Earth Science Archives - AstroWatch.net

It’s funny how, as a child, my friends and I used to lie on the grass and look up at the starry night sky and watch as all the stars and the moon seemed to slowly drift by, like a decorated float in the Christmas parade. We always just assumed that we were in a fixed position, and everything in space was revolving around the Earth. Yet, in all our musings, we could never figure out how we would have such cold, harsh winters and such hot, humid summers. So why does the Earth have seasons? Most people think that the reason the Earth experiences different seasons is that part of the Earth is facing the Sun, which would make it warmer, and part of the Earth is facing away from the Sun, which would make it colder. But this is incorrect. The real reason we have warm summers and cold winters has to do with the Earth’s axis of rotation. You see, if there were a long stick that went through the center of the Earth from top to bottom in a perpendicular fashion, then each part of the Earth would be facing the Sun equally no matter where it was in its revolution around the Sun. But the axis is not perpendicular. It’s actually tilted at an angle of 23.5 degrees. With this in mind, the Northern Hemisphere is tilted toward the Sun for half of the year – making it warmer – and tilted away from the Sun for the other half of the year – making it colder. So How Does the Earth’s Tilt Make It So Cold for Some People and So Hot for Others? First, let’s explore the Earth’s tilt. The tilt of the Earth has a lot to do with how the mass is distributed over the planet. There are larger amounts of land mass and ice sheets in the Northern Hemisphere, and this makes the Earth topheavy. As an example, my kids love to swing. We have a huge tree swing in our front yard, and actually, more than just swing, they love to spin. One child will sit on the swing while the other will wind him around tighter and tighter until the ropes can’t twist anymore. Then he will back away and watch as his brother gains momentum while the rope unwinds. Except there’s a catch. My younger son doesn’t like to go too fast. And when the swing is wound up tight, it goes fast! So eventually, my son will lean back by stretching his arms while holding the rope in an effort to slow down the momentum. The funny thing is that, by doing this, his spin becomes wobbly and crooked. He is no longer sitting straight on the swing, so his spin, and his imaginary axis, are now tilting. This is what happens with the Earth. The heavier places cause it to tilt, so its axis is at an angle of 23.5 degrees no matter its position in orbit around the Sun. How We Transition to Winter and Summer In the diagram above, the Earth is in a position in orbit around the Sun, where the northern part of the planet is tilted at an angle farther away from the Sun than the southern part. Thus, The North is getting less heat from the Sun because it is farther away, and the South is getting more of the Sun’s heat because it is closer to the Sun. Now, remember, the Earth is continually spinning on its own axis as it revolves around the Sun. So, every part of the planet that is closer to the Sun at any given time, day or night, as the Earth continues to spin on its axis, will experience warmer weather, or summer. Likewise, any part of the planet in the North that is facing the Sun, whether day or night, will experience colder weather or winter. But What Happens When the Earth Has Rotated to the Other Side of the Sun? There are 365 days in a year. This means it takes the Earth 365 days to orbit around the Sun. In approximately 182 days, half a year, the Earth will be on the other side of the Sun, but its axis will still be at a 23.5-degree tilt. So when the Earth has made it to the other side, all the areas that are in the northern hemisphere of the Earth will be closer to the Sun and will experience their summer. All the areas of the planet that are in the southern hemisphere will be tilted away from the Sun and experience winter. Are All Areas of the Hemisphere That Are Pointed Toward the Sun Going to Be Equally Hot? In the picture above, the Northern Hemisphere is pointing toward the Sun, so that area is experiencing summer. However, not all areas of the Northern Hemisphere will experience equal amounts of heat. The point where the Sun is shining directly on the Earth will be the hottest. But as the sunlight spreads out at an angle over the surface of the planet, the energy from the Sun is also spreading out over a larger surface area, and it is not as hot. So even though it’s the same amount of energy shining on the Earth, the places where the Sun shines indirectly are more spread out, so it will feel cooler than where the Sun is shining directly on the planet. Do All Parts of the Earth Have All Four Seasons? Though all parts of the Earth experience some variation of the four seasons, they do not all experience the four seasons equally. As you get closer to the North and South Pole, daylight and temperatures change with the seasons. This means that in the summer, days are longer, and the temperatures are warmer than in the winter when days are shorter and temperatures are colder. However, as you get closer to the Equator, days are typically always 12 hours long, and the season generally remains warm, though they do experience a wet and dry season. What Is the Equator? The Equator is like a great circle around the middle of the Earth at a point that is equally distant from the North and South Pole. It divides the Earth into the Northern Hemisphere and Southern Hemisphere. At the Equator, the Earth doesn’t really tilt when the planet rotates on its axis. For this reason, its position relative to the Sun remains relatively constant throughout the entire year. So the idea of having Spring, Summer, Fall, and Winter is nonexistent. The only thing that changes is the division between wet and dry periods. Not hot and cold. When Do the Seasons Begin? The position of the Sun tells us when each of the seasons will begin. However, there are two ways that we define seasons. The Astronomical Start of a Season The astronomical start of a season is based on where the Earth is in relation to the Sun. So the beginning of each season is either a solstice, such as a winter solstice or summer solstice, or an equinox, which is used for Spring and Autumn. When the Sun is at the most northerly point in the sky or the most southerly point of the sky, it is either a summer or winter solstice. An equinox is when the Sun passes over Earth’s Equator. Because our calendar system has leap years, the dates of the equinoxes and solstices can change by a day or two, which causes the start dates of the seasons to vary. The Meteorological Start of a Season The meteorological start of a season is based on the yearly temperature cycle and the 12-month calendar. With the meteorological start, each season begins on the first of a specific month and lasts for three months. Thus, Spring begins on March 1, Summer begins on June 1, Autumn begins on September 1, and winter begins on December 1. The reason for the meteorological start is because it is easier to keep records of the weather since the start date will be the same each year. Conclusion This sums up why the Earth has seasons. Unless you live around the Equator, whatever season you are experiencing, the people on the opposing pole from where you live will be experiencing its opposite. Where I live in the Northern Hemisphere, it is now Summer, and on this June day, the temperature reached a high of 91 degrees Farenheight. Yet in Australia, which is in the Southern Hemisphere, the high today was 48 degrees Farenheight. I hope the above illustrations and examples help you better understand what is happening when the Earth experiences different seasons. Related Articles Why Is Earth the Only Planet With Life? Why Is Earth Called the Blue Planet? Why Do Planets Have Moons? What Are the Different Types of Planets?