Download Reason for the Seasons: Earth's Tilt and Revolution and more Summaries Earth Sciences in PDF only on Docsity!
Reason for the Seasons Facts
Revolution
The Earth revolves around the Sun ( revolution, revolve - The circling of one object
around another object in space. A planet revolves around the sun. A moon revolves
around a planet). Here’s a diagram of the Earth revolving around the Sun.
(Alder Planetarium, http://www.adlerplanetarium.org/education/resources/sunearth/section06i.shtml, accessed November 1, 2008)
The Earth is revolving around the Sun counter-clockwise.
Tilt
The Earth tilts 23.5o^ from the perpendicular ( tilt - The earth’s axis is not straight up
and down, instead it leans at a 23.5-degree angle). Say what? This means that instead
of the Earth being straight up-and-down, it tilts 23.5o^. Here’s the tilt of the Earth.
The Earth is tilted. Sometimes the Northern Hemisphere is tilted towards the Sun.
Sometimes it’s tilted away from the Sun. Look at this diagram.
The tilt of the Earth affects the amount of heat energy we receive from the Sun. Look
at the Earth when it is winter. The Northern Hemisphere is tilted away from the Sun.
Distance from the Sun
As the Earth revolves around the Sun, it is not always the same distance from the Sun.
During January, Earth is closest to the Sun. During July, the Earth is farthest from
the Sun. The Earth’s orbit is not a perfect circle. It is an ellipse. Even though it’s not
a perfect circle, it’s very, very close. This diagram shows you the distances to the Sun.
January July 91 million miles (^) 94 million miles
Reason for the Seasons Facts
Tilt and Direct/Indirect Heat Energy
The Earth is tilted. Sometimes the Northern Hemisphere is tilted towards the Sun.
Sometimes it’s tilted away from the Sun. Look at this diagram.
The tilt of the Earth affects the amount of heat energy we receive from the Sun. Look
at the Earth when it is winter. The Northern Hemisphere is tilted away from the Sun.
We receive the same amount of heat energy from the Sun, but it has to heat a larger area. To see how this works, we need to look at some diagrams.
Indirect heat energy strikes the ground at an angle. The light and its heat are spread out over a larger surface area than if it had struck the ground directly. The picture shows that the same amount of heat energy hits the surface. However, this energy
heats a larger surface area. This means winter temperatures are cooler.
What happens in the summer? Look at this diagram.
Direct heat energy is energy at or near right angles to the ground. The picture shows
that, during the summer, the heat energy heats a smaller surface area. This means
that summer temperatures are warmer.
There are two times when the heat energy is between direct and indirect. That is when it is spring and autumn. At that time, the Earth gets an equal amount of heat energy. This is why it is warming up in spring. It is why it is getting cooler in autumn.
There are four important dates as the Earth revolves around the Sun.
; December 21 st^ – Winter Solstice. Solstice means "Sun standing still." On the Winter Solstice, the Sun appears at its most southern position along the horizon at sunrise and sunset and reaches its lowest midday altitude for the year. The South Pole is tilted most toward the Sun.
Reason for the Seasons Facts
Rotation
Fact: The Earth rotates on its axis ( axis - An imaginary line going from the North Pole
to the South Pole. The earth spins on this line). Look at this picture:
The Earth rotates once every 24 hours (or once a day!).
Why do things appear to move in the sky? Why does the Sun and Moon move across our
sky? Why do we see different stars at 10:00 p.m. and at 4:00 a.m.? The Earth rotates.
As the Earth rotates, objects in the sky appear to move. They really don’t, you know!
As you watch objects throughout the day and night, you will see everything rises in the
east and sets in the west. This is because the Earth rotates counter-clockwise.
Changes in Daylight Hours Thru the Year
How does the length of daylight change throughout the year? Look at this diagram.
When it’s winter, the Northern Hemisphere is tilted away from the Sun. The North Pole is in darkness 24 hours a day. Daylight hours increase as you move south. In Utah, our daylight hours are around eight hours a day. Shorter daylight hours and indirect heat energy from the Sun make for a cold season.
After the Winter Solstice, the daylight hours start increasing. By the Vernal Equinox,
daylight hours are about twelve hours long. Daylight hours continue to increase until the Summer Solstice. Look at this diagram.
The Northern Hemisphere is now tilted towards the Sun. The North Pole is getting 24 hours of daylight. Daylight hours decrease as you move south. In Utah, our daylight hours are around 16 hours a day. Longer daylight hours and direct heat energy make for a warm season.
After the Summer Solstice, daylight hours start decreasing. By the Autumnal Equinox,
daylight hours are about twelve hours long. Daylight hours continue decreasing until the Winter Solstice.
How do the daylight hours change? Look at this table that shows daylight hours throughout the year.
Latitude 40°N
Date Jan 1 Jan 16 Feb 1 Feb 16 Mar 1 Mar 16 Hours light 9.21 9.46 9.92 10.48 11.02 11.
Date Apr 1 Apr 16 May 1 May 16 Jun 1 Jun 16 Hours light 12.41 13.07 13.68 14.21 14.63 14.
Date Jul 1 Jul 16 Aug 1 Aug 16 Sep 1 Sep 16 Hours light 14.81 14.58 14.13 13.59 12.92 12.
