The Wave Nature of Light
Light is a form of energy. Scientists have two ways of explaining how light travels: in waves or as energy parcels called photons. The wave form of energy is called electromagnetic radiation.
The electromagnetic spectrum is the full range of electromagnetic radiation, organized by wavelength from very long to very short; examples include radio waves, microwaves, infrared, visible light, ultraviolet radiation, and X rays.

10. The Wave Nature of Light

11. The Electromagnetic Spectrum
Humans can only see a small fraction of the electromagnetic spectrum, a rainbow of colours known as the visible light spectrum.
Each of the different colours of the visible light spectrum, from red through to yellow and green and on to violet, varies in wavelength. The wavelength of red light, for example, is longer than the wavelength for blue light.
Technology enables us to detect other forms of electromagnetic radiation that are not readily visible to humans.

12. Analyzing the Movement of Light Waves
Observing the light emitted by a galaxy allows scientists to measure the light’s wavelengths to determine whether the galaxy is moving and in which direction. This phenomenon is called the Doppler effect.

13. Theories
In science, although a theory can be supported by evidence, it is never considered to be proven correct. If new evidence conflicts with a theory, scientists may need to rethink or revise the theory.
This happens quite often. This is why there are so many theories and so few laws in the field of science.
To date, the Big Bang theory continues to be the only theory for the universe’s formation that is supported by the entire body of scientific information gathered so far.

14. Planetary Motion
From our perspective on Earth, everything in the sky appears to be in motion. The Sun rises and sets. The Moon, in its ever-changing phases, travels across the sky. Planets shift against a background of stars. Even constellations appear to change position.

15. Geocentric Model
More than 2000 years ago, the widely held belief among astronomers was that Earth sat at the centre of the universe.
Models that have Earth at the centre of the universe are called geocentric models, or Earth-centred.
You know that the Sun moves across the sky along a path known as the ecliptic. Planets also appear to follow a path close to the ecliptic. However, Mars, Jupiter, and Saturn appear to loop backward for a period of days or weeks before resuming their normal path.

16. Geocentric Model
This apparent backward motion is a celestial phenomenon known as apparent retrograde motion. Various astronomers proposed changes to the geocentric model to explain apparent retrograde motion, but most of these adaptations were extremely complicated.

17. Geocentric Model

18. Heliocentric Model
The geocentric model of the solar system was accepted for almost 2000 years. The heliocentric model, which places the Sun at the centre was proposed as far back as 500 BCE.
Nicolaus Copernicus (1473–1543) reintroduced a simplified heliocentric model. Copernicus’s model placed the Sun, stationary, near the centre of the solar system and everything revolved around the Sun, including Earth. This was a very radical idea at the time.

19. Heliocentric Model
Two key ideas about planetary orbits helped add support to the heliocentric model. One was the relationship between a planet’s orbital radius and its speed of orbit. The other was the fact that planetary orbits are elliptical and not circular.

20. Orbital Radius
In the heliocentric model, all planets orbit the Sun in the same counterclockwise direction but at different distances. A planet’s distance from the Sun is called the planet’s orbital radius.
The shorter the orbital radius, the faster the planet moves in its orbit.
Therefore, Earth, which is closer to the Sun than Mars, orbits the Sun more quickly than Mars. This is not just because Earth’s orbit is shorter than Mars’ orbit. Earth is also moving faster than Mars. In turn, Mars is moving faster than Jupiter, the next planet out from the Sun.
This pattern is true for all the planets, dwarf planets, and even asteroids in the solar system. The reason is that the farther an object is from the Sun, the weaker is the effect of the Sun’s gravity on that object.

21. Apparent Retrograde Motion
Mars is moving eastward across the sky, stopping, and moving westward before stopping again and resuming its eastward motion.
This change in direction is just an optical illusion.
To help visualize how apparent retrograde motion occurs, imagine you are in a track race. You pass three other runners, not only because you are moving faster, but also because you have the inside turn position. When you pass the other runners, they appear to be moving backward relative to you.
Heliocentric models were better able to explain apparent retrograde motion than the Geocentric models. This was key evidence in support of heliocentric models of the solar system.

22. Elliptical Orbits
Early heliocentric models could still not predict planetary motion very accurately. These models assumed that planets had a circular orbit.
A circle has a single focus point: the centre. If the Sun were placed in the centre, astronomers’ predictions of planetary motion did not match their observations.
Johannes Kepler realized that the orbits of the planets were ellipses.

23. Elliptical Orbits
An ellipse is somewhat like a slightly flattened circle.
Today, all astronomical observations continue to support the heliocentric model of our solar system. It is also the guide we use when studying other star-and-planet systems.