Rome Star Gazers

The Eta Aquarid Meteor Shower is produced by the remnants of Halley’s Comet.  Each year, as the Earth orbits the Sun, it passes through the debris trail of the comet producing the meteor shower.  Over time the debris trail has spread widely and the meteor shower is reported to be active from April 19 through May 28.  During the first week in May, if you are in ideal conditions, you may see as many as 30 meteors per hour. On the evening and early morning of May 5/6, the peak of the shower will produce as many as 60 meteors per hour.

Since the Moon is not very favorable for early evening observing, I suggest you get up early Wednesday morning and observe from 4 a.m. until sunrise.  This will give you the best advantage for seeing quite a few meteors.

When you go out, wear plenty of clothing, remain comfortable and watch the skies.  Remember, you do not need a telescope to view meteors.  You just need a good dark, safe location away from city lights.

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Wednesday morning, before sunrise, we have a chance to see a beautiful meteor shower.  The Lyrid Meteor Shower is caused by the Earth passing through the dusty tail of Comet Thatcher (C/1861 G1).  The grains of sand strike the Earth’s atmosphere and vaporize producing brilliant streaks of light, what we call a meteors or shooting stars.

With the Moon nearly New, the conditions are perfect for viewing this meteor shower.  There are only three things we need to see the meteors.  First, the sky must be clear, lately this has not been the case.  Secondly, you need to go to a fairly dark location to view them.  Lastly, you need to get up, out of bed, and go into the night to see them.  I feel this year may be just the year to see some pretty bright and exciting meteors.

For a good website to show you where they originate and some additional information go to:

http://spaceweather.com/meteors/lyrids/lyrids.html

In preparation of your meteor watching party, dress warmly, take along a nice folding chair or blanket to lie on.  It would not hurt to bring along some friends and hot chocolate.  This is a great time to bring a friend who might not be as familiar with the sky as you are so you can introduce them to the wonders you have discovered.  You can also bring some binoculars to view the sky between meteors.  Binoculars will not help you to see any meteors; they come and go in the blink of the eye but they will give you something else to observe.

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What is a Valentine’s Day Star, you ask?  Well, a Valentine’s Star is a big red star that is shining brightly in the night sky, just for you and your sweetie.  Astronomers classify these stars as Red Giants.  They are stars that have advanced in age and are consuming their outer layer of Hydrogen atoms, giving off a beautiful red glow.  There are several of them in the night sky for you to pick from; just waiting for you to point them out to your sweetie: Aldebaran, Arcturus, and Betelgeuse are among my favorites.

If you want to take your sweetie out in the early evening to show them the star that you have selected for them, then Aldebaran or Betelgeuse are your best options.  Aldebaran is the brightest star in Taurus.  You will find it almost directly overhead in the “Vee” of Taurus at 9:00 p.m. EST.   Betelgeuse is the brightest star in Orion.  It will also be high on your meridian at this time of the early evening.  Looking at Orion, Betelgeuse is the hunter’s left shoulder star.

If you and your sweetie are early risers then Arcturus is your star.  By 5:00 a.m. Arcturus, the brightest star in Bootes, will be very high on your meridian and it should be pretty easy to spot from its bright reddish glow.  However, my all-time favorite is CA116 in Cancer, the Crab, but as precious as it is, it is hard to find.

So on Valentine’s Day this year, take your sweetie out under the stars and give her (or him) a Valentine’s Day Star; it will make for a heavenly experience.

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If it is not the brightest star, what is so special about the North Star?

Most people are surprised to learn that what you heard is correct, the North Star is not the brightest star in the night sky.  In fact, Sirius (the Dog Star) is the brightest star.  The following website shows us that the North Star (Polaris) is actually the 48th brightest star in the sky: http://www.cosmobrain.com/cosmobrain/res/brightstar.html

So, if the North Star isn’t so bright, just what is so special about it?

The answer is given in its name, North Star.  To explain we need to remember that the Earth spins on it axis one complete turn every 24 hours, a day.  You can see the spinning action of the Earth by watching the Sun, Moon and stars move from east to west over a period of hours.  If you watch more closely, you will begin to notice that stars that are in the northern sky do not seem to move as much as the stars in the southern sky.  As the Earth spins on its axis, we find that the star that resides directly over the Earth’s axis does not appear to move in the regular procession of all of the other stars.  To get a wonderful time-lapsed picture of this phenomenon go to the following website: http://antwrp.gsfc.nasa.gov/apod/ap060915.html

By Author: John Elder

Notice that all of the stars form concentric circles around the North Star.  Now, as if that were not enough, we will learn that the North Star is not really just one fixed star.  In reality, it is any star that the Earth’s axis points toward in the northern sky.  Right now, the North Star is Polaris but in the past, it was other stars (Thuban in Draco, Vega in Lyra, and Alpha Cephei in Cepheus).

What happens is, as the Earth spins, its axis also slowly rotates like the axis of a toy top.  To put it another way, it has a slow wobble.  This wobble or precession, as it is formerly called, takes about 26,000 years for one cycle.  That means starting today, the North Star is Polaris, in 5,000 years, it will be Alpha Cephei, in 12,000 years, it will be Vega, in 21,000 years, it will be Thuban, and 26,000 years from now it will be Polaris again.  Now, as you can imagine, the time we are talking about is so great we can see very little change over the period of a single lifetime.

For many years, the North Star has been used as a navigational aid to travelers because it does not move in the sky during the daily rotation of the Earth.  It is a very useful fixed point of reference in the sky.

Many stories and mythology have been told explaining its fixed, stationary position.  One of the most famous stories, which explain why the star is motionless, is a Native American myth.  According to the story, a brave son, Na-Gah, tried to impress his father by climbing the tallest cliff he could find.  Through difficult conditions, he persisted until he found himself at the top of a very high mountain.  The mountain was so tall that Na-Gah looked down on all the other mountains.
Unfortunately, there was no way down.  When his father came looking for him, he found Na-Gah stuck high above.  Not wanting his son to suffer for his bravery, he turned Na-Gah into a star that can be seen and honored by all living things.

To find Polaris in the sky, locate the Big Dipper and look northward from the two stars at the end of the bowl.  This will lead you directly to Polaris (North Star).  Polaris is in the constellation Ursa Minor and is the last star in the tail of the Little Dipper.

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This is a great question for several reasons.  To begin with, the Horsehead Nebula, in Orion, is considered by astronomers to be the closest nebula to our star, the Sun.  Secondly, it is a beautiful nebula and looks just like its namesake, a horse’s head.  Finally, this is a good question, because it is a similar question that many stargazers have asked over the years: “How would something look if I were somewhere else?”

Close!

To begin with, we need to understand that a nebula is a huge collection of dust and gas.  The etymological root of “nebula” means “cloud.”  The cloud is large enough that it may have enough collective mass to begin to collapse in on itself, due to its gravity.  When this happens, stars and solar systems are created.  We often call large nebulas star nurseries, because that is where stars are born.  The Eagle Nebula and the Orion Nebula are two prime examples of star forming regions in the night sky.

Now that we know what a nebula is, let us consider the different types of nebulae.  We can describe most nebulae as “diffuse nebulae.”  This means that they are extended and contain no well-defined boundaries.  In visible light, these nebulae may be divided into emission nebulae and reflection nebulae.  The distinction between emission and reflection nebulae depends on how the light we see is created.  Emission nebulae contain ionized gases that emit light.  In contrast to emission nebulae, reflection nebulae do not produce significant amounts of visible light by themselves but instead reflect light from nearby stars.

Getting away!

The Horsehead Nebula is an example of a dark nebula.  Dark nebulae are similar to diffuse nebulae, but we cannot see their emitted or reflected light.  Instead, we see them as dark clouds in front of stars that are more distant or in front of emission nebulae.

Now to the question at hand: “What would we see if we were in the middle of a nebula?”  We must remember first that when we look at a nebula from a great distance away, we are seeing a collection of light from the entire nebula all at once.  This is a hugely vast area from which the light originates.  If we were in the middle of a typical nebula, it would be so diffuse and thin that we would not be aware of its presence.  It would take very sensitive and elaborate equipment to detect its presence.

I suppose that if we were at the far edge of such a nebula and we looked though the long axis of the nebula, thereby looking through a great deal of depth, one might be able to detect a faint haze.  However, it would not appear to be the thick cloud that one might expect.  A good discussion on this matter was filmed by “Thebadastronomer”.  In this film clip, he discusses this very topic.  To view this presentation, go to http://www.ustream.tv/recorded/252198.

Where did it go?!?

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Venus: Why So Bright?

I’ve noticed, as most folks have, that Venus is very bright.  Why is Venus so bright?

Venus is one of the brightest objects in the night sky. Venus can often be seen within a few hours after sunset or before sunrise as the brightest object in the sky (other than the moon). It looks like a very bright star. It is for this reason that it is often called the Morning Star or Evening Star.

When you “Google” the question, “Why is Venus so bright?”  The answer you always seem to get is: Venus is the brightest planet because it has the highest surface reflectivity.  That is to say, its thick clouds reflect most of the sunlight that reaches it, about 70% reflects back into space.  The other factor that is often mentioned is that Venus is the closest planet to Earth.  If Venus were deep in our solar system, then the reflectivity would be overcome by the remote distance.

As I ponder this question, I wonder if the size of the planet and the distance it is from the Sun and Earth may also have an effect on its brightness in our night sky.  The answer is, of course they do.  If the reflectivity of all of the planets were identical then the distance from the Sun and Earth and their relative size would definitely affect the planet’s relative brightness.

I created a spreadsheet which listed each planet and its relative distance to the Sun and its relative diameter (see attached).  We know that the surface area of a circle with a diameter of “d” is equal to Pi times ½ d squared: (area = π ½ d2).  Therefore, everything else being equal (reflectivity and distance), the larger the planet, the brighter it would appear.  In fact, if it had twice the diameter, it would reflect four times the light or 2 squared.

Now we have the relative size of the planets computed for brightness, we need to figure in the relative distances from the Sun to the planet and then the planet to the Earth.  This is where we must consider the way light radiates out from an object.  In Physics we learn the light radiates out in all directions, in a sphere.   Calculating the area of a sphere at a certain radius “r” we find that it is equal to four times pi times the radius squared (area = 4 π r2).  This appears similar to our surface area formula but this time, greater the radius (distance from the sun) the dimmer the planet will be by squared the radius ratio.  That is to say, if a planet (A) is three times further away than planet (B), planet A will be one-ninth (1/9) as bright as planet B.

Given this notion, I took the relative distances from the planet to the sun and calculated the relative sunlight that struck the planet’s surface.  Taking into consideration the planets relative size, I calculated the amount of sunlight that would be reflected from its surface; the further out the planet, less sunlight that would strike it by the relative distance squared.  Next I took the amount of sunlight reflected from the planet and calculated the amount that would head to Earth, using a mean relative distance from the planet to the Earth.  This relative distance will also follow the inverse squared law with respect to the amount of light that would reach Earth.

When all was said and done, I found that Venus is the brightest planet when only considering distance and disc diameter, ignoring relative reflectivity.  Roughly figuring, I calculated that the brightest would be Venus by 8 times brighter than Mercury.  Mars would be third and Jupiter would be fourth.  But as we know, the reflectivity of the planet’s surface or atmosphere has a huge effect as well.  In conclusion, Venus is the brightest planet because of its reflectivity, size and relative distance from the sun and earth.

Comment: If anyone thinks they see an error in my logic or analysis, I would love to hear your comments and thoughts – please be specific.

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To put is another way: do they have an effect on each other’s orbits?

Last week, we learned that it is common for moons to become “tidal locked” with their parent planet and rotate on their axis at the same rate they orbit the planet.  As such, they always show the same surface to the planet.
In addition to being tidally locked to the parent planet, many moons become locked to each other.  When this happens, the moons begin to orbit the parent planet in a resonance or cyclical pattern.

The closest three of the Galilean moons of Jupiter are an example of this resonance pattern: Io, Europa, and Ganymede.  The three moons are in a 1:2:4 resonance of each other, respectively.  That means, that Ganymede takes exactly 4 times longer to orbit Jupiter than Io and Eurpoa takes twice as long as Io to orbit the planet.
Furthermore, this resonance pattern is not unique in our solar system.  The three pairs of moons around Saturn (Mimas-Tethys, Enceladus-Dione and Titan-Hyperion) interact gravitationally in such a way as to maintain stable relationships between their orbits.  The period of Mimas’ orbit is exactly half that of Tethys, they are thus said to be in a 1:2 resonance.  The orbits of Enceladus-Dione are also 1:2 and the Titan-Hyperion pair are in a 3:4 resonance.

With the tidal locking and resonance of moons, it is interesting to note how “our” understand of the heavens changes.  When each of us first began to look into the night sky, all seem peaceful and static.  Then we began to notice that the skies change.  Everthing seemed to be erratic and random.  The more we studied and observed, the more we begin to detect regular patterns and see that, frequently, there is a nice dynamic action going on in the heavens; that there seems to be some kind of plan unfolding.

One moon is locked, the other is not!

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Why do we see it, and do other planets have similar apparitions in their moons?

The “Man in the Moon” is the figure that people have imagined over the centuries resembling a human face, head or body.  This image is generally observed during the bright disc of the full Moon.  The image is formed by the blend of dark and light areas of the surface of the lunar surface.   For more about this go to: http://en.wikipedia.org/wiki/Man_in_the_Moon.

The reason that people have observed this figure in the Moon is that the Moon orbits the Earth at the same rate it turns on its axis.  In this way, we always see the same side of the Moon.  Until humankind sent a spaceship around the Moon, no one had ever seen the back side of the Moon.

This effect, where the period of rotation about an objects axis is the same as it period of orbit around a primary planet is said to be synchronous rotation.  As mentioned earlier, this implies that the satellite always keeps the same hemisphere facing its primary (e.g. the Moon). It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite’s motion while the other (trailing) one always faces backward.

What is really interesting, the Moon is not the only moon that behaves in this way.  In fact, most of the satellites (moons) in the solar system rotate synchronously around their respective planet!

The reason that the moons orbit their respective planet is due to “Tidal Locking”.  Tidal locking occurs when the gravitational gradient makes one side of an moon always face another; for example, one side of the Earth’s Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This synchronous rotation causes one hemisphere constantly to face the partner body.

Usually, only the moon becomes tidally locked around the larger planet, but if the difference in mass between the two bodies and their physical separation is small, both may become tidally locked to the other.  This is observed in the case between Pluto and Charon (its moon).  What we observe between them is, they both show the same side always.  If you lived on Pluto, you would always see Charon in the same part of the sky, it would never move.  The same goes if you lived on Charon, Pluto would always be in the same part of the sky.  If you lived on the back side or either, you would never see the other.

Just how common is the synchronous rotation of a moon about a planet in our solar system?  Of all of the moons in our solar system, of the ones we know their precise orbital information, and there are more than a hundred, there are only (4) four that are not tidally locked with their parent planet: Himalia and Elara around Jupiter and Hyperion and Phoebe around Saturn!

So, in answering the original question: Do other planets have an image like the “man in the moon”?  The answer is, “perhaps yes”.

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If you have been fortunate enough to see through clear skies in the early mornings and looked at Saturn, you will have noticed that the rings of Saturn are not at large as they were a few months ago.  This shrinking of the rings is normal.  This is caused by our angle of view of the rings from Earth as Saturn orbits the Sun.  The tilt in Saturn’s axis (or axial tilt) is 26.73 degrees.

• In astronomy, axial tilt is the inclination angle of a planet’s rotational axis in relation to its orbital plane. It is also called axial inclination or obliquity. The axial tilt is expressed as the angle made by the planet’s axis and a line drawn through the planet’s center perpendicular to the orbital plane.

  
  

Saturn is about 9.5 times further from the Sun than Earth and it takes Saturn about 29.5 years to orbit the Sun (one Saturn year).  During a Saturn year (one orbit around the Sun), there are two times when the Earth is in the plane of Saturn’s equator and ring system.  There are also two times during each Saturn year when Saturn is tilted toward or away from the Earth and we can get the best view of the ring structure.  The last time Saturn had a maximum tilt from Earth was on March 7, 2003.  Since then, it has slowly been moving to its current perspective.  As Saturn continues to orbit the Sun, the ring system will continue to level out until, on September 4, 2009, we will be in the plane of Saturn’s ring system and they will disappear for about three months.  In astronomy, this is called the “ring plane crossing”.  We, here on Earth, have enjoyed a southern hemisphere exposure of Saturn for the past 15 years or so.  After we pass through Saturn’s ring plane again, our view will from a northern hemisphere perspective.

Huygens explained that the rings around Saturn are difficult to view, because they are very thin. Every fourteen to fifteen years the Earth moves into the same plane as the ring system. If you tried to observe Saturn from Earth at this time, Huygens said, you would be viewing the outer edge of the ring head-on, making it virtually invisible. This explained why Galileo was unable to see “the handles” around Saturn in 1612. It was a year in which Earth passed through Saturn’s ring plane.

There are several planetary alignments that cause Saturn’s rings to be invisible to Earth observers. One of these is when Earth passes into the Saturn ring plane. A similar effect occurs when the Sun passes through Saturn’s ring plane and when the Sun and Earth are on opposite sides of the ring plane. The next Saturn ring plane crossing will occur in August and September of 2009. Throughout history ring crossings have been the best times to discover new moons around Saturn.

I’ve tried to use the plane-crossing table to estimate the maximum tilt dates of Saturn’s rings from Earth.  I also happen to have the date for the last southern exposure date from earlier reports, March 7, 2003.  My estimation for the northern exposure maximum ring angle is in December 2016.

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