Chapter 12 Section 1 – pg 464 Earth in Space

Astronomy: the study of the moon, stars, and other objects in space – The Egyptians were the first to do this They used this knowledge to predict the flooding of the Nile Pg 464

How Earth Moves Earth moves through space in two major ways: rotation and revolution Pg 465

Rotation Axis: the imaginary line that passes through Earth’s center and the North and South poles Rotation: the spinning of Earth on its axis – Causes day and night It is day on the side of the earth facing the sun and night on the side facing away – It takes the Earth about 24 hours to rotate once Pg 465

Revolution Revolution: the movement of one object around another – One complete revolution of the Earth around the Sun is called a year The Earth follows a path, or orbit, as it revolves around the sun – Slightly elongated circle, or ellipse Pg 465

Calendars Calendar: a system of organizing time that defines the beginning, length, and divisions of a year – Egyptians created one of the first ones They counted the number of days in a year to be 365 Early people used moon cycles to divide up the year – The cycle of the moon is about 29.5 days – The Egyptians split up the year into 12 “moonths” 12 months of 30 days each, plus 5 extra days at the end Pg 466

The Egyptians were a little off with their count – The Earth orbits around the sun in about days – The Romans adjusted to this by adding a extra day every four years (leap year) Using this system ensures that annual events, such as the beginning of summer, occurs about the same day each year The Romans calendar was off by a little more than 11 minutes a year – Over the centuries, this added up – By the 1500s, the beginning of spring was about ten days too early Pope Gregory XIII dropped ten days from the year 1582 to straighten things out – He also made some minor changes to our calendar, making the calendar we use today Pg 467

The Seasons on Earth Most places (besides the tropics and polar regions) have four distinct seasons: winter, spring, summer, autumn There is a great differences in temperature from place to place Pg 468

How Sunlight Hits Earth Sunlight hits the Earth more directly at the equator – This is why it is always warmer there Away from the equator, sunlight arrives at steep angles causing the light (and heat) to have to be distributed over more area – This is why it gets colder the further from the equator you are Pg 468

Earth’s Tilted Axis Earth has seasons because its axis is tilted as it revolves around the sun – If the Earth’s axis were straight up and down relative to its orbit, temperatures would remain fairly constant year-round As Earth revolves around the sun, the north end of its axis is tilted away from the sun for part of the year and towards the sun for part of the year Pg 468

Earth in June In June, the north end of the Earth’s axis is tilted toward the sun – In the Northern Hemisphere, the noon sun is high in the sky and there are more hours of sunlight The combination of direct rays and more hours of sunlight heats the surface more (causing summer) – South of the equator, the sun’s rays are less direct The combination of less direct rays and fewer hours of sunlight heats the surface less (causing winter) Pg 470

Earth in December In December, the north end of the axis is tilted away from the sun – Winter for Northern Hemisphere – Summer for Southern Hemisphere Pg 470

Solstices Solstice: when the sun is farthest north or south of the equator – Summer solstice: the day when the sun is the farthest north (June 21) Longest day for Northern Hemisphere Shortest day for Southern Hemisphere – Winter solstice: The day when the sun is the farthest south (December 21) Shortest day for Northern Hemisphere Longest day for Southern Hemisphere Pg 470

Equinoxes Equinox: when the noon sun is directly over head at the equator, “equal night” – Day and night are both 12 hours long – Vernal Equinox: March 21, beginning of spring for Northern Hemisphere – Autumnal Equinox: September 22, beginning of fall for Northern Hemisphere Pg 471

Chapter 12 Section 1 Homework-Pg 471

1A. What are the two major motions of Earth as it travels through space?

1B. Which motion causes night and day?

2A. What causes the seasons?

2B. What are solstices and equinoxes? How are they related to seasons?

2C. How would the seasons be different if Earth were not tilted on its axis?