Eclipses

Previously, we learned about the phases of the Moon. We saw how it goes from new Moon to full Moon back to new Moon again. We know that during the new phase, the Moon is between us and the Sun. During the full phase, the Earth is between the Sun and the Moon. We also know the apparent diameter of the Sun and the Moon are nearly identical, at about 0.5°.  So how come we don't see the Moon blocked out when its full and why isn't the Sun always eclipsed during the new Moon phase?

The main reason why is the Earth and the Moon do not orbit in the same plane.  The Moon's inclination is about 5° compared to the ecliptic.  Therefore, during most new Moon phases or most full Moon phases, the Moon is not in the same alignment with the Earth and the Sun.  This 5° inclination leads to the eclipses to happen every six months.

 



There are two types of eclipses: solar eclipses and lunar eclipses.  Solar eclipses occur when the Moon crosses in front of the Sun, blocking us from seeing the Sun on Earth.  Lunar eclipses occur when the Moon passes through the shadow of the Earth.  A lunar eclipse (or a solar eclipse) follows two weeks after the associated solar (or lunar) eclipse.  Solar eclipses and lunar eclipses never occur more than 14 days apart.

Lunar eclipses occur during the full Moon phase as the Earth is between the Moon and the Sun.  If the Moon is completely in the shadow, or umbra, of the Earth.  Since the Earth is so much larger than the Moon, the shadow of the Earth completely envelopes the Moon during a lunar eclipse.  Also, this allows a lunar eclipse to occur even if the Earth, Sun and Moon are not completely in line.  If the Moon is slightly out of the umbra and partially in the penumbra, partial lunar eclipses will occur.  From viewing partial lunar eclipses, ancient astronomers could tell that the Earth was round, not flat, because of the shape of the Earth's shadow on the Moon.

Solar eclipses occur during the new Moon phase when the Moon crosses in front of the Sun.  These are only total if the Moon is near perigee and the Sun, Moon, and Earth are completely aligned.  If the Moon is nearer apogee than perigee, a solar eclipse will be annular, or ring-like, because there will be a ring of sunlight around the outline of the Moon.  As with lunar eclipses, if the Moon is slightly off center of the ecliptic, then a partial solar eclipse will occur.

Total Solar Eclipse (NASA)

Annular Solar Eclipse (Wikipedia)

In the future, total solar eclipses will not longer occur because the Moon is moving away from the Earth at a rate of 2 cm/century.  We will learn more about the recession of the Moon later.

Apollo 11

Forty-five years ago today, a human stood on another celestial body for the first time.  Remember that Neil Armstrong, Buzz Aldrin, and Michael Collins weren't sure if they were going to come back.

Also, remember that NASA performed this feat with computers that had the computing power of today's smart phones, but took up a lot of room.

We need to go back.

Our Nearest Neighbor

The nearest celestial body to the Earth is obviously our Moon.  We've already discussed the phases of the Moon and how they relate to the Moon's position with respect to the Sun and the Earth.  In the next few posts, we will learn about the Moon.

For example, how come eclipses don't happen every month? Why is there a supermoon? Where did the Moon come from?  What would life be like if the Moon had never existed or what would life be like if we decided to blow up the Moon (it was actually thought of at one point)?  Why should did we go to the Moon originally, why haven't we gone back, and should we go in the future?

Before we get into these topics, let's learn a little about our Moon. 

• It is the only natural satellite of the Earth and is the fifth largest satellite in the solar system, behind Ganymede, Titan, Callisto, and Io. It is the only celestial object, besides the Earth, that humans have stood on.
• It is approximately 238,900 miles from Earth or about 384,400 kilometers, and it took Neil Armstrong, Buzz Aldrin, and Michael Collins about 4 days to reach it from Earth. 
• It has a sidereal period of 27.3 days and a synodic period of 29.5 days. We will talk about this in a future post. 
• It has no atmosphere to speak of and therefore, no life. 
• The Moon has a radius of 1,737 km (27.3% of Earth's) and a mass of 7.35x10²² kg (1.23% of Earth’s). On the Moon, you would weight approximately 1/6 of what you weigh on Earth. That’s why in videos, the astronauts on the moon always looked like they were bouncing.  It would actually be more difficult to walk normally.
• At one point in its history, the Moon was active. This was likely to the cooling it went under after it formed and its constant bombardment by asteroids early in its life.
• It has an inclination of about 5° to the ecliptic, which I realize now I have never talked about.  To learn about the ecliptic and what it is, click here.

 

A Day on Venus

Venus has a unique day.  Granted, Mercury can have a day where the Sun travels from west to east in the sky, can even set again in the east only to rise again later, but this does not happen every day on Mercury.  Venus day is even weirder.

VENUS HAS A DAY LONGER THAN ITS YEAR



Yes, that is correct. It takes Venus less time to complete one revolution around the Sun than it takes to complete one full rotation on its axis.  Its orbital period is 224.7 Earth days.  Its rotational period is 243.0 Earth days.  Recall that both these are in reference to distant stars.

The orbital period, or sidereal period, of Venus is how long it takes for Venus, the Sun, and a distant star to be in the same configuration.  This is generally called its year.

The rotational period, or sidereal day, is how long it takes for a star to appear at the same longitude in the sky.  For Venus, this is 243 Earth days.  However, the solar day is how long it takes for the Sun to go from noon to noon.  For Venus, this is actually 117 Earth days.  How are these so different?

*This is my awesome artistic skills

 

As you can see, as Venus goes around the Sun, it's rotation is slow enough that it takes just over half a solar year for Venus to go from noon to noon.  However, it has just completed over half a rotation in that time.  Therefore, it takes almost another half a solar year to complete one full rotation on its axis.

 

There is another strange phenomena about the rotation of Venus.  On almost all the planets, the Sun generally rises in the east and sets in the west.  We've already discussed Mercury's strange day and Uranus day is strange as well, but Venus is the weirdest.  The Sun exclusively rises in the west and sets in the east.  We call this retrograde rotation.  If we view the solar system from above (i.e. looking down on the Earth's north pole), all the planets rotate counterclockwise (also called anticlockwise).  Venus, however, rotates clockwise.  

The way this is explained is by describing the inclination of Venus' orbit.  The Earth is tilted 23.5° with respect to its orbital axis.  Venus's inclination is 177°.  Why not 3°?  A 3° inclination would suggest that Venus rotates like all the other planets, counterclockwise (west to east).  By saying Venus has an inclination of 177°, we know that Venus rotates clockwise (east to west).  That is why the Sun rises in the west and sets in the east on Venus. 

 

 

Both the slow rotational speed of Venus and its almost 180° inclination are probably explained by the same thing: early in its creation, Venus was hit but a large planetoid body which caused it to flip upside down and considerably slowed down its rotation.

 

 

*Forgive my horrible drawings, as I am not an artist.  But I feel I should probably start using my own images instead of resorting to Google to find them.

 

The Morning Star and The Evening Star

Have you ever heard the terms "Morning Star" or "Evening Star"?  These terms are actually misnomers.  Neither object is actually a star.  They both refer to planets, either Venus or Mercury.  Today, we will answer why they are referred to either of these terms.

The ancients knew that both Mercury and Venus were close to the Sun.  They could only see them either in the morning or the evening.  This is how they got the name of Morning Star or Evening Star.  When they are the Morning Star, they rise just before the Sun and therefore, are only seen in the eastern sky.  When they are the Evening Star, they set just after the Sun and therefore, are only seen in the western sky.

If the ancients believed that everything in the solar system orbited around the Earth, how did they explain how Venus and Mercury stayed close to the Sun?  In the Ptolemaic model, the epicycles for both Mercury and Venus were connected to the Sun by a line.

Copernican Revolution



 

This line was needed in the model to keep Venus and Mercury close to the Sun in the sky.  Otherwise, in this model, Venus or Mercury and the Sun could be at opposition, which we know is not true.

 

When the Copernican Revolution occured and the heliocentric model came to prominence, the line was no longer needed.  Since Mercury and Venus orbit the Sun and in the model, are closer to the Sun than the Earth, their proximity to the Sun in the sky is easier to explain.

Copernican Revolution



We discussed the Phases of Venus in our last post and mentioned the greatest eastern elongation and the greatest western elongation.  For Mercury and Venus, we use these terms to tell us the point in Mercury's and Venus' orbits when they appear the farthest from the Sun in the sky depending on their position relative to the Sun.

Greatest eastern elongation occurs when Venus (or Mercury) is the farthest east of the Sun.  When either planet is at greatest eastern elongation, the planet is only visible from Earth just after sunset, which means we only see it when it’s in the western sky.  We call this the Evening Star.

Greatest western elongation, however, occurs when Venus (or Mercury) is the farthest west of the Sun.  When either planet is at greatest western elongation, the planet is only visible from Earth just before sunrise, which means we only see it when it’s in the eastern sky.  We call this the Morning Star.

Remember that the terms refer to the planet’s position with respect to the Sun in the sky, and not its location in the sky when it is visible.

By comparing the position of Mercury or Venus to the Sun, we can determing the angle between the planet and the Sun at either greatest elongation.  Venus cannot be any more than 47.8° from the Sun in the sky. At greatest western elongation, Venus rises about 2.5 hours before the Sun and at greatest eastern elongation, Venus sets about 2.5 hours after the Sun.  Mercury cannot be any more than 27.8° from the Sun in the sky.  At greatest western elongation, Mercury rises about 1.5 hours before the Sun and at greatest eastern elongation, Mercury sets about 1.5 hours after the Sun. 

Note that these elongations occur when the planet is at aphelion.  If either planet is at perihelion, the angle is smaller.  For Mercury, greatest elongation at perihelion is only 18° and is visible an hour before or after the Sun rises or sets.  For Venus, greatest elongation at perihelion is 45° which doesn't change its appearance before or after sunrise or sunset by much.

The reason for the huge difference for Mercury's elongations is its relatively elliptical orbit.  Venus' orbit is closer to circular so the angles are much closer.

The Phases of Venus

When Galileo first looked at the heavens with his telescope, little did he expect to show that the heliocentric model was the more correct model than the geocentric model.  As mentioned in the previous post, there were many things that he was able to show just by looking at the sky.  Here, we will focus on looking at the phases of Venus and why the heliocentric model is the correct model of the solar system.
In the geocentric model, Venus orbits the Earth on an epicycle, and must be always close to the Sun in the sky (this will be explained in the next post). 

Copernican Revolution



So what does this show?  It shows that from Earth, we never see Venus "full" or its face being totally illuminated by the Sun.  What we see is Venus either new or in crescent phase.

When Galileo looked at Venus, he observed that Venus also had quarter phases and gibbous phases and assumed (based on his observations), that if we could see Venus, it would be full. 

 

Copernican Revolution

 

Looking at the above image, we see a couple of things.  When Venus is closest to the Earth, it is in what is called inferior conjunction*. 

 

*Conjunction is a term when a planet and the Sun are in the same direction in the sky. If the planet and the Sun are in opposite directions (i.e. 180° away from each other), they are said to be in opposition.  Obviously, based on Venus' location in the solar system, it can never be in oppostion.

At inferior conjunction (E in the above picture), Venus is considered to be in its new phase (as seen from Earth).  The unluminated portion of Venus' surface is facing Earth, much like during the new Moon, we see the darkened face of the Moon.  Venus is between us and the Sun.

At superior conjuntion (A in the above picture), Venus is considered to be in full phase (as seen from Earth), if we could see Venus.  As shown, the Sun is between Earth and Venus.

There are two other locations on Venus' orbit that will be discussed in the next post: greatest eastern elongation and greatest western elongation.  These are the points in the orbit where Venus is the farthest east from the Sun or the farthest west from the Sun as seen from Earth.  These are the "quarter" phases of Venus.

Galileo was able see Venus go through phases as he observed it just before sunrise and just after sunset.  By watching how much of Venus face was illuminated and recording what he saw, he could show that Copernicus and Kepler were right.  Venus did not orbit the Earth; but instead, orbited the Sun.  To the same degree, if you could observe Mercury, you will be able to see Mercury go through the same phases as Venus, and also show that Mercury orbits the Sun.

Galileo Galilei

Galileo Galilei was a famous Italian mathematician and astronomer.  He is well-known for being charged with heresy by the Catholic Church in Italy for teaching and promoting the heliocentric model of the solar system.  He was never officially declared a heretic, but he spent the rest of his life under house arrest.  He was not forgiven for this teaching until 1992 by Pope John Paul II.

Why did Galileo believe that the heliocentric model was correct and not the geocentric model, which had been the official Church canon for centuries?  In one word: telescopes.  He used his telescope to look to the skies and observe some things that he saw.  Based on his observations, he concluded that Copernicus and Kepler were right and that Ptolemy was wrong.

Some of his observations were:

1. Jupiter had moons going around it.  It had always been argued that the only object that things could orbit around was the Earth.  Jupiter had four moons that countered this.  I'll discuss the Galilean moons later.
2. The Moon had a bumpy surface.  The Church believed, as did scientists up to this point, that all celestial objects were perfect spheres.  By observing craters and mountains on the Moon, Galileo showed that this was incorrect.
3. He found sunspots on the Sun.  As mentioned in #2, the Sun was thought to be perfect and had no blemishes.  This was incorrect.
4. The milky strip across the night sky was found to contain many stars.  For that matter, the stars on the "fixed sphere" were found to not be fixed, but moved with respect to other stars.  This will be addressed in a future post.
5. Venus experienced phases.  Based on the geocentric model, Venus always had its darkened face towards the Earth, but when Galileo looked at Venus, he discovered that this was not true.  The next post will go into more detail about this and help explain why the phases of Venus could explain why Venus goes around the Sun and not the Earth.

Heliocentric Model

In the 1500s, Nicolaus Copernicus developed what is now known as the heliocentric model of the solar system.  He took the idea of the Ptolemaic model of the solar system with the Earth at the center and realized how convoluted it was and introduced the idea that the Earth and the planets all orbited the Sun.  The only exception to this was the Moon, which orbits the Earth.

This was a revolutionary idea.  For more than 1500 years, the geocentric model was accepted as fact, and the Catholic Church refused to budge, even calling the heliocentric model heresy.  Ironically, Copernicus was a canon of the Catholic Church who had vowed to stay celibate.  His work, the Commentariolus, was a short 40 page essay outlining his ideas.

1. Celestial bodies do not all revolve around a single point, i.e. the Moon orbits around one point, the Earth and the planets around another point
2. The Moon orbits around the Center of Earth
3. The Earth and planets orbit around the Center of the Sun
4. Parallax to stars cannot be observed because the Earth-Sun Distance is so much smaller than the distance to the stars. In reality, parallax is so small that they couldn't be measured until almost 400 years later.
5. The stars don't move.  The motion we see in the stars is due to Earth's motion.
6. Earth moves around the Sun, which causes the apparent annual motion of the Sun.  Earth rotation causes the apparent motion of the Sun in the sky over the course of a day.
7. Earth's orbital motion around the Sun causes the retrograde motion we see in the planets, i.e. why the planets seem to reverse direction and move east to west in the sky relative to the stars rather than west to east.

However, his model did not get rid of a couple of things.  He still felt that the orbits were circular and because of fluctuations in the observation of the planets, he had to keep epicycles.

Bucknell

 

It wasn't until Johannes Kepler came along that a more modern heliocentric model arrived.  Johannes Kepler was an apprentice of Tycho Brahe, who was a strange fellow and may be featured in a later post, and was able to use the thorough and accurate observations made by Brahe to come up with his three laws of planetary motion.

 

1. Planets have elliptical orbits, i.e. circular or oval and the Sun is at one focus.

   

   Hankwang

    
2. A line connection the Earth and the Sun will sweep out equal areas in equal times.  This was discussed earlier here.
   
   
   
   
   
3. Shashi M. Kanbur, Professor of Physics and Earth Science, SUNY Oswego
4. The cube of a planet's semi-major axis distance equals the square of the planet's orbital period, as long as they are measured relative to Earth.  The distance is measured in astronomical units, AU, which is the average distance from the Earth to the Sun. The period is measured in Earth year's.  This was actually proved by Isaac Newton with calculus and physics.
   
   
   
   
   

 

How do we use this to explain retrograde motion of the planets?

Saint Mary's University, Canada

As the Earth overtakes Mars because it is moving faster, Mars "appears" to be moving backwards.  As explained here, it is like a faster car overtaking a slower car on the highway. The slower car appears to be moving backwards with respect to someone in the faster car.

 

I think this post might raise some questions, and if you do, please leave a comment.  I will try to explain them in a future post.

 

Geocentric Model

The geocentric model of the solar system is a model that has the Earth at the center of the solar system and all the planets, Moon, and Sun orbiting around the Earth.

This model made sense to the ancients who could only see the sky from the ground.  Imagine if you didn't know aby better, it looks as if the Sun, the planets, and the Moon all orbited around us.  We know that only the Moon goes around the Earth. But to an ancient, it would make sense that the Sun and planets went around the Earth.  This is why they created a model called the geocentric model.

 

Let's define some terms:

Deferent - the actual orbital path of the planet, Sun, or Moon around the Earth. Everything moves on these orbits counterclockwise looking down on the North Pole of the Earth.

Epicycles - the smaller circles that are centered on a point on the deferent.  These are used for the planets to help explain retrograde motion in the sky.  Without these, retrograde would be difficult to explain.

There is also a line connecting Mercury, Venus, and the Sun so that they stay in relatively the same location in the sky.  This will be explained later.

Lastly, the Earth is offset from the center to account for the changes in size of the Moon and planets as they "orbited" the Earth.

The most famous geocentric model was produced by Ptolemy in the 2nd century.  This model persisted for 1500 years until Copernicus and later Kepler and Galileo were able to prove heliocentric model, which will be our next post.

Both images above are from ScienceU.com.

The Phases of the Moon

We are going to take a break from talking about Venus to explain about the phases of the Moon.  This background is necessary when we get into the next post about Venus.

The Moon goes through basically eight phases as it orbits the Earth.  These phases are a result of the Moon's alignment with both the Earth and the Sun.  Before we go into the descriptions of the phases, we should define two terms:

The sidereal lunar cycle, or "sidereal month" is how long it takes the Earth, Moon, and a background star to line up.  This period last about 27.3 days.  After another 2.2 days, the Earth, Moon, and Sun are lined up in what is called the lunar month, or synodic month.  This period is approximately 29.5 days.  The difference between these periods is because as the Moon orbits the Earth, the Earth is also orbiting the Sun.

The first phase during the lunar month is called the "New Moon".  Basically, it is when the Moon is between the Earth and the Sun.  The unilluminated face of the Moon is pointed towards the Earth.  However, the new Moon is not completely dark.  Earthshine, reflection of sunlight off the Earth's surface can illuminate the surface of the Moon.

Farmer's Almanac Online

 

The next phase is the Waxing Crescent phase.  Waxing in astronomy means that something is increasing in size.  As the Moon moves around the Sun, a sliver of the Moon on the right limb is illuminated by the Sun as seen from the Earth.  What we see is the crescent shape of the Moon.





 

Farmer's Almanac Online

The Moon is said to be aging as it goes from New Moon to Waxing Crescent.  About a week after the New Moon, the Moon reaches the First Quarter phase because it has reached the end of one quarter of the lunar cycle.  The entire right half of the Moon is illuminated as seen from the Earth.

10 Minute Astronomy



 

The next phase is called Waxing Gibbous.  The illuminated surface of the moon crosses the center of the moon and spills over into the left half of the Moon.  The illuminated portion of the Moon resembles an oval with pointed ends.

Farmer's Almanac Online



Halfway through the lunar cycle, we hit the Full Moon. No, werewolves do not appear during the Full Moon.  People do not get crazy during the Full Moon, though it may appear to be so.  The Full Moon is the phase where the Earth is between the Sun and the Moon and from Earth we see the entire face of the Moon illuminated.

Farmer's Almanac Online

After the full Moon passes, the illuminated portion of the Moon begins to decrease, or wane.  The next phase is the Waning Gibbous phase.  The right limb has a darkened crescent shape, and the left half and a portion of the right half of the Moon are illuminated, opposite the Waxing Crescent phase.



Farmer's Almanac Online

After three weeks in the lunar cycle, the Moon reaches the Last Quarter or sometimes called Third Quarter.  It is called this because either the Moon is at the beginning of the last week of the lunar cycle or the end of the third week of the lunar cycle.

Astropixels



In the final week before the Moon is "reborn" into the New Moon, the Moon goes through the Waning Crescent phase.  Only a sliver on the left limb of the Moon is illuminated. 

Farmer's Almanac Online

 

Here is an image showing the relative position of the phases with respect to the Earth and the Sun.





From Swinburne Astronomy Online

 



Venus' Atmosphere

The atmosphere of Venus is one of the thickest atmospheres in the Solar System, and is by far, the thickest of the terrestrial planets.  It is mainly carbon dioxide (CO2) with 96.5% of the atmosphere made up of CO2.  It has about 3.5% nitrogren (N2) and trace other elements with sulfur compounds beign a major portion.  Compare this to the Earth with 78% N2, 21% O2, and trace other gases (argon being the chief among those gases).

The atmosphere is so thick that the atmospheric pressure at the surface (what we would call sea level on Earth) is 92 times that of Earth.  A cubic meter of air on Earth has a mass of about 1.2 kilograms, or weighs about 10 pounds.  On Venus, the same volume of air has a mass of 67 kilograms, or weighs 600 pounds on Venus (on Earth, that volume of air would weigh 670 pounds).  This weigh is so heavy, that its atmosphere at the surface would squash you flat and kill you, if the oppressive heat didn't get you first.

Besides being oppressive, the heat is the most impressive thing about Venus' atmosphere.  Despite being farther from the Sun than Mercury, its surface temperature is hotter.  Because Mercury has such a thin, if lacking, atmosphere, it does not retain heat well.  With Venus thick atmosphere composed of mostly carbon dioxide, the atmosphere does a great job of retaining heat reflected and emitted by the surface of the planet.  Carbon dioxide a really good job of preventing infrared radiation from escaping into space which in turn heats up the atmosphere.  This lead to a runaway greenhouse effect which increases the heat on Venus' surface.  On Venus, surface temperatures can reach 462°C (864°F) where on Mercury, in sunlight, reaches "only" 420°C (788°F).  Mercury does drop below freezing on the side facing away from the Sun at -220°C (-364°F) because the lack of an atmosphere.

At the same time, it is nearly impossible to see the surface of Venus without some help.  The intense cloud cover does not allow visible light to escape.  On Earth, our clouds are made of water vapor and droplets.  Venus' clouds are hydrogen sulfide and sulfuric acid.  Not easy material for visible light to traverse.  These clouds allow 50% of the visible light to come through and heat the ground, leading to the reflection and emission of infrared light, while the other 50% is reflected into space.  What we see when we look at Venus is the cloud cover.

To see Venus itself, we use radio waves which have long enough wavelengths to travel through the clouds.  The reflected radio waves can then be detected and map the surface.  This was what the spacecraft Magellan did to show us the planetary features.  Any images of the surface of Venus are all false color.

The Surface of Venus

Venus was bombarded by meteoroids in the past much like Earth was.  We know this by looking at Mercury, Earth, the Moon, and Mars.  They were all pelted continuously by asteroids and comets that left behind craters when they impacted the surfaces.  But why don't we see many craters on Venus?

The main reason is volcanism.  There are three objects in the solar system with active volcanism: Earth, Venus, and Jupiter's moon Io.  There is evidence that both Mars and minor planet Vesta had volcanism in the past, but now are dormant bodies.  On Venus, the volcanism is ongoing; this is evidenced by the amount of sulfur compounds in the atmosphere.  The volcanism is continually reshaping the surface of Venus, much like it does on Earth.  On Venus, the volcanism can lead to interesting features.

The first are flat-topped mountains called farra.  On Earth, volcanoes form calderas and spew forth lava which can make the volcano taller, or in most cases, widen the base of the volcano.  Farra, on the other hand, are tall (100 m to 1000 m high), but very wide (20 km to 50 km).

 

 

There are also features called novae.  They get the name from the star-like appearance they take on.  The nova are fractures emanating radially from a central region.

 

There are coronae.  These are concentric circles centered on a central depression.

 

 

 

Finally, there are arachnoids.  These are spiderweb like features that combine the look of novae and coronae.

 

 

Venus also has two main landmasses, similar to continents on Earth.  They are Ishtar Terra, named after the Babylonian goddess of love and Aphrodite Terra, after the Greek goddess of love.  It is on Ishtar Terra where we find the the taller mountain on Venus, Maxwell Montes, 11 km tall (above average elevation, what we refer to as sea level).  Note that Mount Everest is only 8.85 km tall.  The weaker gravity on Venus allowed Maxwell Montes to grow taller.  We will see the largest mountain in the solar system on Mars, with weaker gravity than both Earth and Venus.  Maxwell Montes is named after the physicist James Clerk Maxwell who is well known for his laws of electricity and magnetism.  Using his four equations, he was able to predict the existence of radio waves, electromagnetic radiation with the longest wavelengths.  Using radio waves, scientists were able to determine what the surface of Venus looks like.  Maxwell Montes is one of the four features on Venus not named after a female; mythological or historical.  The other three features are:

• Alpha Regio: a tessera in the southern hemisphere
  
• Beta Regio: a volcanic rise in the northern hemisphere
  
• Ovda Regio: the western portion of Aphrodite Terrra that has a complex surface dominated by a large caldera in the far west
  

All of these features were formed by volcanism.

Next time, we will learn about the atmosphere on Venus.





Venus

 

Venus, Earth's twin, is the second planet from the Sun.  In terms of size, it is the sixth largest planet, larger than Mercury and Mars.  It is 95% the size of Earth and 81.5% the mass of Earth.  If you were to stand on Venus, besides dying, you would weight 90% of what you do on Earth.

 

Venus has a relatively young surface which indicates that it is geologically active.  It has active volcanoes, as evidenced by the amount of sulfur in its atmospher.

 

Venus has two contintents, named after two goddesses of love, the Greek Aphrodite and the Babylonia Ishtar.  It has a large mountian, Maxwell Montes, one of the few features not named after historical or mythological females.

Venus also has a much thicker atmosphere than Earth, even though it is smaller than Earth, as mentioned above.  This thick atmosphere actually makes Venus the hottest planet, hotter than Mercury.

 

When Galileo Galilei first turned his telescope to the sky, little did he know that he would prove the heliocentric model of the solar system, and disprove the geocentric model.  One of his key discoveries was the phases of Venus.  The phases helped show that Venus went around the Sun and not around the Earth as had been previously believed.

The last thing that is strange about Venus, and may be the strangest of them all, is how the Sun transverses the Venutian sky.  One most planets, the Sun rises in the east and sets in the west.  Sometimes, it can go retrograde (but only on Mercury), but on Venus, the Sun only goes from west to east in the sky, rising in the west and setting in the west.

Space Station Idea

This is something I came up with about 8 years ago.  The physics is sound.  However, the engineering is not quite there yet.

Rim Space Station

Mercury and General Relativity

Albert Einstein proposed General Relativity in 1916. In it, he states that time and space re not separate geometric entities, but combined into one entity called space-time. Space-time is not flat, but curved and is heavily influenced by mass sitting in space-time. The larger a mass is, the larger its gravitational influence on space-time.

from Space.com

 

A consequence of General Relativity is that the apside of an object orbiting another object will move as the smaller object moves in its orbit. The apside in our Solar System can be thought of as being similar to the perihelion of a planet relative to the Sun. However, apsides are the point closest to the center of mass of a two-body system, like the Sun and a planet. Since the Sun is much more massive than any planet, we can equate the apside of a planet to the perihelion.

 

What we mean by the precession of the apside is that as the planet moves in its orbit, the planet goes deeper into the space-time well as shown above. This causes the planet's apside to move farther along as it orbits the Sun.

 

This is why full moons vary in size from month to month and why solar eclipses can vary from total, annular, or partial.  The apside of the Earth-Moon system precesses as the Moon orbits the Earth.

 

Since this post is about Mercury proving General Relativity, we should discuss how this is so. Back in 1859, French Mathematician Urbain Le Verrier discovered that Mercury's orbit had some strange anomalies. It was at first thought that the anomalies were due to a planet closer to the Sun than Mercury. This explanation helped in the discovery of Neptune by Le Verrier and two others: John Couch Adams and Johann Galle earlier in the century. But once Einstein's theory came out, the equations describing the precession of the asides calculated the anomalies present in Mercury's orbit.

 

There are other examples of observations proving the Theory of General Relativity. They can be found in the above Wikipedia link.

 

 

 

Mercury's Orbit

Mercury has a unique orbit around the Sun.  It was long believed that Mercury had a 1:1 resonance with the Sun, much like the Moon has a 1:1 resonance with Earth, i.e. the same face is always facing the Sun.  However it was discovered that Mercury has a 3:2 resonance, i.e. for every three complete rotations, Mercury completes two full orbits around the Sun.  This resonance, its proximity to the Sun, and its highly eccentric orbit leads to some strange phenomena.

Mercury, by far, has the highest eccentricity of all the planets (e=0.206). What does this mean?  An orbit that is eccentric means that the orbit is elliptical, or oval. The closer the eccentricity of an orbit is to zero, the more circular the orbit is. For reference, the orbital eccentricity of the Earth is 0.017.

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Eccentricity leads us to mention Johannes Kepler's three laws of planetary motion, specifically Kepler's Second Law:

• A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.

This is just a fancy way of saying the closer the planet is to the Sun, the faster it goes.

Shashi M. Kanbur, Professor of Physics and Earth Science, SUNY Oswego

 

Isaac Newton later proved this with physics and calculus in his three laws of motion,

 

We also know that Mercury is 0.4 AU, on average, from the Sun. Because it is so close, it is by far the fastest moving planet in terms of orbital speed.  And from its highly eccentric orbit, when Mercury is at perihelion, it is moving its fastest.

Before we move on to the funky stuff, let's discuss more about Mercury's sidereal rotation and sidereal period, or its day and its year, A sidereal day is how long it takes a planet to rotate once on its axis completely, or how long it takes a star to appear at the same spot in the sky.  For Earth, this is about 23 hours and 56 minutes.  On Mercury, it takes about 58.5 Earth days.  A sidereal year is how long it takes to complete one complete orbit around the Sun.  Earth takes about 365.25 days to complete one sidereal year. Mercury has a sidereal year of approximately 88 days.  So for every three sidereal days on Mercury, Mercury completes two full orbits, hence 3:2 resonance.

But because of its unique position in the solar system, funny things happen on Mercury. As Mercury approaches perihelion, its orbital angular velocity increases.  At around four days before perihelion, the orbital angular velocity equals the rotational angular velocity. When these two angular velocities are equal, the Sun appears stationary in the Mercury sky.  As Mercury gets closer to perihelion, the angular orbital velocity increases and becomes larger than the rotational velocity. The Sun then appears to move backwards in the sky!  In some cases, it is possible that the Sun could actual set in the east.  At perihelion, the orbital angular velocity begins to decrease. Four days after reaching perihelion, the orbital rotational velocity once again equals the rotational orbital velocity. The Sun again appears stationary then again begins its apparent movement to the west.

Another interesting fact about Mercury's orbit, is that the time from true noon to true noon, what we call a solar day, is larger than a Mercury year. One Earth, our solar day is 24 hours. On Mercury, it is 176 Earth days, or two Mercury years.  Mercury has one of the slowest rotational angular velocities in the solar system, comparable to that of Venus, which is a topic for later.

NOAA

 

In conclusion, because of its proximity to the Sun, its highly eccentric orbit, and its tidally locked 3:2 resonance with the Sun, strange things happen in the sky of Mercury.

 

Evolution vs. Creationism

Here is a link to a post I made in January of 2006 where I give my thoughts on evolution vs. creationism.

Creation vs. Evolution

Barren Mercury

As mentioned before, Mercury is a planet.  But it is unique in our Solar System in that it is the only planet without any type of atmosphere.

There may be a couple of reasons for it:

1. It may be too small to hole an atmosphere.  Mars is the second smallest planet but has a very thin atmosphere.  Because of their small sizes, the escape velocity for the planets are relatively small.  Most gas molecules have a relatively fast speed when moving about in the atmosphere, so they could escape easily.  However, this explanation may not be true. Callisto, as you recall, is similar in size to Mercury. But it does have a thin atmosphere.  The next bullet point is probably the correct explanation.
2. Mercury is extremely close to the Sun, approximately 0.4 AU out.  The Sun has a very active solar wind and that solar wind is extremely hot.  Possibly in the early formation of the Solar System, the primordial solar wind stripped Mercury of its atmosphere, which would have likely been very tenuous to begin with.  If Mercury had an atmosphere, it would have been similar to that of Callisto, with probably and composition similar to that of Mars, mostly carbon dioxide.

Next time, we will learn about Mercury's strange orbit.

Lopsided Mercury

Generally, planets will be wider at their equatorial region than at their poles. Being flatter at the poles of a sphere creates what is called an oblate spheroid.  However, Mercury is strange.

There is a large crater on the surface of Mercury called the Caloris Basin or Caloris Planitia .  It is a large impact crater on the surface that was formed about 4 billion years ago.  The impact was so great, that it is believed that seismic waves from the original impact pushed up the area directly opposite the Caloris Basin.  This area is on the antipode of the impact crater and is referred to the Chaotic Terrain, or my favorite, Weird Terrain.  There are relatively few impact craters in this area which tells us that the area is relatively young.

Because of the uplift on the antipode of the Caloris Basin and the Basin itself, the radii from the center to the Basin and to the antipode are not the same.  So the planet is a little lopsided.

NASA.gov

 

NASA.gov

There is also a series of concentric rings around the Caloris Basin, much like any crater in the solar system.  These rings were created from ejecta from the impact.

And here are craters on Mercury that have a strange shape:

Space.com