Motions of Earth, the Moon and Planets

Models of our Solar System Our solar system consists of our sun and the objects that travel around it. Early models of the solar system established the Earth as the center, with the sun, moon, planets and stars traveling around it as part of a celestial sphere – this is known as the Geocentric model.

This model went unchallenged for nearly 2000 years, until the 1500s, when an astronomer by the name of Nicholas Copernicus proposed a radical new model known as the Heliocentric model. In this model, the Sun was established as the centre of the solar system with the earth and other planets travelling around it.

Planetary Motion Axial Tilt: the Earth’s axis is tilted 23.5o from the plane of the Earth’s orbit. So the Sun and planets appear to move along a tilted line in the sky.                                               

Rotation – the turning of an object around its own axis **The Earth’s rotation is 24 hours  creates day and night

Earth’s rotation is in a counter-clockwise direction (this is why stars appear to move in an east to west direction) Since the Earth’s axis is tilted 23.5°, it creates the 4 seasons (the Northern hemisphere is tilted toward the sun in the summer and away from the sun in the winter; this is reversed in the Southern hemisphere).

Revolution – the movement of an object around another object (orbit); this movement is not in a perfect circle, but in an elliptical shape Since the Sun in not directly in the centre of the ellipse, the Earth is actually closer to the Sun in December than it is in June!!! The Earth revolves around the sun every 365.24 days  this is our year

The Earth-Moon-Sun System

The motion of the Moon                                                                  The Moon revolves around the Earth in a nearly circular orbit.

the revolution period is 27 the revolution period is 27.3 days; this is the same as its rotation period  for this reason the Sun only illuminates the same ½ of the moon (the ‘near’ side) The Moon is illuminated by the Sun: one side is always bright and one dark. the position of the moon during it’s revolution around the Earth, and the fact that the moon’s orbit is tilted ~5° from Earth orbit, determines how much of the illumination we observe  this creates the phases of the moon

Gravity at close range: Tides                                               The Earth and Moon are (relatively) close. Gravity’s pull is much stronger on one side of each body than the other, distorting the Earth and the Moon.

Tides The water moves more than the solid Earth does. There are two high tides and two low tides per day.

Solar vs. Lunar Tides The Sun is more massive but very far away: Solar tide = ½ Lunar tide Solar and Lunar tides may work together or against each other.

Eclipses If the Sun, Moon, and Earth line up, you can have an eclipse:

Eclipses Lunar Eclipse: the Earth prevents sunlight from striking the Moon Solar Eclipse: the Moon prevents sunlight from striking the Earth

Only a small region right behind the Moon gets a Total Solar Eclipse Only a small region right behind the Moon gets a Total Solar Eclipse. Other observers might see a Partial Solar Eclipse.                                                                                                                                                                                  

Eclipses We don’t see eclipses every month because the Moon’s orbit is inclined at 5o to the Ecliptic.

The Night Sky

The Earth’s rotation makes the sky appear to revolve around us.                                                                                      

Polaris: does not appear to move because it's aligned with the Earth's axis.

Star Constellations: Constellations are groups or patterns of stars that early sky-watchers saw in the night sky Most were named after animals, mythical heroes or Gods People have used constellations for thousands of years as calendars, timekeepers and direction finders

Seasonal Constellations Seasonal Constellations: stars closer to the equator will rise and set and we will see different constellations through the year.

Homework Create a Star Wheel to take home Read section 8.5, pages 320-328 of your text book Answer pg 328 #2, 7, 8, 10, 11, 13 Read section 8.6, pages 329-333 of your text book Answer pg 333 #1, 2, 5