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By Juan Ivaldi, November 29, 2009 7:24 pm

Quick answer: What we see as the lunar phase is the portion of the Moon which appears illuminated by sunlight.  The angle of illumination changes as the Moon orbits around the Earth, thereby changing the phase.

The Moon is about 240 thousand miles away from the Earth.  As the Moon orbits around the Earth, we see a complete cycle of phases taking place every 29½ days.  It is easy to understand why the word “month” and the word “moon” have a common root.  The lunar cycle has been recognized since the time of ancient civilizations and became an important element in the construction of calendars.

When we look at the Moon, we can observe that the portion which appears illuminated changes throughout the course of a month.  As the Moon orbits the Earth, the angle of the Sun’s illumination changes and more or less of the Moon appears to be lit up.  The progression has a familiar pattern which repeats in nearly the same way each cycle.

At the very beginning of each lunar cycle, the Moon is new which means it appears relatively near the Sun in the sky.  During the new phase the Moon is not visible since the part of the Moon facing the Earth is in shade.  About two weeks after the new phase, the Moon appears in full phase in the sky.  During the full phase the Moon appears located opposite the Sun in the sky. This is why a full moon can often be seen rising in the east just as the Sun sets in the west. A full Moon appears as a fully round disk since the portion facing us is completely illuminated.

The other two important phases are called “quarters” which fall exactly midway between the full and new phases.  During a first quarter moon the disk of the Moon appears half illuminated.  The word “quarter” reminds us that the Moon is one quarter of the way through its cycle from either the new or full phase.  Observers may sometimes get confused by the fact that during quarter phases the moon appears half lit.  To avoid confusion it helps to remember that the full moon phase is the halfway point in the lunar cycle.

By observing the Moon on different days we can see the combined effect of the lunar orbit and the changing illumination.  The orbital motion of the Moon causes changes in where we see it in the sky and at what times.  That same motion is what changes the illumination angle of the sunlight from our viewpoint.  So the lunar phase and position are intimately linked.

A common misconception is that the shadow of the Earth cast on the lunar surface causes the phases of the Moon. Of course this is not true at all.  On rare occasions the shadow of the Earth does fall on the Moon.   This is a special and beautiful occurrence known as a lunar eclipse. Lunar eclipses are a completely different phenomenon from the regular observation of the lunar phase.

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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 5, 2009 7:38 pm

Quick answer: Stars shine because they are hot.

Stars shine because their surface temperatures are very hot.  The temperatures of stars are so high that they defy human comprehension.  The Sun has a surface temperature of about 5,700°C.  Many stars visible to the unaided eye in the night sky are known to have surface temperatures which range from around 3,000°C  to 40,000°C.   The lowest temperature stars appear red in color whereas and the highest temperature stars are bright blue.

The incredible temperatures of stars are much hotter than what is generated in household ovens, stove tops, or wood fires.   Even volcanic lava is not as hot as the surfaces of stars.  However, temperatures of 20,000 to 30,000°C  can occur naturally within lightening strikes.  Man can artificially generate star-like temperatures in specially engineered and relatively small environments.  Examples of those environments are halogen arc lamps, camera flash bulbs, arc-welders, or plasma science apparatus.  A tungsten filament used in household light bulbs can reach temperatures just over 3000°C.

The surface of stars visible to observers is called the photosphere.  The photosphere is a spherical shell where light can escape away from the star.  Just beneath the photosphere a star is opaque.   Astronomers can estimate a star’s temperature by measuring the amount of light at different colors in the spectrum of starlight.  The spectral signature obeys certain physical laws which relate the temperature of the photosphere.

The process of nuclear fusion is the energy source that keeps stars shining.  Deep within the core of a star, temperatures can reach millions of degrees.  The initial source of heat is the crushing weight of the star’s great quantity of hydrogen compressed by the force of gravity.  When core temperatures reach about 10 million degrees, hydrogen fusion occurs spontaneously.

The great energy released in the core is exchanged with the intermediate layers within the star.  Eventually the energy reaches the photosphere where it can be released as starlight.  Those photons which fly away from the star begin a journey traveling the vast distances of interstellar space.

Some of the photons released from distant stars reach our man-made telescopes on Earth today.  The journey of these photons is ended when they are captured by the retinas of human eyes or by the detectors in astronomical cameras.  Some of these photons are known to have traveled for billions of years through space.

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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.