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Seasons: Contrary to popular belief, seasons are not caused by changes in Earth’s distance to the sun. In fact, in the Northern hemisphere, we are actually closer to the sun during our winter!
The tilt of the Earth’s axis is what causes it to experience seasons. Currently, the earth’s axis is tilted 23.5o off vertical. See the diagram to the right. This axial tilt causes varying sunlight hours and intensity as the earth completes its 365.24 day revolution of our sun. This axial tilt does not change direction over the year, however its relative position compared to our sun does change. In January, the axial tilt points the northern hemisphere away from the sun. This significantly decreases the solar irradiance< the amount of sunlight power or intensity striking the earth per unit area> values for the entire northern hemisphere. With less sunlight energy, the earth’s surface and atmosphere experience less heating, resulting in lower temperatures. Check out this interactive animation to visualize this change over the course of a year.
Implications for Life on Earth: The presence of seasons on Earth provided the environmental pressures for life to evolve and adapt to constant changes. From the life cycles of deciduous trees, which shed their leaves to remain dormant during winter, to the hibernation strategies of animals ranging from bears to amphibians, seasons directly affect the lives of all living creatures. Whether hot to cold, wet to dry, food abundance to scarcity, seasons demand that organisms have the ability to flourish in cyclically changing, yet often unpredictable, environmental conditions.
On average, the earth lies approximately 149.6 million km from the sun. This distance changes as the earth follows its elliptical
Implications for Life on Earth: Everyone has read the story of Goldilocks and the Three Bears. Just as Goldilocks had a preference for the temperature of her porridge, so too, it appears, does life itself. As scientists have found, life has a set of requirements in order to flourish. The most important of these requirements is believed to be the presence of liquid water. So far, all studies and explorations of other planets and moons in our solar system have yet to produce concrete evidence of liquid water. Earth is fortunate to be located in an orbit around the sun that is close enough for H2O to exist as a liquid. Any closer to our sun, and all water would likely evaporate as on the hot planets, Mercury and Venus. Any further away, and we would only find solid water in the form of ice, such as what Mars currently displays. Earth is located at a very special position in our solar system, one that we often take for granted in our busy lives.
Want to know more? Check out the link to explore the most recent exoplanet discoveries of the Kepler Space Telescope.
The presence of a large moon makes Earth a special place in our solar system. Our moon is approximately 27% the size of the earth, but only weighs 1.2% of the earth! That may not seem like much, but it has some major implications for the development of life on Earth. The moon’s gravity acts to stabilize the wobble of the earth’s axis.
Implications for Life on Earth: By smoothing out the wobble of the earth’s axis, the moon acts to stabilize the climate of Earth. This has had a positive impact on the development of life by softening the cyclical temperature extremes experienced over thousands of years. Imagine that. Without the moon, you may not have been here to take this course!
The presence of a large moon also directly relates to the occurrence of tides in all the oceans of the world. Tides are the predictable phenomena of ocean levels falling and rising twice daily. Each day, a coastline will experience two high tides as well as two low tides. The moon has a strong gravitational pull on the earth and tugs unevenly at the land and water. The side of the earth that is closest to the moon experiences more pull than others. This results in the tidal “bulge” seen below. The area displaying light blue represents high tide.
In the animations on this website, you can clearly see that the areas of the earth experiencing high tide are those both on the nearest side to the moon, and the opposite side.
As the moon orbits the earth every 28 days, the tidal cycle predictably shifts in accordance to the position of the moon. Below are pictures taken by the author of this course at Hopewell Rocks, New Brunswick. The Bay of Fundy on Canada’s east coast experiences the world’s highest tidal differences with a peak change of 16m (50 ft) between high and low tide!
Note: Not all tides are equal! You may have come across the terms Spring Tides and Neap Tides. The moon is not the only astronomical influence on our earthly tides. When the moon and sun align as seen in the animation below, we experience especially high tides known as Spring Tides. This is due to the the additional gravitational pull from both the sun and the moon acting from the same direction. When the moon and sun are pulling from perpendicular< to meet at a 90o angle> positions, they act to cancel each other out, resulting in lower than normal tidal differences known as Neap Tides.
Implications for Life on Earth: Tides have tremendous impacts on both aquatic and terrestrial life. Tides transport nutrients necessary for the survival of non-moving life such a corals. They moderate ocean temperatures by directing the flow of trillions of tonnes of seawater over large distances. This allows waters of varying temperatures to mix. Tides are in part responsible for the incredible diversity in coastal sea life such as birds, crustaceans, and mammals, which flock to the emptied sea beds to collect food at low tide. If you have eaten oysters, clams, mussels, kelp (seaweed) lately, you can likely thank fisherpeople that have taken advantage of the collecting conditions only experienced at low tide.
OPPORTUNITY TO CONNECT TO NATURAL HAZARDS! Tidal cycles play a crucial role in predicting how devastating the effects of a hurricane storm surge can have on the natural and human features of coastlines. When hurricanes hit at high tides, the accompanying storm surge is amplified in height as both the forces of low pressure within the storm, as well as the pull of the moon and sun, act to increase the ocean height. Needless to say, many people that live in hurricane-prone areas should always wish that if a hurricane is to hit, it does so at low tide! Check out this article if this topic has sparked your interest about how the moon can make a hurricane more deadly.
The earth does not revolve around the sun following a perfect circle. It travels in what is known as an ellipse.< oval shaped> As discussed in the proximity to the sun section, we spend portions of our year traveling toward (closer) to the sun, and other times moving away (farther) from our host star. The term eccentricity< the amount an orbit of a planet varies from a perfect circle> is used to describe the true shape of a planet’s orbit. The more eccentric a planet’s orbit, the greater the shape varies from a circle. See the diagrams below.
Implications for Life on Earth:The characteristics of the earth’s orbit play a crucial role in the the presence of life as we know it. As was stated earlier, the orbit is very near circular, however just slightly elliptical in shape. This results in relatively consistent inputs of solar energy from our sun throughout the year. As for life on our planet, this consistency has resulted in minimal changes in how much the earth and its atmosphere heat up or cool down throughout the year, making conditions more suitable to life. Over time, the earth’s orbital eccentricity changes on a cycle of 100 000 years. In theory, this long term cycle has the ability to affect Planet Earth as severely as causing ice ages.
Impact events consist of the collision of the earth with space rocks ranging from the size of grains of sand to those several kilometres wide. Each day, many thousands of tiny particles enter our atmosphere and burn up as meteors or what many call “shooting stars.” Occasionally, larger objects ranging in size from basketballs to small cars will collide with Earth. This results in powerful explosions within our atmosphere. One such event, popularly called “The Tunguska Event,” occurred in 1908 in a remote area of Russia known as Siberia. Though no crater was created, this space rock exploded in the atmosphere with the power of 185 atomic bombs. It flattened over 2000 km2 of forest. Eyewitnesses claimed the shockwave knocked them off their feet in a town 35 miles from the explosion. Check out this link to learn more.
When space rocks are larger in size, they may have enough strength to remain intact long enough to strike the surface of the earth. At this point, meteors can unleash a massive amount of energy upon contact, and are capable of creating impact craters. When a meteor strikes the surface of the earth, it is called a meteorite. Seen below is Meteor Crater in Arizona, USA. This crater is approximately 1 km in diameter, 200 m deep, and 50 000 years old. It is believed that a meteor only 20-30 metres wide created this massive scar. The photograph below clearly shows the danger and destruction experienced during and after an impact event.
Luckily for us, the frequency of impacts between the earth and larger space rocks is far less common than that of smaller bodies.The power and destruction found at Meteor Crater truly pale in comparison to that of the most famous impact event in the earth’s history. Sixty-five million years ago, some 50% of all plant and animal life vanished from the fossil record in a very short time period. This event marked the end of the dinosaurs, and countless other life forms the world over. It is hypothesized (with great evidence) that the cause of this mass extinction was the result of an impact event on a massive scale. Found off the coast of Mexico’s Yucatan Peninsula, the Chicxulub Crater is 180 km in diameter. It has been dated to the same time period as the extinction of the dinosaurs. A crater of this size could only be created by an object in excess of 10 km in diameter. An impact of this magnitude would have propelled dust and rock kilometres into the atmosphere. Over time, this debris would be carried around the world by wind currents, eventually blanketing the planet in a layer of dust. The effects of an impact event of this size are similar to that of the large volcanic eruptions such as Krakatoa discussed in this unit.
Though most large asteroids maintain predictable orbits that do not intersect with the earth, scientists are tracking some 15 000 near earth objects and are always on the hunt for more. Still interested? Check this out!
Implications for Life on Earth: It is not hard to make connections between impact events and their effect on life on Earth. We have evidence of repeated large impacts on Earth and can study the fossil record linking their occurrence to large-scale changes to life on our planet. Whether is it the 50% extinction that occurred 65 million years ago, or the Tunguska Event of 1908, we and all life on Earth are at the mercy of astronomical influences. Though we seldom think about it, there are many dangers in the form of asteroids, comets, and other celestial bodies that could force major global changes to life on this planet. You witnessed above what Meteor Crater did to the unpopulated desert of Arizona, but what would happen if the same kind of object struck one of the ever-growing megacities of today? If this question has frightened you into thinking about the effects of impact events on our species and planet, rest assured that impacts the size of the Chicxulub event occur - on average - once every 50-100 million years.
Side Note: Scientists believe that it was the destruction of the dinosaurs by way of impact events, that allowed mammals such as our ancient ancestors to begin their rapid evolution, and transition towards becoming the top predators of Planet Earth! So, on behalf of all humans - thank you, Chicxulub!
Connections to Natural Hazards: Can you think of possible disasters already learned about in this course that could result from a meteor or asteroid striking the earth? Think of both land and ocean impact locations. After considering any connections you can think of, check out this link.