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By Juan Ivaldi, November 15, 2009 1:38 am

Quick answer: The seasons occur because of the tilt of the Earth’s axis of rotation.

Each year the Earth travels through space orbiting around the Sun.  As it does so we observe the passing of the seasons.  Although the amount of variation in the seasons depends on where we live, the trends are the same.

During summer the length of the day exceeds 12 hours.  So in summer, nighttime is shorter than daytime.  During summer is when we experience the longest days and shortest nights compared to any other time of year.  The Sun is higher in the sky at midday in summer than in winter.  The combination of longer days and the high Sun explains the heat of summer.

By contrast, winter is characterized by shorter days and longer nights.  Nighttime in winter exceeds 12 hours.  With the Sun lower in the sky at midday, the sunlight striking the ground is less concentrated.  This results in the significantly cooler temperatures of winter.

Another fact is that summer in the northern hemisphere coincides with winter in the southern hemisphere.  Likewise, winter in the northern hemisphere coincides with summer in the southern hemisphere.  In between summer and winter we have the seasons of spring and autumn during which the length of day and night are roughly equal over most of planet Earth.

All these effects are linked to the same cause, the tilt of Earth’s rotation axis.  The rotation axis of the Earth is inclined by about 23½° away from the axis of Earth’s orbit around the Sun.  Like a spinning top, the Earth’s rotational axis points in the same general direction in space.  So as Earth orbits around the Sun, one hemisphere (northern or southern) may be tilted toward the direction of the Sun while the opposite hemisphere tilts away.

Summer in the northern hemisphere occurs when the north pole is the most tilted toward the Sun.  At this same time the southern hemisphere is in winter with the south pole tilted away from the Sun.  Six months, or half an Earth orbit later, the northern hemisphere points away from the Sun, and we observe winter in the northern hemisphere and summer in the southern hemisphere.

There are special dates on our calendars to precisely mark the key locations in the orbit of the Earth which coincide with the seasonal changes.  The summer solstice occurs around June 21 and marks the beginning of summer in the northern hemisphere.  It is also the longest day of the year.  At the summer solstice the north pole is tilted as much toward the Sun as it ever gets.  The winter solstice occurs around December 21 and is the beginning of winter in the northern hemisphere.  It is also the shortest day of the year.  The winter solstice is when the north pole is tilted as far away from the Sun as it ever gets.

It is easy to think of the equinoxes of spring and autumn as the midway points between the solstices.  The name equinox implies equal nighttime and daytime.  At the moment of equinox, the poles are pointing neither toward nor away from the Sun.  This is because the spin axis of the Earth is at right angles to the Sun’s illumination during an equinox.  The vernal equinox occurs around March 21 and marks the start of spring in the northern hemisphere.  The autumnal equinox occurs around September 21 and is the beginning of autumn.

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By Juan Ivaldi, November 1, 2009 11:23 pm

Quick answer:  Yes, this has happened many times over the history of the Earth.  It will happen again but no one knows when.  Fortunately, such big events are extremely rare and unlikely to occur in our lifetimes.

Earth is about 4.6 billion years old.  Throughout this enormous stretch of time, Earth has been impacted by countless solar system objects.  Fortunately, in recent Earth history, major impact events occur many millions of years apart.  So the chance of any of us observing one of these big impacts is incredibly small.  However, much smaller less significant impacts happen more frequently.

There is plenty of evidence for past impacts, although weather and active geology on planet Earth erases most of the obvious impact remains.  This is the reason why impact craters are so rare on the surface of the Earth.  The Moon, by contrast, lacks geological activity and weather.  The long impact history is therefore recorded in the cratered lunar surface for all to see with the aid of a telescope or binoculars.

Among the most famous reminders that impacts can still occur on Earth, is Meteor Crater in Arizona.  This crater is believed to have been formed by the impact of an asteroid about 80 feet across.  The impact is estimated to have occurred around 20 to 50 thousand years ago.  Early human beings were walking the Earth at that time and it is quite possible that early humans witnessed the consequences of this impact event .

To understand where these impacting bodies come from, it is important to review the history of formation of the solar system.  Between 4.6 and 3.8 billion years ago, when the solar system was forming, impacts occurred at a very heavy rate.  This period of heavy bombardment happened as material in the solar nebula clumped into bigger pieces and ultimately impacted the young planets and their moons.  As the planets grew larger the strength of their gravity fields became larger and they swept up ever more of this solar system debris.  Over time, the orbital paths of the planets were cleared out and the rate of impacts in the inner solar system reduced significantly.

Today, there is plenty of left over material from that early time which did not get swept up but stayed in stable orbits around the Sun.  The most important of these debris bands are the asteroid belt, which lies between the orbits of Mars and Jupiter, and the Kuiper belt which lies beyond the orbit of Neptune.

The asteroid belt is composed mostly of rocky bodies also called minor planets.  Because of the greater distance away from the Sun, the Kuiper belt is the cold home of millions of dirty ice balls.  Occasionally one of these Kuiper belt objects gets flung toward the Sun to become a comet.  According to the International Astronomical Union, over 60 million minor planets and comets have been observed in the solar system.

Astronomers using powerful telescopes and cameras are keeping an eye on a subset of about 6000 objects which are on a path of close approach to planet Earth.  They are called Near Earth Objects (NEOs).  About 1000 of these are on a special list of potentially hazardous objects.  At present, one asteroid named Apophis is due to make a close approach on April 13, 2029.  Apophis is predicted to fly by the Earth at a safe distance of about one Earth diameter away.

In 1994 the world observed Comet Shoemaker-Levy 9 slam into Jupiter creating massive impact scars in the atmosphere of the great planet which lasted for months.  These scars were large enough to be visible in amateur telescopes.  The event caused great excitement in the scientific community since no human had previously predicted and subsequently witnessed the collision of two solar system objects.   There was a flood of media coverage for the event.  A wealth of scientific data was collected but the famous collision was a grim reminder to the human race that violent impacts can still happen in the solar system today.

Impacts of a less spectacular scale happen more regularly.  The Tunguska blast which happened over Siberia on June 30, 1908 is believed to have been caused by an asteroid or comet that entered Earth’s atmosphere.  The object exploded high above the ground and never had a chance to impact.  However, the explosion blew down trees for hundreds of miles around the point of entry and carried the sound of the blast even further away.

A more recent event occurred on October 7, 2008 when a boulder sized asteroid named 2008 TC3 was tracked and predicted to enter Earth’s atmosphere over Africa.  It followed the predicted path, entered the skies over Sudan, and disintegrated high in the atmosphere.  In a rare find, a university team recovered fragments of the broken up asteroid.  Scientists are now studying these fragments for clues about the formation of the solar system.  This is a unique and rare opportunity to directly examine pieces of the left over building blocks which formed our solar system 4.6 billion years ago.