Chapter 10 What we know about the Universe

Chapter 10: What we know about the Universe has taken us thousands of years to learn. Unit 4: Space

�Astronomy is one of the oldest sciences. �Within the past 2000 years, men and women began making careful observation of the motion of objects in the sky and they created and tested models to explain this motion.

�Celestial Body: a natural object in space. Examples: The Sun, the Moon, a planet, or a star. �A star is a celestial body made of heat and light and is very far away. Example: the Sun, Planets (moving stars)

�For years, people made note of the movement of the celestial bodies. �When certain stars were visible at a certain time of the night, they knew it was time to plant their crops. When other stars were visible, they knew it was time to harvest their crops. �For ancient people the sky was sometimes a source of fear.

Constellations �Thousands of years ago, people noticed that the stars made certain patterns that appeared night after night. �Constellation: patterns made up of stars. Examples: Orion, Ursa Major, Ursa Minor �The people told stories of kings and queens, heroes and villains, animals and mythical creatures, and put them in the sky.

�People observing the sky also noticed five bright “stars”. They called these moving stars planets (the Greek word for wanderer). The planets were given the names Mercury, Venus, Mars, Jupiter, and Saturn. �Planet a celestial body that orbits one or more stars, is large enough that its own gravity holds it in a spherical shape, and is the only body occupying the orbital path

Constellations

Asterism �Asterism : Smaller groups of stars forming patterns within the constellations. Example: the Big Dipper § The end stars in the bowl are called pointer stars. They point to the North Star (Polaris).

Constellations �There are 88 constellations, but not all are visible in Canada. � 20 never rise above our horizon.

Stars �Magnitude: the brightness of a star. �The magnitude of a star depends on 2 things: 1. How close it is to Earth 2. How bright the star is § Some stars are much brighter than our Sun, but since they are far away, they appear very dim. § Sirius, the brightest visible star, is not as bright as Rigel, a nearby star, because Rigel is 100 times further away.

Stars � The first person to develop a scale to describe the brightness of a star was Hipparchus of Rhodes. The lower the magnitude, the brighter a star appears. §

Stars §Stars move from East to West §Looking south, the stars appear to rise in the east and set in the west, some moving directly overhead. Looking north, we see something different. The stars seem to slowly rotate around a common point §The point at which the stars rotate is called Polaris. Earths axis points to Polaris.

Circumpolar Constellations �The constellations in our north sky, including the constellations of Ursa Major and Ursa Minor, are called circumpolar constellations. They never go below the horizon. � They appear in the north sky all year!

Motion of the Sun, Moon, Planets �The sun travels along the same path every day, also known as the ecliptic. �As planets revolve around the sun, they travel at different rates. �Mercury and Venus always appear closer to the sun.

�Because Earth moves around the Sun every year, we see a different part of the sky every season �Some constellations are visible all year long but change positions, and some constellations are visible in certain seasons only.

Early Models of the Universe Aristotle (383 -322 B. C. E. ) �Visualized the universe as being geocentric. This means the Earth is the centre of the universe and everything else revolves around it. �He believed that the Earth was a sphere due to the curved edges observed during a lunar eclipse.

Aristotle’s Model

Early Models of the Universe Ptolemy (83 -168 C. E. ) �Based his model on his observations of Mars. �Mars orbital path creates a loop or s-shape in the sky. �His model showed each planet attached to a crystal sphere with its centre at Earth.

Ptolemy Model §Each planet was not attached directly to its sphere but to an offcentre wheel (epicycle). §Accepted for nearly 15 oo years.

Early Models of the Universe �Copernicus (1473 -1543) �Believed in a heliocentric universe (Sun at the centre). �The Earth rotated on its axis once daily and revolved around the Sun once a year.

Copernicus’ Model

Early Models of the Universe Galileo (1564 -1642) �First person to view the “heavens” through a telescope. �Allowed him to see objects about 20 x closer. He observed: �Craters on the Moon �Spots on the Sun �Four “stars” orbiting Jupiter (called the Galilean Moons)

�Now that the Sun was placed at the centre of our solar system, other astronomers could work to understand the motion of the planets. Improvements in the telescope would show new and strange objects never imagined!

Johannes Kepler (1571 -1630) Developed 3 Laws of Planetary Motion 1. All planets move in ellipses(oval shapes) with the Sun at one focus. 2. Planets sweep out at equal areas of their elliptical orbit in equal times. 3. The time a planet takes to revolve around the Sun is directly related to how far away it is from the Sun.

Planetary Motion

Sir Isaac Newton (1643 -1727) �Developed the three laws of motion. �First to show that the force of gravity affects all celestial bodies, causing them to stay in orbit. �Invented the reflecting telescope which uses a curved mirror to focus the light to a point at an eyepiece. �Moons revolve around planets

The Planets Inner Planets (small and have solid cores and rocky crusts. You could stand on any of these planets) � Mercury � Venus � Earth � Mars Jovian Planets (known for their large gaseous atmospheres, cold temperatures, and lack of a solid surface) § Jupiter § Saturn § Uranus § Neptune

Scale of the Solar System

Scale of the Solar System �The planets Mercury, Venus, Mars, Jupiter, and Saturn are visible without a telescope, and have been observed for many years �The planet Uranus is barely visible to the naked eye.