Are You at Rest?

Jack Dempsey

Our Solar System

• Our Solar System orbits our Galaxy, the Milky Way

• Our Galaxy is part of the Local Group of Galaxies

• Galaxies exist throughout the Universe.

Our Knowledge of the Solar System and the Universe

• Aristotle in 340BC wrote book “On the Heavens”. Postulates Earth is a sphere not a flat disk. If flat shouldn’t see sails of ships first on horizon.

• He could see five lights moving across sky at night and called them planets - Greek name for wanderer.

• Aristotle claimed Sun and five planets move around a stationary Earth; Mercury, Venus, Mars, Jupiter, Saturn.

Our Knowledge of the Solar System

• Eratosthenes, Chief Librarian, Great Library of Alexandria in 200BC calculates the diameter of the Earth

• Ptolemy in 200AD created a model of the heavenly bodies that was reasonably correct

• Copernicus in 1514AD, claims Sun at centre of Solar system and planets move in a circular orbit.

• Galileo in 1600s, uses telescope to chart the skies• Kepler, in 1600s claims planets move in ellipses.

Our Knowledge of the Solar System

• Roemer in 1676AD is first to calculate speed of light using Jupiter’s moons

• Newton in 1687AD publishes “Principia Mathematica”

• Einstein in 1915 published his “General Theory of Relativity”

• Hubble in 1929 was first to detect other Galaxies.

Are You at Rest?

• The Earth's circumference is 40,075 km• A person on the Equator is travelling at 1,700 km/hr• A person in Kirkcaldy, Latitude 57 degrees North is

travelling at 900 km/hr• The earth circles the Sun at 107,000 km/hr

Are You at Rest?• The orbital speed of the Solar System is 792,000

km/hr• To complete one orbit around the Galaxy takes our

Solar System about 225–250 million years • The Sun is thought to have completed 20 orbits

during its lifetime• It has completed 1/1250 of a revolution since the

origin of humans

The Universe

• The human eye can observe about 5,000 stars• This is about 0.0001% of all stars in Milky Way• There are about 100 billion galaxies in the Universe • Each Galaxy has about 100 billion stars

A Galactic Collision

• The Milky Way and Andromeda are separated by 2.5 million light years

• The two galaxies are slowly moving towards each other at 120 km/s

• They will collide in approximately 4 billion years

The Universe?

• In the 1920s astronomer Edwin Hubble discovered that galaxies are all moving away from us at a rate proportional to their distance

• This relationship is called the Hubble constant• It establishes an expansion rate that is critical for

estimating the age and size of the universe, so far it is estimated to be 14 billion light years

Any Questions?

• Absorption lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to absorption lines in the optical spectrum of the Sun (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decreases).

Hubble was first to detect other Galaxies

• A theory is a good theory if it satisfies two requirements;

• It must accurately describe a large class of observations

• It can be used to predict future observations

Are You Resting?

• The Orbital speed of our Solar System around the galactic nucleus is 217 km/s

• It takes 226 million years to complete one lap