The reason for autumn in the Northern Hemisphere and spring in the Southern Hemisphere is because Earth's tilt is sideways to the sun, and hours of daylight and darkness are the same in both hemispheres on both September 22 Many people believe that Earth is closer to the Sun in the summer and that is why it is hotter. And, likewise, they think Earth is farthest from the Sun in the winter. Although this idea makes sense, it is incorrect. It is true that Earth's orbit is not a perfect circle. It is a bit lop-sided. During part of the year, Earth is closer to the Sun than at other times. However, in the Northern Hemisphere, we are having winter when Earth is closest to the Sun and summer when it is farthest away! Compared with how far away the Sun is, this change in Earth's distance throughout the year does not make much difference to our weather. There is a different reason for Earth's seasons. Earth's axis is an imaginary pole going right through the center of Earth from "top" to "bottom." Earth spins around this pole, making one complete turn each day. That is why we have day and night, and why every part of Earth's surface gets some of each. Earth has seasons because its axis doesn't stand up straight. But what caused Earth to tilt? Long, long ago, when Earth was young, it is thought that something big hit Earth and knocked it off-kilter. So instead of rotating with its axis straight up and down, it leans over a bit. By the way, that big thing that hit Earth is called Theia. It also blasted a big hole in the surface. That big hit sent a huge amount of dust and rubble into orbit. Most scientists think that that rubble, in time, became our Moon. As Earth orbits the Sun, its tilted axis always points in the same direction. So, throughout the year, different parts of Earth get the Sun’s direct rays. Sometimes it is the North Pole tilting toward the Sun (around June) and sometimes it is the South Pole tilting toward the Sun (around December). It is summer in June in the Northern Hemisphere because the Sun's rays hit that part of Earth more directly than at any other time of the year. It is winter in December in the Northern Hemisphere, because that is when it is the South Pole's turn to be tilted toward the Sun. Earth's lopsided orbitEarth's perihelion (point closest to Sun) = 91,400,000 miles from Sun Earth's aphelion (point farthest from Sun) = 94,500,000 miles from Sun While that is a difference of over 3 million miles, relative to the entire distance, it isn’t much. And, believe it or not, aphelion (when Earth is farthest from the Sun) occurs in July, and perihelion (when we are closest) occurs in January. For those of us who live in the Northern Hemisphere where it's summer in July and winter in January, that seems backwards, doesn't it? That just goes to prove that Earth's distance from the Sun is not the cause of the seasons. Related Resources for Educators Seasons (Educator Guide to go with Seasons Spotlite video)
Throughout the course of the year, most places on Earth goes through four noticeable seasons: summer, autumn (fall), winter and spring, each lasting for about 3 months. The seasons experienced by the northern and southern hemisphere always differ by six months – when it is summer in the northern hemisphere, it is winter in the southern hemisphere, and so on. Seasons are a direct consequence of the Earth’s tilted rotation axis, which makes an angle of about 23.5 degrees to a line drawn perpendicular to the plane of the ecliptic. The direction of the Earth’s axis stays nearly fixed throughout one orbit, so that at different parts of the orbit one hemisphere ‘leans’ towards the Sun (summer), while the other ‘leans’ away (winter). Six months later, the Earth is leaning in the opposite direction.
The Earth’s tilt causes the Southern Hemisphere (SH) to lean towards the Sun during SH summer. Meanwhile, it is winter in the Northern Hemisphere (NH) which leans away from the Sun. Six months later, the situation is reversed. For locations north or south of the equator, the main feature accompanying each season is a change in temperature caused by the varying amount of sunlight that falls on each hemisphere of the Earth throughout its annual orbit. The hemisphere tilted towards the Sun will experience longer hours of sunlight, and more direct sunlight.
left: Winter in the Northern Hemisphere – the amount of sunlight falling on the Northern Hemisphere is much less than than… right: …the amount of sunlight falling on the Southern Hemisphere during Southern Hemisphere summer. As the Sun is higher in the sky during summer, the sunlight reaching the surface is more concentrated. In winter, the Sun is lower in the sky, and sunlight is spread out over a larger area. During spring and autumn, both hemispheres receive about the same amount of sunlight.
At the equator, the temperature variation is much smaller throughout the year, and it is common to consider just two seasons: dry and wet (or monsoon). For observers right at the north pole and the south pole, there are only two seasons – an almost six-month long winter night followed by an almost six-month long summer day! Within the Arctic circle and the Antarctic Circle (latitudes 66.5 degrees north and south), there will be at least one polar day (24 hours of continuous daylight, sometime called the ‘midnight sun’) and one polar night (24 continuous hours of darkness). The date of the start of the seasons is often chosen to start on the dates of the solstices (summer and winter) and equinoxes (autumn and spring). Alternatively, the start of a new season may be associated with the first day of the month (December, March, June and September) in which a solstice or equinox occurs. The Earth’s changing distance from the Sun due to the Earth’s elliptical orbit is sometimes thought to cause the seasons. This is incorrect! The Earth’s distance from the Sun varies by about 3% from closest (perihelion distance = 147.09 million km) to furthest approach (aphelion distance = 152.10 million km). This small change in distance cannot account for the temperature differences between summer and winter, and cannot explain how it can be winter in one hemisphere and summer in the other hemisphere.
If you have ever gone swimming in summer or had a snowball fight in winter, then you know something about seasons. Seasons are times on Earth that have very specific weather patterns and hours of daylight. Earth’s four seasons are spring, summer, fall, and winter. Seasons are caused by Earth’s changing position as it revolves around the Sun. Some people think that the seasons occur because of Earth’s distance from the Sun. However, Earth is closer to the Sun in December and January and farther away from the Sun in July and August. So what’s going on? All of the planets and bodies in our solar system orbit the Sun. This is called a heliocentric (sun-centered) orbit. Almost all of these objects also travel in the orbital plane. Think of this as a flat disk, with the Sun at the center, extending out toward the edges of our solar system. As Earth travels in its orbital plane, it is held in place by the Sun’s gravitational field. Earth also tilts on its axis at 23.5 degrees to this orbital plane. This means that the north and south poles of our planet are not straight up and down as we orbit the Sun; they are always at an angle. Earth’s rotation on its axis takes approximately 24 hours, which is why we have day and night. It takes Earth about 365.25 days to complete one entire revolution around the Sun, which gives us 1 year. As Earth revolves around the Sun, it rotates on its axis. Sometimes Earth tilts toward the Sun, which is when summer occurs. In the winter, Earth tilts away from the Sun.
Summer Solstice In North America, around June 21, Earth tilts on its axis toward the Sun. This is called the summer solstice, and it is when the Northern Hemisphere has the most daylight of any time of the year. It is also when the Sun appears to be the highest in the sky, and it has its greatest angle to Earth. The higher the Sun is, the more sunlight and heat Earth receives. This is when the days are longer and the nights are shorter. The seasons in Earth’s Northern Hemisphere are always opposite those in the Southern Hemisphere. This means that at the northern summer solstice, North America will experience summer, and winter will start in in the Southern Hemisphere. Winter Solstice Around December 21, the Northern Hemisphere tilts the farthest away from the Sun. This is called the northern winter solstice, and it is when we have the least amount of daylight of any time of the year. During this time, the Sun appears lowest in the sky, with the least angle to Earth, and we receive less sunlight and warmth. This is also when the days are shorter and the nights are longer.
Equinoxes Around March 21 and September 21, Earth is angled approximately 90 degrees away from the Sun (a right angle). This is when the spring and fall equinoxes occur. During this time, both the Northern and Southern Hemispheres are exactly the same angle from the Sun. Therefore, we all experience about the same amount of daylight and darkness. Having four seasons is typical for people that live above and below Earth’s equator. People that live at the equator do not experience four seasons. This is because the middle of Earth does not tilt much as Earth rotates on its axis. Without tilt, the angle of the Sun is always the same, so the equator receives the same amount of warmth and light year round. The farther you get from the equator, the bigger the difference there is in seasons and sunlight. For example, in Barrow, Alaska, the northernmost city in the United States, they experience 4 months of complete sunlight from May to August and complete darkness from November to January. This is an excerpt from the Space Systems Exploration unit of our curriculum product line, STCMSTM. Please visit our publisher, Carolina Biological, to learn more. |
What will be the season in the Southern Hemisphere when the North Pole is tilted away from the sun?
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