Chapter 1 Earth, Moon, and Sun

Section 1

Earth in Space

What Is Astronomy?

• Astronomy is the study of the moon, stars, and other objects in space.

How Earth Moves

• Ancient astronomers believed that the universe moved around the Earth while we stood still. However, we now know this is untrue.

• Earth moves through space in two major ways: – Rotation

– Revolution

How Earth Moves

• Rotation

– The imaginary line that passes through the Earth’s center and the North and South poles is Earth’s axis

– The spinning of the Earth on its axis is called rotation

– Earth’s rotation causes night and day because as the Earth rotates, part of the planet will be facing towards the sun (day) and part of the planet will be facing away (night)

– One full rotation takes 24 hours

Rotation

How Earth Moves

• Revolution

– The movement of one object around another object in called revolution, just as we revolve around the sun

– One full revolution around the sun takes 365 days (or 1 year)

– Earth follows a path, or orbit, as it revolves around the sun

– Earth’s orbit is not perfectly circular, but more like an oval or ellipse

Revolution

How Earth Moves

• A calendar is a system of organizing time that defines the beginning, length, and divisions of a year.

• The Egyptians and Romans used the stars and the moon to help create their calendars, both of which helped to influence our calendar

The Seasons on Earth • How sunlight hits Earth:

– Sunlight hits the most directly at the Equator, strongly focusing its light/heat on a more central location… this is why it is always hotter at the Equator than at the poles

– Near the North and South poles, the sun hits at a steeper angle, weakly spreading the sun’s light/heat over a greater area

The Seasons on Earth • Earth’s tilted axis:

– If the axis were straight up and down, temperatures would be constant all year long… there would be no seasons!

– Earth has seasons because its axis is tilted 23.5° as it revolves around the sun

The Seasons on Earth • Earth’s tilted axis:

– As the Earth revolves around the sun, its tilt allows the North end of the planet to be facing towards the sun for part of the year (our summer) and away from the sun during part of the year (our winter)

– Fall and Spring are just half-way marks between Summer and Winter

– Seasons have NOTHING to do with distance from the sun, it is all about which direction it is tilted

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The Seasons on Earth

• During different seasons, the height of the sun above the horizon varies due to the tilt of the axis.

The Seasons on Earth

• This difference in sun height and angle results in the differences of the seasons:

– Summer = more direct sunlight, warmer temperatures, more hours of sunlight each day

– Winter = less direct sunlight, cooler temperature, less hours of sunlight each day

The Seasons on Earth

• Solstices

– Twice a year the sun is at the farthest point north (our summer) or south (our winter) of the equator, this is known as a solstice

• June 21st = our summer solstice (longest day of the year, first day of Summer)

• December 21st = our winter solstice (shortest day of the year, first day of Winter)

The Seasons on Earth

• Equinoxes

–Halfway between each solstice is an equinox, when the length of day and night are equal

• March 21st = our vernal equinox (days and nights are equal, first day of Spring)

• September 22nd = our autumnal equinox (days and nights are equal, first day of Fall)

The Seasons on Earth

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