What we know about the universe has taken us thousand of years

Celestial Bodies Celestial Body: a natural object out in space such as a planet, a moon, an asteroid, a comet, or a star Star: like our sun, give off heat and light and are thousands of years away Planets: 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 Constellations: a distinctive pattern in the night sky formed by a group of stars. It often looks like a familiar object like an animal. Examples: Ursa Major or the great bear, Ursa Minor or the little bear, Orion the hunter, Cassiopeia the queen and Leo the lion. Asterisms: smaller groups of stars forming patterns within the constellations. Example: the big dipper (found in Ursa Major)

Ursa Major

Ursa Minor

ORION THE HUNTER

CASSIOPEIA THE QUEEN

LEO THE LION

Motion of the Sun Sun follows the same path through the sky everyday and the path is called ecliptic. Ecliptic: the path followed by the Sun through the sky, as seen from Earth.

Apparent path of the Sun as seen from Earth

Motion of planets They follow an ecliptic path called orbits around the Sun. The orbits are like long, slightly flattened circles. The planets move at different rates due to their distance from the Sun. These orbits result from gravitational forces As the Earth revolves around the Sun, it appears that the Sun is moving against the background of stars As the planets revolve around the Sun, they move at different rates along an ecliptic path.

Planet motion

Motion of Planets Those inside Earth’s orbit (Mercury and Venus) appear to race through the sky and appear to be close to the Sun Those beyond Earth’s orbit are slower (Mars) and appear far from the Sun in the sky. Seem to move against the background of stars in the sky

Motion of Asteroids and Comets Both Orbit around the sun Asteroids: one of many small rocky bodies in our solar system, most of which orbit the Sun between Mars and Jupiter.

Comet orbit Comet: Discussed later in Chapter 11, but basically a “dirty snowball” The orbits of comets can be above or below the orbit of Earth, for this reason we do not always see the comet on the ecliptic They can orbit in different planes (unlike all other planets) Visit to explore the orbits of comets and asteroids bits_cnsr.pdf

Comets Orbit the Sun

Motion of the Moon The moon orbits around Earth once every 28 days.

Other motion.. Planets, suns (stars) and moons spin on a central axis. Earth spins on its axis, which is an imaginary line from the North Pole to the South Pole Example: the Earth spins on its axis towards the east. Stars, including the Sun, all generally move from east to west.

Early Models and Evidence Gathered about the Universe Aristotle: saw the universe as geocentric – the Earth at the center and the sun, moon, planets and stars revolving around it. Patterns of stars does not change so he thought the Earth was in a fixed place During a lunar eclipse, the shadow cast on the moon had curved edges so Earth must be a sphere.

Aristotle’s Evidence that Earth is Sphere

Early models... Later, Aristarchus proposed that the universe was heliocentric, revolved around the sun but not widely accepted at this time Erathosthenes of Cyene: first person to accurately measure the diameter of Earth.

Early models… Ptolemy: He noticed that the motion of Mars in the sky over several weeks creates a loop in the sky. This change in direction is called retrograde motion.

Early models… To explain this motion, Ptolemy proposed that each planet revolved around a point on its orbit, called an epicycle, to explain its motion through the heavens

Early models… Copernicus: he thought the earlier observations could be explained in a simpler way by a model in which Earth rotated on its axis once daily and revolved around the Sun once a year. (Heliocentric model) The Earth’s motion around the Sun is relatively recent (1543) Galileo: First person to turn a small telescope towards the sky and publicly report what he saw. Observed craters on the moon, spots on the sun, and four “stars” orbiting Jupiter.

Galileo... Galileo also observed that Venus had phases like our moon does and this could only happen if Venus orbited the Sun

Galileo published his findings and was imprisoned for his work by the Catholic church because they taught an Earth centered universe. Johannes Kepler: with the help of Tycho Brahe, made observations of Mars’ orbit and found that it modeled an ellipse (oval shape) Kepler developed three laws of planetary motion All planets move in ellipses, with the sun at the center The speed of the planet as it revolved around the sun is not constant. Closer to the sun it speeds up The time it takes the planet to revolve around the Sun is related to how far away it is from the Sun

Sir Isaac Newton Sir Isaac Newton: developed three laws to predict and describe motion and explained how celestial bodies move through the universe. He was the first to show mathematically that the force of gravity reaches far beyond the surface of Earth and affects all celestial bodies This force causes the bodies to remain in orbit around larger bodies Invented the reflecting telescope which allowed more precise observations.

Early Technologies for observations Stone Circles: possibly used to make simple astronomical observations. For example, Stonehenge. On midsummer’s day the Sun rises directly over a particular stone in Stonehenge. Marking the longest day of the year. Sun rising over a stone

Early Technologies… Astrolabe: around the time of Ptolemy, they were used to help locate and predict the positions of the Sun, Moon and stars Using a compass and an astrolabe, they were able to describe the position of any celestial body in relation to the direction North as well as in relation to the horizon. ASTROLAB ACTIVITY: y_07.html

Technologies… Telescope: Refracting telescope (uses lenses to gather and focus light) was used by Galileo to make his observations (Sun centered universe) Reflecting Telescope: (uses mirrors to gather and focus light) was developed by Newton to make even more precise observations and this design is used today, called the Newtonian design

Core Lab Strolling Through the Solar System