Rome Star Gazers

It is true that over time, the planets not only change their positions, relative to the stars, but they change in their appearance.  They change in how bright appear and they change in how large they appear.

There are two factors that affect a planet’s size and brightness.  For this discussion, we are ignoring the Earth’s atmosphere and particulate matter that is suspended in the atmosphere (clouds, dust, and pollution).  The factors are: how far the planet is from Earth and how much of the lit surface of the planet is facing the Earth.

The further a planet is from Earth the smaller it will appear.  Mars is a classic example of this fact.  During August before last, Mars was as bright and large as it ever gets because it was as close to Earth as it ever gets.  Since Mars is an outer planet, for all intents and purposes, it is always full, that is, we can see its fully lit surface.
One characteristic of an outer planet is, they do not go through phases, like the Moon.  For this discussion, we will say that we can always see its fully lit surface.  So for outer planets, size and brightness is determined by distance from Earth alone.

For the inner planets, the distance from Earth makes a difference but also the phase of the planet is important.  As was just mentioned, the inner planets (Mercury and Venus) go through phases, as they travel around the Sun, relative to Earth.  It stands to reason that the smaller the portion of the lit side of the planet we can see, the dimmer the planet will appear.  The odd thing about the inner planets is, the less we can see of the lit surface of that planet, the closer it is to Earth.  The more we can see of its lit surface, the further it is away from Earth.

If we consider these two conflicting pieces of information, the more of the lit surface we can see the further away it is and less of the lit surface we can see the closer it is, we find it hard to make a hard and fast rule about the planets’ brightness.  Obviously, the further way the planet is from Earth, the smaller it will appear.  To get the actual data about the planets, you can go to magazines such as Astronomy and Sky&Telescope.

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No, the tail does not always trail the comet.  To understand how this can be so, you must first understand what causes the tail of the comet.  First, a comet is composed of two parts, the core and the tail.  The core is a lump of frozen gas, pebbles, dust, and other interstellar stuff, sometimes described as a large, dirty snowball.  This “snowball” travels around the Sun in an elliptical orbit.
That means that the comet travels in a long, narrow oval shaped orbit with the Sun close to one end of the oval.  When the comet is far away from the Sun it travels relatively slowly and is extremely cold and frozen.  As it continues in its orbit, it eventually returns close to the Sun at an extremely rapid rate of speed, swinging around the Sun and whipping back out to the distant regions of the Solar System again.  Some Comets have orbital periods of hundreds, even thousands of years.

Comet Halley

As the comet approaches the Sun, it begins to warm up and give off gases and small particles that form the comet’s tail.  These gases and small particles are slowly pushed away from the comet’s core by “solar wind” or the pressure of light shinning from the Sun. As the comet approaches the Sun, its tail will appear to stream behind the comet like a vapor trail of an airplane.  On the other hand, as the comet recedes back out into deep space, the comet’s tail will appear to lead the comet, as the tail is pushed along by the sunlight.

In any case, the tail of a comet always streams away from the Sun, on the opposite side of the comet from the Sun.  One last point I wish to make here.  It is this debris trail left behind the comet’s passing that produces a meteor shower.  What happens is, the Earth, moving in its orbit about the Sun, happens upon the comet debris trail and cuts through it.  The debris left behind by the comet burns up in the Earth’s atmosphere, producing a meteor shower, as the Earth collides with it.

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Will Mars be so close to Earth that it will appear as large as a Full Moon on August 27th of this year?  This is very interesting because since 2003, this has become a perpetual astronomy question that resurfaces each year in August.   Somehow this rumor keeps popping up each year.  The short answer is “NO!”

Now for the long answer.  Most rumors have a kernel of truth in them.  This is no different.  The fact is, on August 27, 2003, Mars and Earth were as close to each other as they have been in several tens of thousands of years and closer than they will be again for another 60 thousand years.  If you were around in 2003, you may remember that Mars was not the size of a Full Moon.  You may say, well I don’t remember if it was or not.  I submit to you that if it were the size of a Full Moon, you WOULD remember!  I really have no clue as to how that part of the rumor developed.  The diameter of Mars is actually two times the diameter of the Moon, so that can’t be it.

Ok, so how close is Mars now?   Well, let me give you some comparisons.  Currently, Mars is 157 million miles from Earth (1.696 AUs).   On August 27, 2003, it was only 34.5 million miles from Earth (0.3727 AUs).   This means that it is four-and-a-half times further from Earth now than it was back in August ’03.

It is interesting also to note that currently Earth and Mars are getting closer to each other.  In five (5) months (January 27, 2010) Mars and Earth will be in another opposition.  This means that Mars will pass between the Earth and Sun.  This is when Mars and Earth are at their closest point in their orbits around the Sun.  This year is not an especially close approach (0.664 AUs from Earth).  This is roughly 61.6 million miles or 1.78 times further away from Earth than it was back on August 27, 2003.   The opposition of Earth and Mars occurs every 26 months or so.

So now you know.  When someone tells you about Mars, you can say: “Actually that is not true.  The real story is..……”.

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How can the stars be moving in different directions and yet we have star maps that tell us where they are.  If the stars are moving, how can star maps be accurate?

To begin with, “yes, the stars do move relative to each other.”  Some groups of stars move together in space, we call them star clusters.  Massive numbers of stars move in an orbit about the center of the galaxy that they belong.  There is constant turmoil of motion in deep space and the stars are moving very rapidly in that turmoil.

The reason that star maps can remain accurate in all of that motion and change is, the distances involved in the galactic motion are staggering.  Because the distances are so vast, stars would have to move tremendous distances to be observed from Earth.

The truth is, star maps must be updated to compensate for the motion of the stars.  If you are a serious astronomer, the star map updates are critical to your work.  For the rest of us, we can’t see the position changes even when we try.  Now, for those of you who plan to stick around another 50,000 years or so, you will easily be able to see changes in the star positions.  In that length of time, many of the constellations will be unrecognizable, assuming you can recognize them now.

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I was watching the original Star Wars with my sons and noticed that Luke Skywalker was looking at a horizon with two setting suns.  It looked quite strange to see two suns in the sky.  Although we are used to thinking of stars (and suns) coming as individuals because our own Sun is alone but the truth is, this is not the norm. The evidence from observations is that most stars that we see in the sky are parts of multiple star systems revolving around a common center of mass. If there are two stars in the system, we call it a binary star system.

Since multiple star systems are the norm, it is easy to site examples: Castor System in Gemini, Sirius System in Canis Major, Polaris System in the Ursa Minor, and the beautiful Albireo System in Cygnus.

It is important to mention that there is a difference between a visual binary and a true binary system.   A visual binary is a situation where two stars simply, because of line of sight, appear to be together.   The other situation is where the two are actually close together and orbit each other.

Just imagine what our sky would look like with two or three stars orbiting each other.  The orbital configurations and the Earth’s orbit would be quite complicated in deed.  As I ponder it, it may not be possible to have life as we know it on a planet where there are multiple stars (suns).

As you look into the night sky, remember that most of the stars have companion stars that are either too close or too dim to see with the naked eye.

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This is a great question and its answer is not real obvious.  The three assumptions that are made in asking this question are true.  First, in order for a satellite dish to work and have TV reception, the dish must be pointed directly at a satellite.  Secondly, satellites orbit the Earth at a velocity that is specific to their altitude.  Thirdly, there is something special about a satellite that does not appear to move in the sky while at the same time orbits the Earth.

Motion in space is a tricky thing, to say the least; especially when you are talking about orbital velocities and altitude.   There is a paradox when talking about altitude, velocity, and apparent velocity.  The fact is, the faster you go, the higher will be your orbit, but the longer it will take you to orbit the Earth.  So, if you want to go around the Earth faster, you slow down.  If you want to slow your apparent ground speed, you increase your actual velocity.  The reason for this is, as you increase your actual velocity, your altitude from Earth increases and the distance around the Earth is greatly increased.  Since the orbit is higher and distance is much longer, it takes longer to make the orbit.  Therefore, higher the orbit, longer it takes.

To simplify the discussion, I am assuming the satellite is above the equator and is moving in the same direction as the Earth’s rotation.

Now, if we look at the orbit of a satellite and look at the spinning of the Earth, we find something interesting.  Low orbit satellites appear to move forward.  Extremely high orbit satellites appear to move backward because the Earth spins underneath them.  There is a magic altitude where the satellite orbits the Earth at the same rate that the Earth spins on its axis.  This is the geostationary orbit.  Communication satellites are placed in this geostationary orbit so the antenna dishes can be pointed to them and they appear to, as their name suggests, remain stationary.

There is an interesting website that you will want to read on this: http://en.wikipedia.org/wiki/Geostationary_orbit.

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This question is often asked and has several possible answers.  First it depends on what your skies are like and second, it depends on whether or not you are using any optical aid.

To judge how good your skies are, you can look at the Little Dipper.   It is often used as a measure of sky darkness.  Knowing the magnitude of each star and noting which stars you can see, tells you what your limiting magnitude is for that part of the sky.  If we assume you have reasonably dark clear skies and are simply looking with your naked eye, then you can see about 2,000 stars in the sky.

On the other hand, if you have the same skies and are looking through a pair of binoculars, you will be able to see about 20,000 stars.  Finally, if you are using a small to moderately sized telescope under the same conditions of good, dark skies, you can probably see as many as 2,000,000 stars.  In any case, the main thing is to get out and … LOOK!

Hubble Ultra Deep Field

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On Earth the longest-lasting Atlantic tropical storm was “Ginger”.  It worked its way around the open ocean for 28 days in 1971.  The longest lasting Tropical Cyclone was “Hurricane/Typhoon John” which lasted 31 days in 1994.

Now these storms seem quite long until we consider other planets.  In 2008, astronomers observed a storm on Saturn that lasted more than five months!  The collosoal storm of all time is the storm on Jupiter.  I know you’ve heard of it but you might not realize that it is storm rather than a solid landmark.   Of course I refer to the Great Red Spot on Jupiter.  The Great Red Spot was first observed on the surface of Jupiter in the 17th century and has been raging ever since, 400 years later!

In reality, the ’spot’ is a hurricane in Jupiter’s atmosphere. It is the largest known storm in our solar system.  It is unbelievably twice as far across as the diameter of Earth!

Hubble Views Ancient Storm in the Atmosphere of Jupiter

The next time you go out to observe Jupiter, look for the Red Spot with a small telescope.  When you see the Red Spot, try to “feel” the raging 250 mile an hour winds which are whipping around the cloud belts of Jupiter.  Never forget, Astronomy is a study of action and motion!

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That is a great question.  The answer is, the light from the Sun strikes the surface of the Moon and it is reflected back out into space.   We see the reflected sunlight that heads toward the Earth.  There is a technical term, “albedo”, that is a measure of the reflectivity of an object.  To be specific, albedo is a ratio of the incident light and the reflected light.  The greater the albedo, the brighter an object would appear.

When we look at the Moon, we see light and dark areas.  The dark areas on the Moon are the lakes or Maria.  When you look at the Moon through a small telescope or pair of binoculars you can see that these are smooth areas.  These areas are relatively young regions on the Moon that were formed by volcanic flows of basalt.  The albedo of the basalt rock is relatively low.

The lighter regions on the Moon are the mountain areas with old and jagged rocks.  The albedo of the mountain areas is fairly high.
It is also interesting to note that we see all the planets by the reflection of sunlight.  We see the stars by the light that they give off themselves.

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First, we need to understand that the color of light is not actually a property of light but rather how our eyes perceive the wavelengths of light energy that strikes our retina.  That may sound like a lot of mumbo-jumbo but it is an important distinction when talking about star color.

The color of a star is determined by two factors: the temperature of the star light and the sensitivity of our eyes to the various wavelengths of light.  Hotter the star, more blue the star appears.  Likewise, cooler the star, the more red the star appears.  According to star temperature and light color, the temperature that a star would have to have to produce green light is about 10,000 degrees Kelvin.  Now, there are plenty of stars with that surface temperature.  The problem comes in when we begin to talk about light color sensitivity of our eyes.

Star Colors

It turns out that light sensitivity of our eyes in the wavelengths of green light and red light are almost overlapping wavelength charts.  While the wavelengths of the colors are widely varied, our eyes sometimes have a hard time distinguishing them.  I am reminded about color blindness charts.  The tables I’ve seen are used to determine red / green color blindness (i.e. http://waynesword.palomar.edu/colorbl1.htm).  Getting back to star color…  If the sensitivity of red and green are nearly the same, then it follows that this means that stars that are the proper temperature to produce green light produce nearly the same amount of red light.  Evidently, this superimposed red and green light results in light that is perceived in our eyes as white.

This is seen when we look at a Temperature vs Star Color Chart.  The table of star color goes from the hottest stars to coolest stars: blue, blue/white, white, yellow/white, yellow, orange, and red (note- no green).  The green star color that one might expect in the mid-range star temperatures are not seen as green, but white.  This explanation is not intended to be a dissertation on the subject but rather a short thumbnail sketch of a more complex explanation that I encourage you to investigate further.
You can go to article by Philip Steffey in the September, 1992 issue of Sky and Telescope (p. 266), for a more detailed analysis of this event.
In the mean time, there are no green stars but your are encouraged to go out and look for them.

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